1911 Encyclopædia Britannica/Reproduction
REPRODUCTION, in biology, the generation of new organisms from existing organisms more or less similar. It is a special case of growth, and consists of an increase of living substance in such fashion that the new substance is either set free as a new individual, or, whilst remaining attached to the parent organism, separated by some sort of partition so as to have a subordinate individuality. Y. Delage has distinguished as multiplication those cases in which the new individual arises from a mass of cells which remain a part of the maternal tissues during differentiation, reserving the term reproduction for those cases in which the spore or cell which is the starting-point of the new individual begins by separating from the maternal tissues; but the distinction is inconvenient in practice and does not appear to carry with it any fundamental biological significance. The general relation between parent and filial organisms is discussed under Heredity and Embryology; many of the details of the cellular processes are dealt with under Cytology and the modes of reproduction exhibited by different kinds of animals and plants are treated of in the various articles describing individual groups. Finally, some of the special problems involved are discussed under the heading Sex. As reproduction is a general biological phenomenon, its manifestations should be dealt with simultaneously in the case of animals and plants, but many of the special details differ so much that it is practically convenient to make two headings.
Reproduction of Animals
A. Asexual.—Many animals possess a more or less limited capacity to repair portions of the body that have been accidentally removed (see Regeneration), and this capacity may be so extensive that, if the whole body be cut in pieces, each portion may grow into a new organism. Such a mode of artificial propagation, familiar in horticultural operations, has been made use of in such animals as sponges, and has been performed experimentally in hydroids and some worms. In many Protozoa asexual reproduction by simple division is a normal event. In Coelentera it is common, the plane of division usually passing through the long axis of the body, as in Actinians and many Hydroids, or being horizontal, as in the repeated divisions by which medusae are produced from an asexual polyp; the new individual may separate completely, or serve to build up a colonial or compound organism. In some Turbellarians (Microstomum) and Chaetopods (Syllis, Myrianida, Nereis, Eunice viridis (the palolo-worm of Samoa), asexual reproduction occurs in a form that is partly fission and partly budding; portions are constricted transversely or laterally, very much smaller than the whole animal, and these grow out into new animals which may separate or remain attached in chains. In Salps, chains are formed sometimes by transverse constriction, sometimes by budding. True budding is much more common than fission; it occurs in Protozoa, Coelentera, Sponges, Polyzoa, Tunicates and some Flatworms and Chaetopods, the bud being a multicellular portion of the tissues which is partly or completely separated from the parent before it proliferates into the new form. In various larval stages of many animals, asexual reproduction by fission or budding may be produced experimentally or may occur naturally. It has been suggested that cases of identical twins in vertebrates and many monstrous forms, including even dermoid cysts, are due to embryonic asexual fission or budding. The artificial subdivision of young embryos has been performed successfully by several investigators (see Heredity). In Lumbricus trapezoides the gastrula stage of the embryo divides and each half produces a complete individual; and multiplication by budding is common at various stages of the life-history of many parasitic worms. Spore formation, or cellular budding, appears to be limited to the Protozoa amongst animals.
B. Sexual.—Apart from the special and probably secondary cases presently to be considered under the subheading parthenogenesis, sexual reproduction or amphimixis may be defined as the production of a new organism from a zygote, and a zygote may be defined as the cell resulting from the conjugation of two gametes or sexual cells derived from the specialized reproductive tissue of the parent or parents. In asexual reproduction by spore formation, the spore proliferates without the aid of another spore; in true sexual reproduction the gametes may be regarded as special kinds of spores which appear in two forms, the egg-cell, ovum or female gamete not proceeding to proliferate into a new organism until it has been stimulated by partial or complete fusion with the other form, the spermatozoon or male gamete. The act of fusion or conjugation in question is usually spoken of as fertilization, and the zygote, or starting-point of the new organism, is the fertilized egg-cell. Among protozoa and the lower plants there occur a series of forms of conjugation leading towards the specialized form characteristic of the sexual reproduction of higher animals. The conjugation may be isogamous, that is to say the conjugating cells may be actually or at least apparently indistinguishable. The fusion between the cells may be complete, or may concern only the nuclei. The conjugation may be followed by reproduction, or may apparently have no relation to reproduction. In true sexual reproduction the conjugation is heterogamous, i.e. the gametes are unlike; the fusion is chiefly nuclear, and the process is the prelude of the development of the zygote into the new organism.
In all the Metazoa the gametes arise from special reproductive tissues which are supposed to contain (see Heredity) the reproductive material or germ-plasm. In the lower (or simpler and possibly degenerate Metazoa) the reproductive or germinal tissue consists of a few cells, sometimes in a group, sometimes scattered and sometimes migratory; in the vast majority of the Metazoa the germinal tissue becomes aggregated in distinct organs, of which those that give rise to ova or female gametes are known as the ovaries, and those that give rise to the spermatozoa or male gametes are known as the testes. The ovary and the testis are the primary reproductive organs; the details of their anatomy and position in the various groups need not be discussed here (see Reproductive System).
The male gamete or spermatozoon was first seen in 1677 by Ludwig van Hammen, a pupil of A. Leeuwenhoek, with the microscope that had been constructed by his master. Leeuwenhoek, under the influence of the current preformationist ideas, interpreted these actively moving bodies in the seminal fluids as preformed germs and described them as animalcule spermetia or spermatozoa. Throughout the 18th century the general tendency was to regard them as parasites of no consequence in fertilization. In 1837 R. Wagner established that they were present in all sexually mature males and absent in infertile male hybrids, and in 1841 A. Kölliker showed that they were cells proliferated in the testes. The spermatozoon is one of the smallest of known cells, frequently being no more than one hundred thousandth of the size of the ovum, although the extraordinary case of a small Cypris has been recorded in which the spermatozoa are longer than the animal. It is produced in enormous quantities and relatively to other minute cells is extremely tenacious of life. It may retain its vitality in the male organism for a long time after it has become a separate cell, and may exist for lengthy periods in the female organism. The queen-bee is impregnated only once, and the spermatozoa may remain functional within her body for three years. Lord Avebury (Sir J. Lubbock) has described the case of a female ant which laid fertile eggs thirteen years after she had been impregnated. It is undoubted that in snakes, birds and many mammals, fertilization may not take place for many days after impregnation. The spermatozoa, with a few exceptions, are actively motile, being elongated in shape, with a vibratile tail sometimes provided with a swimming membrane. In a few cases, chiefly of crustaceans, the spermatozoa are spherical with radiating processes, but are capable of amoeboid movements. The cell nucleus is generally situated near the rounded or pointed extremity, with a centrosome immediately behind it, whilst the scanty protoplasm forms the body and vibratile tail; but there appears to be no general significance in the various configurations that occur amongst different animals. The process of spermatogenesis, or production of spermatozoa from the permanent cells of the testis, varies extremely amongst different animals and has been the subject of many elaborate investigations and much confusing nomenclature. Two factors are involved: first, the arrangements to produce a very large crop of cells so to provide for the enormous numbers of spermatozoa produced by most animals; and second, the final changes of shape and of nucleus by which the ripe spermatozoa arise from the indifferent testis-cells, and these processes may to a certain extent overlap. The point of general significance relates to the nuclear changes. The nuclear matter that occurs in the tissue cells of animals, when these cells divide, breaks up into a number of chromosomes constant for each kind of animal, and the final stage of cell division is such that each chromosome splits and contributes a half to each daughter cell, so that the latter come to contain the number of chromosomes peculiar to the animal in which they occur. In the case of spermatozoa, however, a “reducing” division occurs, in which the chromosomes instead of dividing distribute themselves equally between the two daughter cells, with the result that each of the latter contains only half the number peculiar to the species. In its simplest form, what occurs in the last stage of spermatogenesis is that one cell breaks up into four spermatozoa by two successive divisions, the first of which is normal and the second reducing. The nuclear matter of spermatozoa, therefore, contains half the number of chromosomes normal to the tissue cells of the species, and we shall see later that a similar reduction takes place in the formation of the egg. Further complications, however, exist, at least in certain forms. In 1891 H. Henking showed that in a Hemipteran insect of the genus Pyrrochoris, two kinds of spermatozoa are produced in equal numbers, and F. C. Paulmier confirmed the observation in the case of some other insects a few years later, whilst other observers have extended the observation to over a hundred species, In all these cases half the spermatozoa differ from the other half by the presence of what E. B. Wilson calls the “X-element,” and which, in the simplest cases, occurs as an unpaired chromosome of the mother cell which passes into one and not the other of the two spermatozoa formed from that mother cell. The matter is still obscure, and it is not certain whether the facts are peculiar to insects or have a parallel in spermatogenesis universally. According to E. B. Wilson, the facts demonstrate that eggs fertilized by spermatozoa with the X-element invariably produce females (see Sex). The female gamete or ovum is in a large number of cases expanded by the presence of food-yolk and protective swathings to form the visible mass known as an egg, and the production of embryos from eggs has been studied from the time of Aristotle and Pliny. Galen had described the human ovaries as testes muliebres, and W. Harvey in 1651 showed that the chick arose from the cicatricula of the yolk of the egg, compared these early stages with corresponding stages in the uterus of mammals, and laid down the general proposition—ovum esse primordium commune omnibus animalibus—that the ovum is a starting point common to all animals. In 1664 N. Steno identified the sexual organ of the mammalian female with that of sharks, and first named it the ovary. In 1672 R. De Graaf described the structure of the ovary in birds and mammals, observed the ovum in the oviduct of the rabbit, and repeated Harvey's statement as to the universal occurrence of ova, although he mistook for ova the follicles that now bear his name. In 1825 J. E. Purkyne described the germinal vesicle in the chick, thus distinguishing between the structure of the egg as a whole and the essential germinal area, and in 1827 K. E. von Baer definitely traced the ovum back from the uterus to the oviduct and thence to its origin within the Graafian follicle in the ovary, and thus paved the way for identification of the ovum as a distinct cell arising from the germinal tissue of the ovary. The ovum or female gamete, unlike the spermatozoon, is a large cell, in most cases visible to the naked eye even in the ovary. Also, in definite contrast with the spermatozoon, it is a passive non-motile cell, although in certain cases it is capable of protruding pseudopodia. It is usually spherical, contains a large nucleus, a centrosome and abundant protoplasm, and is generally enclosed in a stout membrane which may or may not have a special aperture known as the micropyle. The protoplasm of all eggs contains nutritive material for the nourishment of the future embryo, and this material may be sufficient in quantity to make the whole cell, although remaining microscopic, conspicuously large, or to expand it to the relatively enormous mass of the yellow yolk of a fowl's egg. Finally, the cellular nature of the ovum is frequently further disguised by its being enclosed in a series of membranes such as the albumen and shell of the fowl's egg. Such complexities are ancillary to the growth or protection of the future embryo, and from the general biological point of view the ovum is to be regarded as a specialized cell derived from the germinal tissue of the ovary, just as the spermatozoon is a specialized cell derived from the corresponding stock of germinal material in the testis. The number of ova produced varies from a very few, as in mammals and birds, to a very large number, as in the herring and many invertebrates, but in all cases the number is relatively small compared with that of the spermatozoa produced by the male of the same species. The details of ovogenesis are more sharply divided than in the case of spermatogenesis into processes connected with the production of a crop of large cells bloated with food-yolk, and the peculiar nuclear changes. The latter changes are generally spoken of as the maturation of the ovum, and in most cases do not begin until the full size has been attained. As in the nuclear changes of spermatogenesis, the details differ in different animals, but the salient feature is that the mature ovum contains, like the ripe spermatozoon, half the number of chromosomes normal to the tissue cells of the animal to which it belongs. The simplest form in which the reduction takes place is that the nucleus of the ovum divides by an ordinary division, each chromosome splitting and sharing itself between the daughter nuclei. Of these nuclei one is extruded from the egg, forming what is called a polar body, and this polar body may again divide by a reducing division, so as to form two polar bodies, each with half the normal number of chromosomes. Finally, the daughter nucleus, remaining in the ovum, also divides by a reducing division, and one of the segments remains to form the nucleus of the ripe ovum, with half the normal number of chromosomes, whilst the other is extruded as a polar body. Very many suggestions as to the meaning of the extrusion of the polar bodies have been made, but the least fanciful of these is to regard the ovum ready for maturation as homologous with the cell about to divide into four spermatozoa; in each case the nucleus divides twice and one of the divisions is a reducing division, so that four daughter nuclei are formed each with half-the normal number of chromosomes. Many spermatozoa are required, and each of the four becomes the nucleus of a complete active cell; relatively few ova are required, but each has a large protoplasmic body, and only one of the four becomes a functional mature egg, the other three being simply extruded and so to say wasted. It must be remembered, however, that there is no inherent probability in favour of the apparently simplest explanation of a very complex biological process. It is also to be noted that in many cases the first polar body does not divide, and it is not clearly established that when the first polar body remains single, it is always the result of a normal nuclear division.
When the mature ova and spermatozoa come together in one of the various ways to be discussed later, fertilization, the conjugation of the gametes to form the zygote, occurs. Alcmaeon (580 B.C.) is believed first to have laid down that fertilization in animals and plants consisted in the material union of the sexual products from both sexes, but it was not until 1761 that it was established experimentally by J. T. Kölreuter's work on the hybridization of plants. In 1780 L. Spallanzani artificially fertilized the eggs of the frog and tortoise, and successfully introduced seminal fluid into the uterus of the bitch, but came to the erroneous conclusion that it was the fluid medium and not the spermatozoa that caused fertilization. This error was corrected in 1824 by J. L. Prevost and J. B. Dumas, who showed that filtration destroyed the fertilizing power of the fluid. In 1843 M. Barry observed spermatozoa within the egg of the rabbit, whilst in 1849 R. Leuckart observed the fertilization of the frog's egg, and in 1851 H. Nelson noticed the entrance of spermatozoa to the egg of Ascaris, whilst in 1854 a series of observations published independently by T. L. W. Bischoff and Allen Thomson finally and definitely established the fact that ova were fertilized by the actual entrance of spermatozoa. Further advances in microscopical methods enabled a series of observers, of whom the most notable were E. van Beneden, H. Fol and O. Hertwig, to follow and record the details of the process. They made it clear that the chief event in fertilization was entrance into the ovum of the nucleus or head of the spermatozoon where it formed the “male pro nucleus,” which gradually approached and fused with the female pronucleus or residual nucleus of the ovum. Still later observers, of whom E. B. Wilson is the most conspicuous, have studied the details of the process in many different animals and have shown that the nucleus of the spermatozoon invariably enters the ovum, that the centrosome generally does so, and that the cytoplasm usually plays no part. The nucleus of the zygote or fertilized ovum, then, possesses the number of chromosomes normal in the tissue cells of the animal to which it belongs, but of these half belong to the female gamete and are derived from the germ plasm of the parental ovary, and half to the male gamete or spermatozoon, derived from the germ plasm of the parental testis. The stimulus which leads to and induces the conjugation of the gametes appears to be chemo tactic and to consist of some substance positively attractive to the male gamete, liberated by the mature female gamete, but the attraction is mutual, and in the final stages of approach a protoplasmic outgrowth of the ovum towards the spermatozoon frequently occurs. The fertilized zygote proceeds to form the embryo (see Embryology).
Parthenogenesis is the production of the new organism from the female gamete without previous conjugation with the male gamete, and is to be regarded as secondary to and degenerate from true sexual reproduction. Aristotle recognized that it occurred in the bee. In 1745 C. Bonnet showed that it must occur in the case of Aphides or plant-lice, in which throughout the summer there were developed a series of generations consisting entirely of females. R. A. F. de Réaumur repeated the observations, but evaded the difficulty by suggesting that the Aphides were hermaphrodite, an explanation soon afterwards disproved by L. Dufour. In 1849 (Sir) R. Owen brought together the facts as they were then known and made a remarkable suggestion regarding them. “Not all the progeny of the primary impregnated germ cell are required for the formation of the body in all animals; certain of the derivative germ cells may remain unchanged and become included in that body which has been composed of their metamorphosed and diversely combined or confluent brethren; so included, any derivative germ cell or the nucleus of such may begin and repeat the same processes of growth by imbibition, and of propagation by spontaneous fission, as those to which itself owed its origin.” Taking hold of the recently published views of J. J. S. Steenstrup on alternation of generations, he correlated the sexual and asexual alternation in hydroids and so forth with the virgin births of insects and Crustacea, and regarded the one and the other as instances of the subsequent proliferation of included germ cells, applying the word parthenogenesis to the phenomenon. His theory was a very remarkable anticipation of the germ-plasm theory of A. Weismann, but further knowledge showed that there was an important distinction between the reproduction of the asexual generations described by Steenstrup and the cases of Aphides and Crustacea, the germinal cells in the latter instances being true ova produced from the ovaries of true females, but capable of development without fertilization. In 1856 C. T. E. von Siebold established this fact and limited Owen's term parthenogenesis to the sense in which it is now used, the development without fertilization of ova produced in ovaries. True parthenogenesis occurs frequently amongst Rotifers, and in certain cases (Philodinadae) males either do not exist or are so rare that they have not been discovered. Amongst Crustaceans it is common in Branchiopods and Ostracods; in the case of Daphnids, large thick-shelled ova are produced towards winter, which develop only after fertilization and produce females; the latter, throughout summer, produce thin-shelled ova which do not require fertilization, and from which towards autumn both males and females are produced. Amongst insects it occurs in many forms in many different groups, sometimes occasional, sometimes as a regular occurrence. Apart from Aphides the classical instance is that of the bee, where eggs that are not fertilized develop parthenogenetically and produce only drones. What is known as pathological parthenogenesis has been observed occasionally in higher animals, e.g. the frog, the fowl and certain mammals, whilst in the case of human beings, ovarian cysts in which hair and other structures are produced have been attributed to the incomplete development of parthenogenetic ova. Finally, it has been shown in a number of different instances, notably by J. Loeb, that artificial parthenogenesis may be induced by various mechanical and chemical stimulations. It has been shown that ova may be induced to segment by the presence of spermatozoa belonging even to different classes of the animal kingdom—as, for instance, the ova of echinoderms by the spermatozoa of molluscs. In such cases the resulting embryos have purely maternal characters. A possible interpretation is that spermatozoa have two functions which may be exercised independently; they may act as stimulants to the ovum to segment, and they may convey the paternal qualities. The former function may be replaced by the chemical substances employed in producing artificial parthenogenesis. Juvenile or precocious parthenogenesis, in which there takes place reproduction without fertilization in immature larvae, has been observed chiefly in insects (Dipterous midges), and to this the term paedogenesis has been applied.
The theory of parthenogenesis remains doubtful. When Weismann and others began to study the polar bodies, they made the remarkable discovery that in some parthenogenetic eggs only one polar body was extruded, but the meaning of this distinction was blurred when other cases were described in which two polar bodies were formed. Later on, Weismann drew attention to the difference between normal and reducing divisions, and it now appears to be clear that, with one set of exceptions, ova which develop without fertilization are those in which no reducing division takes place and which, accordingly, contain the number of chromosomes normal to the tissue cells of the species. Such eggs, in fact, resemble the zygote except that all their chromosomes are of maternal origin and the centrosome which becomes active in the first segmentation is that of the ovum and not, as in normal fertilized eggs, that which came in with the spermatozoon. The case of the bee and other insects in which parthenogenetic development results in the production of males, is doubtful; it appears to be the case that a reduction division has taken place in the maturation of the egg. A. Petrunkévitch has made the ingenious suggestion, that after the reducing division the normal number of chromosomes is restored by the splitting of each into two. Cases of pathological and artificial parthenogenesis would fall into line, on the supposition that the stimulus acted by preventing the occurrence of a reducing division in an ovum otherwise mature. It is to be noticed, however, that such explanations of parthenogenesis are not much more than a formal harmonizing of the behaviour of the chromosomes in the respective cases of fertilized and parthenogenetic development; they do not provide a theory as to why the process occurs.
Accessory Reproductive Organs and Processes.—It has been already stated that the primary organs of reproduction in animals are the germinal tissues producing respectively spermatozoa and ova, and that in most cases these are aggregated to form testes and ovaries. In certain animals there are no accessory organs, and when the reproductive products are ripe, they are discharged directly to the exterior if the gonads are external, as in some Coelentera, or if they are internal, break through into some cavity of the body and escape by rupture of the body-wall or through some natural aperture. In a majority of cases, however, special ducts are developed, which in the male serve primarily for the escape of the spermatozoa, but secondarily may be associated with intromittent organs. Similarly, in the female, the primary function of the gonad ducts is to provide a passage for the ova, but in many cases they serve also for the reception of spermatozoa, for the development of embryos and for the subsequent exit of the young. Associated with the ovary and the oviducts are many kinds of yolk-glands and shell-glands, the function of which is to form nutritive material for the future embryo, to discharge this into or around the ovum, and to provide protective wrappings. Although, in the last resort, fertilization depends on impulses attracting the spermatozoa to the ova, probably chemical in their nature, the necessary proximity is secured in a number of ways. In many simple cases the ripe products are discharged directly into the surrounding water, and impregnation is a matter of accident highly probable because such animals discharge enormous quantities of ova and spermatozoa, are frequently sessile and live in colonies, and are mature about the same time. In other cases, as, for instance, Tunicates and many Molluscs, the spermatozoa are discharged, and, being drawn into the body of the female with the inhalent currents, there fertilize the ova. In yet a number of other cases, there is sexual congress without intromittence. The males of many fish, such as salmon, attend the females about to discharge their ova, and afterwards pour the male fluid over the liberated eggs; whilst amongst other fish the males seek out a suitable locality and prepare some kind of nest to which the female is enticed and which receives first the ova and then the milt. In many other animals, again; as for instance the frog, the male grasps the ripe female, embracing her firmly for a prolonged period, during which ova and spermatozoa are discharged simultaneously. Where internal fertilization occurs, there are usually special accessory organs. In the female, the terminal portion of the gonad-duct, or of the cloaca, is modified to receive the intromittent organ of the male, or to retain and preserve the seminal fluid. In the male, the terminal portion of the gonad-duct may be modified into an intromittent organ or penis, grooved or pierced to serve as a channel by which the semen is passed into the female. In arthropods, ordinary limbs may be modified for this purpose, or special appendages developed; in spiders, the terminal joints of the pedipalps, or second pair of appendages, are enlarged, and are dipped into the semen, which is sometimes shed into a special web, and are used as intromittent organs; in cuttlefish, one of the “arms” is charged with spermatozoa, is inserted into the mantle cavity of the female and there broken off. In many cases there is a temporary apposition of the apertures of the male and female, with an injection from the male without a special intromittent organ. The females are usually passive during coitus, and there are innumerable varieties of clasping organs developed by the male to retain hold of the female. Finally, the various secondary sexual characters which are developed in males and females and induce association between them by appeals to the senses, must be regarded as accessory reproductive organs and processes (see Sex).
Another set of accessory organs and processes are concerned with what may be termed in the widest sense of the phrase “brood-care.” In many cases the relation between parent and offspring ceases with the extrusion of the fertilized ovum, whilst others display every possible grade of parental care. Many of the lower invertebrates choose special localities in which to deposit the ova or embryos, and glands, the viscid secretion of which serves to bind the ova together or to attach them to some external object, are frequently present. In many insects, elaborate preparations are made; special food-plants are selected, cocoons are woven, or, by means of the special organ known as the ovipositor, the eggs are inserted in the tissues of a living or dead host, or in other cases a supply of food is prepared and stored with the young larvae. The eggs or larvae may be attached to the parent and carried about with it, as in the gills of bivalves, the brood-pouches of the smaller Crustacea, the back of the Surinam toad, the vocal sacs of the frog Rhinoderma, the expanded ends of the oviducts or the marsupial pouch. In a large number of cases the young are nourished directly from the blood of the mother by some kind of placental connexion, as in some of the sharks, in Anablebs, a bony fish, in some lizards and in mammals. In other cases, the young after birth or hatching are fed by the parents, by the special secretion of the mammary glands in the case of mammals, by regurgitated food in many birds and mammals, by salivary secretions or by food obtained and brought to the young by the parents.
Reproductive Period.—In a general way, reproduction begins when the limit of growth has been nearly attained, and the instances of paedogenesis, whether that be parthenogenetic as in midges, or sexual as in the axolotl, must be regarded as an exceptional and special adaptation. In lower animals, where the period of growth is short or indefinite, reproduction begins earlier and is more variable. But, in all cases, surrounding conditions play a great part in hastening or retarding the onset of reproduction. Increased temperature generally accelerates reproductive maturity, excess of food retards it, and sudden privation favours it. In a majority of cases it endures to the end of life, but in some of the higher forms, such as birds and mammals, there is a marked decrease or a cessation of reproductive activity, especially in the case of females, as life advances. In most animals, moreover, periods of reproductive activity alternate with periods of quiescence in a rhythmical series. In its simplest form, the rhythm is seasonal; but although at first associated with actual seasonal changes, it persists in the absence or alteration of these. Many animals brought to Europe from the southern hemisphere come into reproductive activity at the time of year corresponding to the spring or summer of their native home. “Heat,” menstruation and ovulation in the higher mammals, including man, are rhythmical, and probably physiologically linked, but the ancestral meaning of the periodicity is unknown.
Reproduction and Increase of the Race.—Two distinct factors are involved in this question—the potential fecundity of organisms, and the chances of the young reaching maturity. The first varies with the actual output of zygotes, and is determined partly by the reproductive drain on the individual, and especially the female in cases where the ova are provided with much food-yolk, partly on the duration of reproductive maturity, and partly on the various adaptive and environmental conditions which regulate the chances of the gametes meeting for fertilization. It is to be noted that as the gametes are simply cells proliferating from the germinal tissue, the potential number that can be produced is almost indefinite; and as it is found that in very closely allied forms the actual number produced varies within very wide limits, it may be assumed that potential fecundity is indefinite. The possibility of zygotes reaching maturity varies first with the individuation of the organism concerned—that is to say, the degree of complexity of its structure—and the duration of the period of its growth; and secondly, with the incidence of mortality on the eggs and immature young. It is plain that a parasite capable of living only on a particular host may give rise to myriads of progeny, and yet, from the difficulty of these reaching the only environment in which they can become mature, might not increase more rapidly than an elephant which carries a single foetus for about two years, and guards it for many years after birth. The probable adaptation of the variable reproductive processes to the average conditions of the race is discussed under the heading Longevity. It may be added here that the adaptation, in all successful cases, appears to be in excess of what would be required merely to replace the losses caused by death, and that there is ample scope for the Malthusian and Darwinian factors. The rate of reproduction tends to outrun the food supply.
Literature.—Almost any zoological publication may contain matter relating to reproduction, but text-books on Embryology must be specially consulted. The annual volumes of the Zoological Record, under the heading “General Subject” until 1906, and thereafter under “Comprehensive Zoology,” give a classified subject-index of the literature of the year in which references to the separate parts of the subject are given. Amongst the older memoirs referred to in this article the following are the most important: A. Leeuwenboek, Epistolae ad societatem regiam Angliam (1719); R. A. F. de Réaumur, Mémoires pour servir á l'histoire des insectes (Paris, 1734-1742); C. Bonnet, Œuvres d'histoire naturelle et de philosophie (Neuchâtel, 1779-1783); L. Spallanzani, Dissertations relative to the Natural History of Animals and Vegetables (Eng. trans., 2nd ed., London, 1789); J. L. Prévost et J. B. Dumas, “Observations relatives a l'appareil générateur des animaux mâles,” Ann. Sci. Nat. i. (1824); K. E. von Baer, Epistola ad Academiam Scient. Petropolitanam; Heusinger, Zeitschrift, ii. (1828); Léon Dufour, Recherches anatorniques et physiologigue sur les Hémiptères (Paris 1833); R. Wagner, “Recherches sur la génération,” Ann. Sci. Nat. viii. (1837); A. Kölliker, Über das Wesen der sogenannten Saamenthiere, Froriep, Notizen xix. (1841); M. Barry, “Spermatozoa observed within the Mammiferous Ovum,” Phil. Trans. (1743); J. J. S. Steenstrup, On the Alternation of Generations (Eng. trans., Ray Society, London, 1845); R. Leuckart, Beiträge zur Lehre der Befruchtung (Göttingen Nachrichten, 1849); (Sir) R. Owen, On Parthenogenesis (London, 1849); H. Nelson, “The Reproduction of Ascaris mystax,” Phil. Trans. (1852); C. T. E. von Siebold, On a True Parthenogenesis in Moths and Bees (Eng. trans., London, 1857); E. van Beneden, “Recherches sur la maturation de l'œuf et la fécondation,” Arch. de biol. (1883); O. Hertwig, “Das Problem der Befruchtung,” Jen. Zeitsch. xviii. (1885).
- (P. C. M.)
Reproduction of Plants
The various modes in which plants reproduce their species may be conveniently classified into two groups, namely, vegetative propagation and true reproduction, the distinction between them being roughly this, that whereas in the former the production of the new individual may be effected by the most various parts of the body, in the latter it is always effected by means of a specialised reproductive cell.
I. Vegetative Propagation.
The simplest case of vegetative multiplication is afforded by unicellular plants. When the cell which constitutes the body of the plant has attained its limit of size it gives rise to two either by division or gemmation; the two cells then grow, and at the same time become separated from each other, so that eventually two new distinct individuals are produced, each of which precisely resembles the original organism. A good example of this is to be found in the germination of the yeast plant. This mode of multiplication is simply the result of the ordinary processes of growth. All plant-cells grow and divide at some time or other of their life; but whereas in a multicellular plant the products of division remain coherent, and add to the number of the cells of which the plant consists, in a unicellular plant they separate and constitute new individuals. In more highly organized plants vegetative propagation may be effected by the separation of the different parts of the body from each other, each such part developing the missing members and thus constituting a new individual. This takes place spontaneously in rhizomatous plants, in which the main stem gradually dies away from behind forwards; the lateral branches thus become isolated and constitute new individuals.
The remarkable regenerative capacity of plant-members is largely made use of for the artificial propagation of plants. A branch removed from a parent-plant will, under appropriate conditions, develop roots, and so constitute a new plant; this is the theory of propagation by “cuttings.” A portion of a root will similarly develop one or more shoots, and thus give rise to a new plant. An isolated leaf will, in many cases, produce a shoot and a root, that is, a new plant; it is in this way that new begonias, for instance, are propagated. The production of plants from leaves occurs also in nature, as, for instance, in certain so-called “viviparous” plants, of which Bryophyllum calycinum (Crassulaceae) and many ferns [Nephrodium (Lastraea) Filix-mas, Asplenium (Athyrium) Filix-foemina and other species of Asplenium] are examples. But it is in the mosses, of all plants, that the capacity for vegetative propagation is most widely diffused. Any part of a moss, whether it be the stem, the leaves, the rhizoids, or the sporogonium, is capable, under appropriate conditions, of giving rise to filamentous protonema, on which new moss-plants are then developed as lateral buds.
In a large number of plants provision is made for vegetative propagation by the development of more or less highly specialized organs. In lichens, for instance, there are the soredia, which are minute buds of the thallus containing both algal and fungal elements; these are set free on the surface in large numbers, and each grows into a thallus. In the Characeae there are the bulbils or “starch-stars” of Chara stelligera, which are underground nodes, and the branches with naked base and the proembryonic branches found by Pringsheim on old nodes of Chara fragilis. In the mosses small tuberous bulbils frequently occur on the rhizoids, and in many instances (Bryum annotinum, Aulacomnion androgynum, Tetraphis pellucida, &c.) stalked fusiform or lenticular multicellular bodies containing chlorophyll, termed gemmae, are produced on the shoots, either in the axils of the leaves or in special receptacles at the summit of the stem. Gemmae of this kind are produced in vast numbers in Marchantia and Lunularia among the liverworts. Similar gemmae are also produced by the prothallia of ferns. In some ferns (e.g. Nephrolepis tuberosa and undulata) the buds borne on the leaves or in their axils become swollen and filled with nutritive materials, constituting bulbils which fall off and give rise to new plants. This conversion of buds into bulbils, which subserve vegetative multiplication, occurs also occasionally among Phanerogams, as for instance in Lilium bulbiferum, species of Poa, Polygonum viviparum, &c. But many other adaptations of the same kind occur among Phanerogams. Bulbous plants, for instance, produce each year at least one bulb or corm from which a new plant is produced in the succeeding year. In the potato, tubers are developed from subterranean shoots, each of which in the following year gives rise to a new individual. In the dahlia, Thladiantha dubia, &c., tuberous swellings are found on the roots, from each of which a new individual may spring.
II. True Reproduction.
This is effected by cells formed by the proper reproductive organs. These cells are of two principal kinds. There are, first, those cells each of which is capable of developing by itself into a new organism: these are the asexual reproductive cells, known generally as spores. Secondly, there are the cells which are incapable of independent germination; it is not until these cells have fused together in pairs that a new organism can be developed: these are the sexual reproductive cells or gametes.
In some exceptional cases the normal mode of reproduction, sexual or asexual, does not take place: instead, the new organism is developed vegetatively from the parent. When sexual reproduction is suppressed the case is one of apogamy; when asexual reproduction by spores is suppressed the case is one of apospory. (Apogamy and apospory are discussed below in the section on Abnormalities of Reproduction.)
Asexual Reproduction.—Reproduction by means of some kind of spore (using the term in its widest sense, so as to include all asexually produced reproductive cells) is common to nearly all families of plants; it is wanting in certain Algae (Conjugatae, Fucaceae, Characeae), and in certain fungi (e.g. some Peronosporeae). The structure of a spore is essentially this: it consists of a nucleated mass of protoplasm, enclosing starch or oil as reserve nutritive material, usually invested by a cell-wall. In those cases in which the spore is capable of germinating immediately on its development the cell-wall is a single delicate membrane consisting of cellulose; but in those cases in which the spore may or must pass through a period of quiescence before germination the wall becomes thickened and may consist of two layers, an inner, the endospore, which is delicate and consists of cellulose, and an outer, the exospore, which is thick and rigid, frequently darkly coloured and beset externally with spines or bosses, and which consists of cutin. In some few cases among the fungi, multicellular or septate spores are produced; these approximate somewhat to the gemmae mentioned above as highly specialized organs for vegetative propagation. In some cases, particularly among the algae, and also in some fungi (Peronosporeae, Saprolegnieae, Chytridiaceae, and the Myxomycetes), spores are produced which are usually destitute of any cell-wall, and are further peculiar in that they are motile, and are therefore termed zoospores; they move sometimes in an amoeboid manner by the protrusion of pseudopodia, but more frequently they are provided with one, two, or many delicate vibratile protoplasmic filaments, termed cilia, by the lashing of which the spore is propelled through the water. The zoospore eventually comes to rest, withdraws its cilia, surrounds itself with a cell-wall, and then germinates.
In the simplest case a single spore is developed from the cell of the unicellular plant, the protoplasm of which surrounds itself with the characteristic thick wall. This occurs only in plants of low organization such as the Schizophyta.
In other cases the contents of the cell undergo division, each portion of the protoplasm constituting a spore. Examples of this are afforded, among unicellular plants, by yeast and the Protococcaceae; and in multicellular plants by the Pandorineae, Confervaceae, Ulvaceae, &c., where any cell of the body may produce spores.
In such cases the spore-producing cell may be regarded as a rudimentary reproductive organ of the nature of a sporangium; In more highly organized plants special organs are differentiated for the production of spores. In the majority of cases the special organ is a sporangium, that is, a capsule in the interior of which the spores are developed; but in many fungi the spores are formed by abstraction from an organ termed a sporophore. In the Thallophyta the sporangium is commonly a single cell. In the Bryophyta it is a multicellular capsule. In the Pteridophyta the sporangium is multicellular, but simple in structure, and this is true also of the Phanerogams.
It is important to note that in all the Bryophyta and in some of the Pteridophyta (most of the Filicinae, all existing Equisetinae, and the Lycopodiaceae and Psilotaceae) there is but one kind of sporangium and spore, the plants being homosporous or isosporous, whereas the rest of the Pteridophyta (Hydropterideae, Selaginellaceae) and the Phanerogams are heterosporous, having sporangia of two kinds; some produce one or a few large spores (megaspores), and are hence termed megasporangia, while others give rise to a larger number of small spores (microspores) and are hence termed microsporangia. In the Phanerogams the two kinds of sporangia have received special names: the megasporangium, which produces as a rule only one mature spore (embryo-sac), is termed the ovule; the microsporangium, which produces a large number of microspores (pollen-grains), is termed the pollen-sac.
The development of spores, except in the simpler Thallophyta, is more or less restricted to definite parts of the body. Thus in the Red Algae (Florideae) there are the organs known as stichidia, nemathecia. In the fungi the number and variety of such organs is very great; they may be described generally as simple and compound sporophores: but for a description the article Fungi should be consulted. In the higher plants the organs are less various. In the Bryophyta the production of spores is restricted to the sporogonium. In the vascular plants (Pteridophyta, Phanerogams) the development of sporangia, speaking generally, is confined to the leaves. In most ferns the sporangiferous leaves (sporophylls) do not differ in appearance from the foliage leaves; but in other Pteridophyta (Equisetaceæ, Marsiliaceae, some species of Lycopodium and Selaginella) they present considerable adaptation, and notably in the Phanerogams. In the Phanerogams the specialization is so great that the sporophylls have received special names; those which bear the microsporangia (pollen-sacs) are termed the stamens, and those which bear the megasporangia (ovules) are termed the carpels. The sporophylls are usually aggregated together on a short stem, forming a shoot that constitutes a flower.
Many terms are employed to indicate the nature of the various kinds of spores, especially among the fungi, but the endless varieties of asexual (and asexually produced) reproductive cells may be grouped under two heads—(1) Gonidia, (2) Spores proper.
The distinction between these two kinds of asexual reproductive cells is as follows.
The gonidium is a reproductive cell that gives rise, on germination, to an organism resembling the parent. For instance, among the algae, the “zoospore” of Vaucheria develops into a Vaucheria-plant. There is thus a close connexion between vegetative multiplication and multiplication by means of gonida. The production of gonida is entirely limited to the Thallophyta, and is especially marked in the fungi, though the nature of all the many kinds of reproductive cells formed in this group has not yet been fully investigated. It is, however, wanting in certain algae (Conjugatae, Fucaceae, Characeae) and fungi (some Peronosporeae and Ascomycetes).
The spore proper is a reproductive cell that as a rule gives rise, on germination, to an organism unlike that which produced it. For instance, the spore of a fern when it germinates gives rise, not to a fern-plant, but to a prothallium. The apparent exceptions to this rule occur only among the Thallophyta, and are explained below in the section on Life-history.
The true spore is developed, usually in a sporangium, after a process of division which presents certain features that call for special notice.
Observation of the process of division of the nucleus (karyokinesis) in plants generally has shown (for details see Cytology) that the linin-reticulum of the resting nucleus breaks up into a definite number of segments, the chromosomes, each of which bears a series of minute bodies, the chromatin-disks or chromomeres, consisting largely of a substance termed chromatin. In the ordinary homotype divisions of the nuclei the characteristic number of chromosomes is always observable; but when the spore-mother-cells are being formed the number of chromosomes is reduced to one-half. This, if the number of chromosomes of the parent plant be expressed as 2x the number in the spore will be x. To take a concrete case: it has been observed by Guignard and others that in the early divisions taking place in the developing anther and ovule of the lily the number of chromosomes is 24; whereas in the later divisions which give rise to the pollen-mother-cells in the one case and to the mother cell of the embryo-sac in the other, the number of chromosomes is only 12. Thus the development of a spore (as distinguished from a gonidium) is always preceded by a reducing- or heterotype-division, a process now more generally termed meiosis (Farmer). The reduced number of chromosomes in the nucleus of the spore-mother-cell persists in the spore, and in all the cells of the organism to which the spore may give rise. (Meiosis is discussed below in the section on Sexual Reproduction.)
It should be explained that cells, to which the name “spore” has also been applied, are formed as the result of a sexual act: such are zygospores, oospores, and some carpospores. But these cells differ from spores proper not only in their mode of origin but also in that their nuclei contain the full double number (2x) of chromosomes; hence they may be distinguished as diplospores.
Sexual Reproduction.—Sexual reproduction involves the development of sexual organs (gametangia) and sexual cells (gametes). When the organism is unicellular, as in the lower Green Algae (e.g. Protococcaceae, Conjugatae), the cell becomes a sexual organ and its whole protoplasm gives rise to one or more sexual cells: in the higher forms certain parts of the body are specialized as sexual organs. In many of the lower plants the organs present no external distinction of sex (e.g. lower Green Algae: the Chytridiaceae, Mucorinae, and some Ascomycetes among the fungi): it is impossible to distinguish between the male and female organs, although it cannot be doubted that the essential physiological difference exists; consequently the organs are merely described as gametangia. The gap between these plants and those with differentiated sexual organs is, however, bridged over by intermediate forms, as explained in the article Algae.
When the sexual organs are more or less obviously differentiated into male and female, they present considerable variety of form in different groups of plants, and accordingly bear different names. Thus the male organ is a pollinodium in most of the fungi, a spermogonium in others (certain Ascomycetes, Uredineae); in all other plants it is an antheridium. Similarly the female organ is an oogonium in various Thallophyta (Green and Brown Algae: Oomycetous Fungi); a procarp in the Red Algae; an archicarp in certain Ascomycetous Fungi and in the Uredineae; an archegonium in all the higher plants.
It is generally the case that the protoplasm of the sexual organ is differentiated into one or more sexual cells. Thus the gametangium usually gives rise to cells which, as they are externally similar, are termed isogametes or simply gametes. Certain forms of the male organ, the spermogonium and the antheridium, give rise to male cells which are termed spermatia when they are non-ciliate, spermatozoids when they are ciliated and free-swimming. Again, the female organs termed oogonia and archegonia produce one or more female cells called oospheres. But there are important exceptions to this rule. Thus the protoplasm is not differentiated into cells in the gametangium of the Mucorinae; in the male organ (pollinodium), of fungi generally; and in the female organ (procarp) of the Red Algae and (archicarp) of the Ascomycetes and Uredineae.
The immediate product of the fusion of cells, or of undifferentiated protoplasm, derived from sexual organs of opposite sex may be generally termed the zygote; but it is not always of the same kind. Thus when two isogametes, or the undifferentiated contents of two gametangia, fuse together, the process is designated conjugation, and the product is usually a single cell termed zygospore. When an oosphere fuses with a male cell, or with the undifferentiated contents of a male organ, the process is fertilization, and the product is a single cell termed oospore. When, finally, a female organ with undifferentiated contents receives a male cell, the process again is fertilization; here the product is not a single cell, but a fructification termed cystocarp (Red Algae), or ascocarp (Ascomycetes) or aecidium (Uredineae), containing many spores (carpospores).
As a consequence of the diversity in the sexual organs and cells, in the details of the sexual act, and in the product of it, several modes of the sexual process have to be distinguished, which may be conveniently summarized as follows:—
I. Isogamy: the sexual process consists in the fusion of either two similar sexual cells (isogarnetes), or two similar sexual organs (gametangia): it is termed conjugation, and the product is a zygospore. Its varieties are:—
(a) Gametes ciliated and free-swimming (planogametes), set free into the water where they meet and fuse: lower Green Algae (Protococcaceae, Pandorineae, most Siphonaceae and Confervaceae); some Brown Algae (Phaeosporeae):
(b) Gametangia fuse in pairs, and a gamete is differentiated in each: the gametes of each pair fuse, but are not set free and are not ciliated (the Conjugate Green Algae): or, no gametes are differentiated, the undifferentiated contents)of the gametangia fusing (Mucorinae among the Fungi).
II. Ooamy: male and female organs distinct: the protoplasm of the female organ is differentiated into one or (rarely) more oospheres which usually remain enclosed in the female organ: the contents of the male organ are usually differentiated into one or more male cells: the process is fertilization, the product is an oospore.
(A) The sexual organs are unicellular (or coenocytic as in certain Siphonaceous Green Algae and in the Oomycetous Fungi); the female organ is an oogonium.
(α) The male organ is an antheridium giving rise to one or more free-swimming ciliated spermatozoids:
- (1) The oogonium contains a single oosphere which is fertilized in situ: higher Green Algae (Volvox, Vaucheria, Oedogonium, Coleochaete, Characeae); some Brown Algae (Tilopteris); among the Fungi, Monoblepharis, the only fungus known to have spermatozoids:
- (2) The oogonium produces a single oosphere which is extruded and is fertilized in the water: Dictyota and some Fucaceae (Brown Algae):
- (3) The oogonium contains several oospheres which are fertilized in situ: Sphaeroplea (Siphonaceous Green Alga):
- (4) The oogonium produces more than one oosphere (2-8) which are extruded and are fertilized in the water: certain Brown Algae (Pelvetia, Ascophyllum, Fucus):
(β) The male organ is a pollinodium which applies itself closely to the oogonium: the amorphous male cell is not ciliated and is not set free:
- (1) The oogonium contains a single oosphere which is fertilized in situ: Peronosporaceae (Oomycetes):
- (2) The oogonium contains several oospheres; Saprolegniaceae: but it is debated whether or not fertilization actually takes place.
(B) The male and female organs are (as a rule) multicellular; the male organ is an antheridium, the female an archegonium: the archegonium always contains a single oosphere which is fertilized in situ.
(α) The male cell is a free-swimming ciliated spermatozoid: the antheridium produces more than one (usually very many) spermatozoids, each of which is developed in a single cell: all Bryophyta (mosses, &c.) and Pteridophyta (ferns, &c.): the only Phanerogams in which spermatozoids have been observed are the gymnospermous species Ginkgo biloba, Cycas revoluta, Zamia integrifolia.
(β) The male cell is amorphous and passes directly from the pollen-tube into the oosphere (siphonogamy): all Phanerogams except the species just mentioned.
It must be explained that in the angiospermous Phanerogams, the male and female organs are so reduced that each is represented by only a single cell: the male, by the generative cell, formed in the pollen-grain, which usually divides into two male cells: the female, by the oosphere. The gradual reduction can be traced through the Gymnosperms.
Attention may here be drawn to the fact (see Angiosperms) that, in several cases, the second male cell has been seen to enter the embryo-sac from the pollen-tube, and its nucleus to fuse with the definitive nucleus (endosperm-nucleus) or with one of the polar nuclei. The significance of this remarkable observation is discussed in the section on the Physiology of Reproduction.
III. Carpogamy: the sexual organs are (as a rule) differentiated into male and female: the protoplasm of the unicellular or multicellular female organ (archicarp, procarp) is never differentiated into an oosphere: in many cases definite male cells, spermatia, are produced and are set free, but they are not ciliated, and frequently have a cell-wall: the process is fertilization: the product is a fructification derived essentially from the female organ containing several (sometimes very many) spores (carpospores): characteristic of the Red Algae and of the Ascomycetous Fungi.
(A) There are definite male cells (spermatia):
- (α) The female organ is a procarp, consisting of an elongated, closed, receptive filament, the trichogyne, and of a basal fertile portion, the carpogonium: on fertilization the latter grows and gives rise directly or indirectly to a cystocarp: the spermatia are each formed in a unicellular antheridium and have no cell-wall at first: they fuse with the tip of the trichogyne: Red Algae (Rhodophyceae or Florideae):
- (β) The female organ (archicarp) resembles the preceding: in fertilization the fertile portion (ascogonium) develops into an ascocarp containing one or more asci (sporangia) each containing usually eight ascospores: the spermatia are formed by abstraction from the filaments (sterigmata) lining special receptacles, the spermogonia, which are the male organs: certain Ascomycetous Fungi (e.g. Laboulbeniaceae, some Lichen-Fungi, Polystigma). For the Uredineae, see Abnormalities of Reproduction, below).
(B) There are no definite male cells: the more or less distinct male and female organs come into contact, and their undifferentiated contents fuse: the product is an ascocarp:
- (α) The male and female organs are obviously different: the female organ is an ascogonium, the male a pollinodium: e.g. Pyronema, Sphaerotheca (Ascomycetes):
- (β) The male and female organs are quite similar: e.g. Eremascus, Dipodascus (Ascomycetes).
It may be explained that carpogamy is the expression of sexual degeneration. In the cases last mentioned, when the sexual organs are quite similar, they have reverted to the condition of gametangia. Still further reduction is observable in other Ascomycetes in which one of the sexual organs, presumably the male, is either much reduced or is altogether wanting. Again in the rusts (Uredineae), there are spermatia, but they are functionless (see section on Abnormalities of Reproduction). In the highest Fungi, the Autobasidiomycetes, no sexual organs have been discovered.
Details of the Sexual Act.—It has been already stated that the sexual act consists in the fusion of two masses of protoplasm, commonly cells, derived from two organs of opposite sex: but this is only the first stage in the process. The second stage is the fusion of the nuclei, which usually follows quickly upon the fusion of the cells; but nuclear fusion may be postponed so that the two sexual nuclei may be observed in the zygote, as “conjugate” nuclei, and even in the cells of the organism developed from the zygote (e.g. Uredineae). The result of nuclear fusion is that the nucleus of the zygote contains the double number of chromosomes—that is, if the number of chromosomes in each of the fusing sexual nuclei be x, the number in the nucleus of the zygote will be 2x. Moreover, this double number persists in all the cells of the organism developed from the zygote, until it is reduced to one-half by meiosis preceding either the development of the spores, or, less commonly, the development of the sexual cells. But there is yet a third stage, which consists in the temporary fusion of the chromosomes belonging to the two sexual nuclei. This always takes place as a preliminary to meiosis; it may be in the germinating zygote, or after many generations of cells have been formed from it. At the onset of meiosis the (2x) chromosomes are seen to be double, one of each pair having been derived from the male and the female cell respectively: the chromosomes of each pair: then fuse so that their chromomeres unite along their length, constituting the pseudo-chromosomes. The paired chromosomes separate and eventually go to form the two daughter-nuclei, one to each, which thus have half (x) the original number of chromosomes. The daughter-nuclei at once divide homotypically, retaining the reduced (x) number of chromosomes to form the four nuclei of a tetrad of spores (more rarely, e.g. Fucus, of sexual cells).
It will have been gathered from the foregoing sections that plants generally are capable of both sexual and asexual reproduction; and, further, that in different stages of their life-history they possess the diploid (2x) number of chromosomes in their nuclei, or the haploid (x) number. It may be at once stated that, in all plants in which sexual reproduction and true meiotic spore-formation exist, these two modes of reproduction are restricted to distinct forms of the plant; the sexual form bears only the sexual organs and is haploid; the asexual form only produces spores and is diploid. Hence all such plants are to this extent polymorphic—that is, the plant assumes these two forms in the course of its life-history. When, as in many Thallophyta, one or other of these forms can reproduce itself by means of gonidia, additional forms may be introduced into the life history, which becomes the more complicated the more pronounced the polymorphism.
The most straightforward life-histories are those presented by the Bryophyta and the Pteridophyta, where there are but the two forms, the sexual and the asexual. In the life-history of a moss, the plant itself bears only sexual organs: it is the sexual form, and is distinguished as the gametophyte. The zygote (oospore) formed in the sexual act develops into an organism, the sporogonium, which is entirely asexual, producing only spores: it is distinguished as the sporophyte. When these spores germinate, they give rise to moss-plants. Thus the two forms, the sexual and the asexual, regularly alternate with each other—that is, the life-history presents that simple form of polymorphism which is known as alternation of generations. Similarly, in the life-history of a fern, there is a regular alternation of a sporophyte, which is the fern-plant itself, with a gametophyte, which is the fern-prothallium.
It is pointed out in the preceding section that, as the result of the sexual act, the nucleus of the zygote contains twice as many chromosomes as those of the fusing sexual cells. This 2x number of chromosomes persists throughout all the cell-generations derived from the zygote, that is, in the cells constituting the sporophyte, up to the time that it begins to produce spores, when meiosis takes place. Again, the cell-generations derived from the spore, that is, the cells constituting the gametophyte, all have the reduced x number of chromosomes in their nuclei up to the sexual act. Hence the sporophyte may also be designated the diplophyte and the gametophyte the haplophyte (Strasburger): in other words, the sporophyte is the pre-meiotic, the gametophyte the post-meiotic generation. Twice in its life-history the plant is represented by a single cell: by the spore and by the zygote. The turning-points in the life-history, the transitions from the one generation to the other, are (1) meiosis, (2) the sexual act.
The course of the life-history in Phanerogams and in those Thallophyta which have been adequately investigated is essentially the same as that of the Bryophyta and of the Pteridophyta as described above, though it is less easy to trace on account of the peculiar relation of the two generations to each other in the Phanerogams and on account of various irregularities that present themselves in the Thallophyta.
In the Phanerogams, as in the Pteridophyta, the preponderating generation is the sporophyte, the plant itself. Inasmuch as they are heterosporous, the gametophyte is represented by a male and a female organism or prothallium, both rudimentary. The male prothallium consists of the few cells' formed by the germinating pollen-grain (microspore); and though it is quite independent, since the microspores are shed, it grows parasitically in the tissues upon which the microspore has been deposited in pollination. The female prothallium may consist of many cells with well-developed archegonia, as in the Gymnosperms, or of only a few cells with the female organ reduced to the oosphere, as in the Angiosperms. In either case it is the product of the germination of a megaspore (embryo-sac) which is not shed from its sporangium (ovule): hence it never becomes an independent plant, and was long regarded as merely a part of the sporophyte until its true nature was ascertained, chiefly by the researches of Hofmeister, who first explained the alternation of generations in plants. This intimate and persistent connexion between the two generations affords the explanation of the characteristic features of the Phanerogams, the seed and the flower. The ovule containing the embryo-sac, which eventually contains the embryo, persists as the seed—a structure that is distinctive of Phanerogams, which have, in fact, on this account been also termed Spermatophyta. With regard to the flower, it has been already mentioned that it is, like the cone of an Equisetum or a Lycopodium, a shoot adapted to the production of spores. But it is something more than this: for whereas in Equisetum or Lycopodium the function of the cone comes to an end when the spores are shed, the flower of the Phanerogam has still various functions to perform after the maturation of the spores. It is the seat of the process of pollination—that is, the bringing of the pollen-grain by one of various agencies into such a position that a part (the pollen-tube) of the male prothallium developed from it may reach and fertilize the oosphere in the embryo-sac. Thus the flower of Phanerogams is a reproductive shoot adapted not only for spore-production, but also for pollination, for fertilization, and for the consequences of fertilization, the production of seed and fruit. However, in spite of these complications, it is possible to determine accurately the limits of the two generations by the observation of the nuclei. The meiosis preceding the formation of the spores marks the beginning of the (haploid) gametophyte, male and female; and the sexual act marks that of the (diploid) sporophyte.
The difficult task of elucidating the life-histories of the Thallophyta has been successfully performed in certain cases by the application of the method of chromosome-counting, with the result that alternation of generations has been found to be of general occurrence. To begin with the Algae. In the Dictyotaceae (Brown Algae) there are two very similar forms in the life-history, the one bearing asexual reproductive organs (tetrasporangia), the other bearing sexual organs (oogonia and antheridia). It has been shown (Lloyd Williams) that the former is undoubtedly the sporophyte and the latter the gametophyte, since the nuclei of the former contain 32 chromosomes, and those of the latter 16. Meiosis takes place in the mother-cell of the tetraspores, which, on germination, give rise to the sexual form. Quite a different life-history has been traced in Fucus, another Brown Alga. Here no spores are produced: there is but one form in the life-history, the Fucus-plant, which bears sexual organs and has, on that account, been regarded as a gametophyte. The investigation of the nuclei has, however, shown (Farmer) that the Fucus-plant is actually diploid, that it is, in fact, a sporophyte; but since there is no spore-formation, meiosis immediately precedes the development of the sexual cells, which alone represent the gametophyte (see below, Apospory).
Similarly, two types of life-history have been discovered in the Red Algae. In Polysiphonia violacea, a species in which the tetraspores and the sexual organs are borne by similar but distinct individuals, it has been ascertained (Yamanouchi) that, as in Dictyota, meiosis takes place in the mother-cell of the tetraspores, so that the nuclei of these spores, as also those of the sexual plants to which they give rise, contain 20 chromosomes: and further, that the nuclei of the carpospores (diplospores) produced in the cystocarp as the result of fertilization, contain 40 chromosomes, as do also those of the asexual plant to which the carpospores give rise. Hence the sporophyte is represented by the cystocarp and the resulting tetrasporangiate plants: the gametophyte, by the sexual plants. Though it is the rule in the Red Algae that the tetrasporangia and the sexual organs are borne on distinct individuals, yet cases are known in which both kinds of reproductive organs are borne upon the same plant; and to those the above conclusions obviously cannot apply. They have yet to be investigated.
The second type of life-history has been traced in Nemalion. Here there is no tetrasporangiate form, consequently meiosis takes place at a different. stage in the life-history. It has been observed (Wolfe) that the nuclei of the sexual plant contain 8 chromosomes; those of the gonimoblast-filaments of the developing cystocarp contain 16, whilst those of the carpospores contain 8: hence meiosis takes place in the carposporangia. Here the plant is the gametophyte; the sporophyte is only represented by the cystocarp. The carpospores here are true spores (haplospores).
Among the Green Algae, Coleochaete is the only form that has been fully investigated (Allen). Here meiosis takes place in the germinating oospore: consequently the plant is the gametophyte, and the sporophyte is represented only by the oospore, so that the life-history resembles that of Nemalion. It is probable that this conclusion is generally true of the whole group; at any rate of those forms (Desmids, Spirogyra, Oedogonium, Chara) which have been more or less investigated.
Turning to the Fungi, somewhat similar results have been obtained in the few forms that have been studied from this point of view. In the sexual Ascomycetes it appears (Harper) that meiosis takes place in the ascocarp just before the development of the spores, so that the life-history essentially resembles that of Nemalion. Again, in certain Uredineae, having an aecidium-stage and a teleutospore-stage, which is apparently a sexual process has been observed (Blackman, Christman) which is described in the section on Abnormalities of Reproduction, and the life-history is as follows. The sexual act having taken place, a row of aecidiospores is developed in the aecidium, each of which contains two conjugate nuclei derived from the sexual nuclei. The mycelium developed from the aecidiospore, as well as the uredospores and the teleutospores that it bears, shows two conjugate nuclei. When, however, the teleutospore is about to germinate, the two nuclei fuse (thus completing the sexual act) and meiosis takes place. As a result the promycelium developed from the teleutospore, and the sporidia that it produces, are uninucleate: so are also the mycelium developed from the sporidium, and the female organs (archicarps) borne upon it. Hence the limits of the sporophyte are the aecidiospore and the teleutospore: those of the gametophyte, the teleutospore and the aecidiospore.
Similar observations have been made upon other Uredineae with a more contracted life-history. Phrogmidium Potentillae-canadensis is a rust that has no aecidium-stage: consequently the primary uredospores are borne by the mycelium produced on infection of the host by a sporidium. It has been observed (Christman) that the sporogenous hyphae fuse in pairs, suggesting a sexual act; then the primary uredospores are developed in rows from the fused pairs of hyphae which thus behave as sexual organs (archicarps), and each such uredospore contains two conjugate nuclei. Although the research has not been carried beyond this point, it may be inferred that in this case, as in the preceding, nuclear fusion and meiosis take place in the teleutospore. Here the sporophyte is represented by the uredo-form.
Finally, in some of the fungi in which no sexual organs have yet been discovered, this method of investigation has made it probable that some kind of sexual act takes place nevertheless. Thus in the Uredine Puccinia malvacearum, which has only teleutospore- and sporidium-stages, it has been observed (Blackman) that the formation of the teleutospores is preceded by a binucleate condition of the hyphae. The same idea is suggested by the binucleate basidia of the Basidiomycetes, which correspond to the teleutospores of the Uredineae.
The life-histories sketched in the preceding paragraphs show that one of the complexities met with in the Thallophyta is that meiosis does not always take place at the same point in the life-history. In the higher plants the incidence of meiosis is generally, though not absolutely, constant: it may be stated as a rule that in the Bryophyta, Pteridophyta and Phanerogams it takes place in the spore-mother-cells. In the Thallophyta this rule does not hold. In some of them, it is true, meiosis immediately precedes, as in the higher plants, the formation of certain spores, the tetra spores (Dictyotaceae, Polysiphonia), the teleutospores (Uredineae): but in others it immediately precedes the development of the sexual organs (Fucaceae), or follows more or less directly upon the sexual act (Green Algae, Nemalion, Ascomycetes).
The life-history of most Thallophyta is further complicated by the capacity of the gametophyte of the sporophyte to reproduce themselves by cells termed gonidia, a capacity that is wholly lacking in the higher plants. The karyology of gonidia has not yet been sufficiently investigated: but when, as in the Green Algae and the Oemycetous Fungi, the gonidia are developed by and reproduce the gametophyte, it may be inferred that they, like the gametophyte, are haploid. One case, at any rate, of the reproduction of the sporophyte by gonidia is fully known, that of the Uredineae just described, in which the uredoform, which is a phase of the sporophyte, is reproduced by the uredo-spores which are binucleate, that is diploid, and may be distinguished as diplogonidia. In any case the result is that whereas in the higher plants each of the alternating generations occurs but once in the life-history, in these Thallophyta the life-history may include a succession of gametophytic or of sporophytic forms. This is, in fact, a distinguishing feature of the group. The higher plants present a regular alternation of generations: whereas, in the Thallophyta, though they probably all present some kind of alternation of generations, yet it is irregular in the various ways and for the various reasons mentioned above.
Sufficient information has been given in the preceding pages to render possible the consideration of the origin of alternation of generations. To begin quite at the beginning, it may be assumed that the primitive form of reproduction was purely vegetative, merely division of the unicellular organism when it had attained the limits of its own growth. Following on this came reproduction by a gonidium: that is, the protoplasm of the cell, at the end of its vegetative life, became quiescent, surrounded itself with a proper wall, or was set free as a motile ciliated cell, having in some unexplained way become capable of originating a new course of life (rejuvenescence) on germination. Then, as can be well traced in the Brown and Green Algae (see Algae), these primitive reproductive cells (gonidia) began to fuse in pairs: in other words, they gradually became sexual. This stage can still be observed in some of these Algae (e.g. Ulothrix, Ectocarpus) where the zoo spores (gonidia) may either germinate independently, or fuse in pairs to form a zygote. Gradually the sexuality of these cells became more pronounced: losing the capacity for independent germination, they acquired the external characters of more or less differentiated sexual cells, and the gametangia producing them developed into male and female sexual organs. But this advancing sexual differentiation did not necessarily deprive the plant of the primitive mode of propagation: the sexual organism still retained the faculty of reproduction by gonidia. The loss of this faculty only came with higher development: it is entirely wanting in some of the higher Thallophyta (e.g. Fucaceae, Characeae), and in all plants above them in the evolutionary series.
With the introduction of the sexual act, a new kind of reproductive
cell made its appearance, the zygote. This cell, as
already explained, differs from other kinds of spores and from
the sexual cells, in that its n
ecleus is diploid; and with it the
sporophyte (diplophyte) was introduced into the life-history. It
has been mentioned that in some plants (e.g. Green Algae) the
zygote is all that there is to represent the sporophyte, giving rise,
or germination and after meiosis, to one or more spores. Passing
to the Bryophyta, in the simpler forms (e.g. Riccia), the zygote
develops into a multicellular capsule (sporogonium); and in the
higher forms into a more elaborate sporogonium, producing many
spores. In the Pteridophyta and the Phanerogams, the zygote
gives rise to the highly developed sporophytic plant.
Thus the evolution of the sporophyte can be traced from the unicellular zygote, gradually increasing in bulk and in independence until it becomes the equal of the gametophyte (e.g. in Dictyota and Polysiphonia), and eventually far surpasses it (Pteridophyta, Phanerogams). Moreover, the increase in size was attended by the gradual limitation of spore-production to certain parts only, the rest of the tissues being vegetative, assuming the form of stems, leaves, &c. These facts have been formulated in the theory of “progressive sterilization” (Bower), which states that the sporophytic form of the higher plants has been evolved from the simple, entirely fertile, sporophyte of the lower, by the gradually increasing development of the sterile vegetative tissue at the expense of the sporogenous, accompanied by increase in total bulk and in morphological and histological differentiation.
In connexion with the study of the evolution of the sporophyte, the question arose as to its morphological significance; whether it is to be regarded as modified form of the gametophyte, or as an altogether new form intercalated in the life-history: in other words, whether the alternation is “homologous” or “antithetic.” In certain plants there is a succession of forms which are undoubtedly homologous: for instance, in Coleochaete where a succession of individuals without sexual organs is produced by zoospores (gonidia). The main fact that has been established is that the sporophyte, from the simple zygote of the Thallophyta to the spore-bearing plant of the Phanerogams, is characterized by its diploid nuclei; that it is a diplophyte, in contrast to the haplophytic gametophyte. Were these nuclear characters absolutely universal, there could be no question but that the sporophyte is an altogether new antithetic form, and not an homologous generation. But certain exceptions to the rule have been detected, which are described under Abnormalities of Reproduction: at present it will suffice to say that such things as a diploid gametophyte and a haploid sporophyte have been observed in certain ferns. It can only be inferred that alternation of generations is not absolutely dependent upon the periodic halving in meiosis and the subsequent doubling by a sexual act, of the number of chromosomes in the nuclei, though the two sets of phenomena usually coincide. It must not, however, be overlooked that these exceptional cases occur in plants presenting an abnormal life-history: the fact remains that where there is both normal spore-formation with meiosis, and a subsequent sexual act, the haploid form is the gametophyte, the diploid the sporophyte. But the actual observation of a haploid sporophyte and of a diploid gametophyte makes it clear that however generally useful the nuclear characters may be in the distinction of sporophyte and gametophyte, they do not afford an absolute criterion, and therefore their value in determining homologies is debatable.
IV. Abnormalities of Reproduction.
In what may be regarded as the type of normal life-history, the transition from the one generation to the other is marked by definite processes: there is the meiotic development of spores by the sporophyte, and the sexual production of a zygote, or something analogous to it, by the gametophyte. But it has been mentioned in the preceding pages that the transition may, in certain cases, be effected in other ways, which may be regarded as abnormal, though they are constant enough in the plants in which they occur, in fact as manifestations of reproductive degeneration.
In the first place, the sporophyte may be developed either after an abnormal sexual act, or without any preceding sexual act at all, a condition known as apogamy. In the second, the gametophyte may be developed otherwise than from a post-meiotic spore, a condition known as apospory.
Apogamy.—The cases to be considered under this head may be arranged in two groups:—
1. Pseudapogamy: sexual act abnormal.—The following abnormalities have been observed:—
- (a) Fusion of two female organs: observed (Christman) in certain Uredineae (Caeoma nitens, Phragmidium speciosum, Uromyces Caladii) where adjacent archicarps fuse: male cells (spermatia) are present but functionless.
- (b) Fusion between nuclei of the same female organ: observed in the ascogonium of certain Ascomycetes, Humaria granulata (Blackman), where there is no male organ; Lachnea stercorea (Fraser), where the male organ (pollinodium) is present but is apparently functionless.
- (c) Fusion of a female organ with an adjacent tissue-cell; observed (Blackman) in the archicarp of some Uredineae (Phragmidium violaceum, Uromyces Poae, Puccinia Poarum): male cells (spermatia) present but functionless.
- (d) There is no female organ: fusion takes place between two adjacent tissue-cells of the gametophyte; the sporophyte is developed from diploid cells thus produced, but there is no proper zygote as there is in a, b and c: observed (Farmer) in the prothallium of certain ferns (Lastraea pseudo-mas, var. polydactyla): male organs (and sometimes female) present but functionless. Another such case is that of Humaria rutilans (Ascomycete), in which nuclear fusion has been observed (Fraser) in hyphae of the hypothecium: the asci are developed from these hyphae, and in them meiosis takes place; there are no sexual organs.
2. Eu-apogamy: no kind of sexual act—
- (a) The gametophyte is haploid:
- (α) The sporophyte is developed from the unfertilized oosphere: no such case of true parthenogenesis has yet been observed.
- (β) The sporophyte is developed vegetatively from the gametophyte and is haploid: observed in the prothallia of certain ferns, Lastraea pseudo-mas, var. cristata-apospora (Farmer and Digby), and Nephrodium molle (Yamanouchi).
- (b) The gametophyte is diploid (see under Apospory):
- (α) The sporophyte is developed from the diploid oosphere: observed in some Pteridophyta, viz. certain ferns (Farmer), Athyrium Filix-foemina, var. clarissima, Scolopendrium vulgare, var. crispum-Drummondae, and Marsilia (Strasburger); also in some Phanerogams, viz. Compositae (Taraxacum, Murbeck; Antennaria alpina, Juel; sp. of Hieracium (Rosenberg): Rosaceae (Eu-Alchemilla sp., Murbeck, Strasburger): Ranunculaceae (Thalictrum purpurascens, Overton).
- (β) The sporophyte is developed vegetatively from the gametophyte: observed (Farmer) in the fern Athyrium Filix-foemina, var. clarissima.
- In all the cases enumerated under Eu-apogamy, apogamy is associated with some form of apospory except Nephrodium molle, full details of which have not yet been published.
- Many other ferns are known to be apogamous, but they are not included here because the details of their nuclear structure have not been investigated.
Apospory.—The known modes of apospory may be arranged as follows:—
1. Pseudapospory: a spore is formed but without meiosis, so that it is diploid—observed only in heterosporous plants, viz. certain species of Marsilia (e.g. Marsilia Drummondii) where the megaspore has a diploid nucleus (32 chromosomes) and the resulting prothallium and female organs are also diploid (Strasburger); and in various Phanerogams, some Compositae (Taraxacum and Antennaria alpina, Juel), some Rosaceae (Eu-Alchemilla, Strasburger), and occasionally in Thalictrum purpurascens (Overton), where the megaspore (embryo-sac) is diploid; in some species of Hieracium it has been found (Rosenberg) that adventitious diploid embryo-sacs are developed in the nucellus: these plants are also apogamous.
2. Eu-apospory: no spore is formed—of this there are two varieties:
- (a) With meiosis: this occurs in some Thallophyta which form no spores; the sporophyte of the Fucaceae bears no spores, consequently meiosis takes place in the developing sexual organs; the Conjugate Green Algae also have no spores, meiosis taking place in the germinating zygospore which develops directly into the sexual plant.
- (b) Without meiosis: the gametophyte is developed upon the sporophyte by budding; that is, spore-reproduction is replaced by a vegetative process: for instance, in mosses it has been found possible to induce the development of protonema, the first stage of the gametophyte, from tissue-cells of the sporogonium: similarly, in certain ferns (varieties of Athyrium Filix-foemina, Scolopendrium vulgare, Lastraea pseudo-mas, Polystichum angulare, and in the species Pteris aquilina and Asplenium dimorphum), the gametophyte (prothallium) is developed by budding on the leaf of the sporophyte, and in some of these cases it has been ascertained that the gametophyte so developed has the same number (2x) of chromosomes in its nuclei as the sporophyte that bears it—that is, it is diploid.
- Apospory has been found to be frequently associated with apogamy; in fact, in the absence of meiosis, this association would appear to be inevitable.
Combined Apospory and Apogamy.—Instances have been given of the occurrence of both apospory and apogamy in the same life-history; but in all of them there is a regular succession of sporophyte and gametophyte. The cases now to be considered are those in which one or other of the generations gives rise directly to its like, sporophyte to sporophyte, gametophyte to gametophyte, the normally intervening generation being omitted.
It is possible to conceive of this abbreviation of the life-history taking place in various ways. Thus, a sporophyte might be developed from a haploid spore instead of a gametophyte as is the normal case, but this has not been observed: again, a sporophyte might be developed from a diploid spore (as distinguished from a zygote or a diploid oosphere), a possibility that is to some extent realized in the life-history of some Uredineae in which successive forms of the polymorphic sporophyte are developed from diplogonidia. Similarly a gametophyte might be developed from a fertilized or an unfertilized female cell: the latter possibility is to some extent realized in those Algae (e.g. Ulothrix, Ectocarpus) in which the sexual cells (isogametes), if they fail to conjugate, germinate independently as gonidia, giving rise to gametophytes.
The more familiar mode is that of vegetative budding, as already mentioned. When a “viviparous” fern or Phanerogam reproduces itself by a bud or a bulbil, both spore-formation and the sexual act are passed over: sporophyte springs from sporophyte. Remarkable cases of this have been observed in certain Phanerogams (Coelebogyne ilicifolia, Funkia ovata, Nothoscordum fragrans, Citrus, sp. of Euonymus, Opuntia vulgaris) in the ovule of which adventitious embryos are formed by budding from cells of the nucellus: with the exception of Coelebogyne, it appears that this only takes place after the oosphere has been fertilized. In other plants it is the gametophyte that reproduces itself by means of gemmae or bulbils, as commonly in the Bryophyta, the prothallia of ferns, &c.
The abnormalities described are all traceable to reproductive degeneration; the final result of which is that true reproduction is replaced more or less completely by vegetative propagation. It may be inquired whether degeneration may have proceeded so far in any plant of sufficiently high organization to present spore-formation, or sexual reproduction, or both, as to cause the plant to reproduce itself entirely and exclusively by the vegetative method. The only such case that suggests itself is that of Caulerpa and possibly some other Siphonaceous Green Algae. In this plant no special reproductive organs have yet been discovered, and it certainly reproduces itself by the breaking off of portions of the body which become complete plants: but it is quite possible that reproductive organs may yet be discovered.
V. Physiology of Reproduction.
The reproductive capacity of plants, as of animals, depends upon the fact that the whole or part of the protoplasm of the individual can develop into one or more new organisms in one or other of several possible ways. Thus, in the case of unicellular plants, the whole of the protoplasm of the parent gives rise, whether by simple division or otherwise, to one or more new plants. Reproduction necessarily closes the life of the individual: here, as August Weismann long ago pointed out, there is no natural death, for the whole of the protoplasm of the parent continues to live in the progeny. In multicellular plants, on the contrary, the reproductive function is mainly discharged by certain parts of the body, the reproductive organs, the remainder of the body being essentially vegetative—that is, concerned with the maintenance of the individual. In these plants it is only a part of the protoplasm that continues to live in their progeny; the remainder, the vegetative part, eventually dies. It is therefore possible to distinguish in them, on the one hand, the essentially reproductive protoplasm, which may be designated by Weismann's term germ-plasm, though without necessarily adopting all that his use of it implies, and the essentially vegetative, mortal protoplasm, the somatoplasm, on the other. In the unicellular plant no such distinction can be drawn, for the whole of the protoplasm is concerned in reproduction. But even in the most highly organized multicellular plant this distinction is not absolute: for, as already explained, plants can, in general, be propagated by the isolation of almost any part of the body, that is vegetatively, and this implies the presence of germ-plasm elsewhere than in the special reproductive organs.
If the attempt be made to distinguish between the organs of vegetative propagation and those of true reproduction, the nearest approach would be the statement that the former contain both germ-plasm and somatoplasm, whereas the latter, or at least the reproductive cells, consist entirely of germ-plasm.
The question now arises as to the exact seat of the germ-plasm, and the answer is to be looked for in the results of the numerous researches into the structure and development of the reproductive cells that form so large a part of the biological work of recent years. The various facts already mentioned suffice to prove that the nucleus plays the leading part in the reproductive processes of whatever kind: the general conclusion is justified that no reproductive cell can develop into a new organism if deprived of its nucleus. It may be inferred that the nucleus either actually contains the germ-plasm, or that it controls and directs the activities of the germ-plasm present in the cell. It is not improbable that both these inferences may be true. At any rate there is no sufficient ground for excluding the co-operation of the cytoplasm, especially of that part of it distinguished as kinoplasm, in the reproductive processes.
Pursuing the ascertained facts with regard to the nucleus, it is established that the part of it especially concerned is the linin-network which consists of the chromosomes. The behaviour, as already described, of the chromosomes in the various reproductive processes has led to the conclusion that the hereditary characters of the parent or parents are transmitted in and by them to the progeny: that they constitute, in fact, the material basis of heredity (see Heredity). They can hardly, however, be regarded as the ultimate structural units, for the simple reason that their number is far too small in relation to the transmissible characters. It has been suggested (Farmer) that the chromomeres are the units, but the number of these would seem to be hardly sufficient. It seems necessary to fall back upon hypothetical ultimate particles, as suggested by Darwin, de Vries and Weismann, which may be generally termed pangens. The chromomeres may be regarded as aggregates of such particles, the “ids” of Weismann.
The foregoing considerations make it possible to attempt an explanation of the various reproductive processes.
Vegetative Propagation.—It is easily intelligible that the two individuals produced by the division of a unicellular plant should resemble the parent and each other; for, the division of the parent-nucleus being homotypic, the chromosomes which go to constitute the nucleus of each daughter-cell are alike both in number and in nature, and exactly repeat the constitution of the parent-nucleus.
In the more complicated cases of propagation by bulbils, cuttings, &c., the development of the new individual, or of the missing parts of the individual (roots, &c.), may be ascribed to the presence in the bulbil or cutting of the necessary pangens.
Reproduction by Gonidia.—In this case a single cell gives rise to a complete new organism resembling the parent. The inference is that the gonidium is a portion of the parental germ-plasm, in which all the necessary pangens have been accumulated.
Reproduction by Spores.—In this case, also, an entire organism is developed from a single cell, but with this peculiarity that the resulting organism is unlike that which bore the spore, a peculiarity which has not yet been explained. It has been already stated that the development of true spores involves meiosis, and this process is no doubt related to the behaviour of the spore on germination; but the nature of this relation remains obscure. It might be assumed that, as the result of meiosis, the nucleus of the spore receives only gametophytic pangens. But the assumption is rendered impossible by the fact that the spore gives rise to a sexual organism, the reproductive cells of which, after the sexual act, produce a sporophyte. Clearly sporophytic pangens must be present as well in the spore as in the gametophyte and in its sexual cells. It can only be surmised that they exist there in a latent condition, dominated, as it were, by the gametophytic pangens.
Sexual Reproduction.—Here, again, as yet unanswered questions present themselves. The essence of a sexual cell is that it cannot give rise by itself to a new organism, it is only truly reproductive after the sexual act: this peculiarity is just what constitutes its sexuality. Minute investigation has not yet detected any essential structural difference between a sexual cell and a spore; on the contrary, the results so far obtained have established that they essentially agree in being post-meiotic (haploid). Why then do they differ so fundamentally in their reproductive capacities? Again, sexual cells differ in sex; but there are as yet no facts to demonstrate any essential structural difference between male and female cells. What is known about them tends to prove their structural similarity rather than their difference. But it is possible that their difference may be chemical, and so not to be detected by the microscope.
The normal sexual act has been described as consisting in the fusion, first, of two cells, then of their nuclei, and finally, often after a long interval, of their chromosomes and of their chromomeres in meiosis. What causes determined these fusions is a question that is only partly answered. It is known in certain cases (e.g. ferns and mosses) that the male cell is attracted to the female by chemical substances secreted for the purpose by the female organ; that it is a case of chemiotaxis. Probably this is more common than experiment has yet shown it to be. It is quite conceivable that the consequent cell-fusion, as also the subsequent fusions of nuclei and of chromosomes, are likewise cases of chemiotaxis, depending upon chemical differences between the fusing structures.
The sexual process can only take place between cells which are related to each other in a certain degree (see Hybridism); that is, it depends upon sexual affinity. It is the general rule that it takes place between cells derived from different individuals of the same species; that is, cross-fertilization is the rule. This is necessarily the case when the male and female organs are developed upon different individuals, when the plant is said to be dioecious. When both kinds of organs are developed upon the same individual (monoecious), self-fertilization may and often does occur; but it is commonly hindered by various special arrangements, of which dichogamy is the most common; that is, that the male and female organs are not mature at the same time. But though these arrangements favour cross-fertilization, they do not absolutely prevent self-fertilization. In some cases, cleistogamic flowers, for instance, self-fertilization alone is possible (see Angiosperms). The general conclusion is that though cross-fertilization is the more advantageous form of sexual reproduction, still self-fertilization is more advantageous to the species than no fertilization at all.
In considering this subject, it must be borne in mind that the terms used have different meanings when applied to certain heterosporous plants from those which they convey when applied to isosporus plants. In the latter cases their meaning is direct and simple: in the former it is indirect and somewhat complicated. In heterosporous plants generally the actual sexual organs are never borne upon the same individual, there is always necessarily a male and a female gametophyte; so that, strictly speaking, self-fertilization is impossible. But in the Phanerogams, where there isaprocess preliminary to fertilization, that of pollination, which is unknown in other plants, the terms and the conceptions expressed by them are applied, not to the real sexual organs, but to the spores. Thus a dioecious Phanerogam is one in which the microspores are developed by one individual, the megaspores by another; and again, self-fertilization is said to occur when the microspores (pollen) fall upon the stigma of the same Bower (see Angiosperms); but this is really only self-pollination.
To return to the sexual process itself. Whatever its nature, two sets of results follow upon the sexual act—(1) a zygote is formed, which is capable of developing into a new organism, from two cells, neither of which could so develop; (2) the hereditary sporophytic characters of the two parents are possessed by the organism so developed. These two results will now be considered in some detail.
(1) The Relation between the Sexual Act and Reproductive Capacity—In the early days of the discovery of the sexual process, it was thought that the capacity for development imparted to the female cell was to be attributed to the doubling of its nuclear substance by the fusion with the male cell. Reproductive capacity does not, however, depend upon the bulk of the nuclear substance, for a spore, like an unfertilized female cell, contains but the x number of chromosomes, and yet it can give rise to a new organism. Again, it has been observed (Winkler) that a non-nucleated fragment of an oosphere of Cystoseira (Fucaceae) can be “fertilized” by a spermatozoid and will then grow and divide to form a small embryo, though it necessarily contains only the x number of chromosomes. From this it would appear that some stimulating influence had been exerted by the male cell, and it is probably in this direction that the desired explanation is to be sought. Some important confirmatory facts have been recorded with regard to certain animals (sea-urchins). It has been observed (Loeb) that treatment with magnesium chloride will cause the ova to grow and segment; and similar results have been obtained (Winkler) by treating the ova with a watery extract of the male cells. Hence it may be inferred that the male cell carries with it, either in its cytoplasm (kinoplasm), or in its nucleus, extractable substances, perhaps of the nature of enzymes, that stimulate the female cell to growth.
It may be mentioned that the stimulating effect of fertilization is not necessarily confined to the female cell; very frequently adjacent tissues are stimulated to growth and structural change. In a Phanerogam, for instance, the whole ovule grows and develops into the seed: the development of endosperm in the embryo-sac is initiated by another nuclear fusion, taking place between the second male nucleus and the endosperm-nucleus: the ovary, too, grows to form the fruit, which may be dry and hard or more or less succulent: the stimulating effect may extend to other parts of the flower; to the perianth, as in the mulberry; to the receptacle, as in the strawberry and the apple; or even beyond the flower to the axis of the inflorescence, as in the fig and the pine-apple. Analogous developments in other groups are the calyptra of the Bryophyta, the cystocarps of the Red Algae, the ascocarps of the Ascomycetes, the aecidia of the Uredineae, &c.
(2) The Relation of the Sexual Act to Heredity.—The product of the sexual act is essentially a diploid cell, the zygote, which actually is or gives rise to a sporophyte. The sexual heredity of plants consequently presents the peculiar feature that the organism resulting from the sexual act is quite unlike its immediate parents, which are both gametophytes. But it is clear that the sporophytic characters must have persisted, though in a latent condition, through the gametophyte, to manifest themselves in the organism developed from the zygote.
The real question at issue is as to the exact means by which these characters are transmitted and combined in the sexual act. There is a considerable amount of evidence that the hereditary characters are associated with the chromomeres, and that it is rather their linin-constituent than their chromatin which is functional (Strasburger): that they constitute, in fact, the material basis of heredity. From this point of view it is probable that the last phase of the sexual act, the fusion of the chromomeres in meiosis, represents the combination of the two sets of parental characters. What exactly happens in the pseudo-chromosome stage is not known; at any rate this stage offers an opportunity for a complete redistribution of the substance of the chromomeres—in other words, of the parental pangens. It is a striking fact that, in the subsequent nuclear division, the distribution of the chromosomes derived from the male and female parents (when they can be distinguished) seems to be a matter of indifference: they are not equally distributed to the two daughter-nuclei. The explanation would appear to be this, that they are not any longer male and female as they were before meiotic fusion; and that it is because they now contain both male and female nuclear substance that their equal distribution to the daughter-nuclei is unimportant.
The nature of this redistribution of the substance of the chromomeres is still under discussion. Some regard it as essentially a chemical process, resulting in the formation of new compounds; others consider it to be rather a physical process, a new material system being formed in the rearrangement of the pangens; here it must be left for the present.
The various ways in which the parental characters manifest themselves in the progeny are fully dealt with in the articles Heredity, Hybridism, Mendelism. It will suffice to say that the progeny, though maintaining generally the characters of the species, do not necessarily exactly resemble either of the parents, nor do they necessarily present exactly intermediate characters: they may vary more or less from the type. It is an interesting fact, the full significance of which has not yet been worked out, that, as a rule, plants that vary profusely are those in which the characteristic 2x number of chromosomes is high (60-100).
Brief reference may be made to the cases of abnormal sexual or pseudo-sexual reproduction described above under Apogamy. Taking first the cases of true apogamy, there is clearly no need for any sexual process, for, since no meiotic division has taken place, the gametophyte is diploid; its cells, whether vegetative or contained in female organs, possess the capacity for both development and the transmission of the sporophytic characters. It is not remarkable that such a gametophyte should be able to give rise directly to a sporophyte; but it is remarkable, in the converse case of apospory, that a sporophyte should give rise to a diploid gametophyte rather than to another sporophyte. In the latter case the tendency to the regular development of the alternate form appears to override the influence of the diploid nucleus.
Turning to the various forms of pseudo-apogamy, there are first those in which fusion takes place between two apparently female organs (some Uredineae; Christman), and those in which it takes place between nuclei within the same female organ (Humaria; Blackman). If these are to be regarded physiologically as sexual acts, it must be inferred that the fusing organs or nuclei have come to differ from each other to some extent; for it is unthinkable that equivalent female organs or cells should be able to fertilize, or to be fertilized by, one another. There are finally those cases in which apparently vegetative cells take part in the sexual act, as in Phragmidium (Blackman), where the female organ fuses with an adjacent vegetative cell, and in the fern-prothallium (Farmer), where the nuclei of two vegetative cells fuse. They would seem to indicate that vegetative cells may, in certain circumstances, contain sufficient germ-plasm to act as sexual organs without being differentiated as such.
An interesting question is that of the origin of apogamy. It is no doubt the outcome of sexual degeneration; but this general statement requires some explanation. In certain cases apogamy seems to be the result of the degeneration of the male organ; as in Humaria, where there is no male organ, and in Lachnea, where the male organ is rudimentary. In others, as in the Uredineae, it is apparently the female organ that has degenerated, losing its receptive part, the trichogyne; the male cells (spermatia) are developed normally, and there is no reason to believe that they might not fertilize the female organ were there the means of penetrating it. In yet other cases the degeneration occurs at a different stage in the life-history, in the development of the spores. In the apogamous ferns investigated, meiosis is suppressed and apogamy results. In the heterosporous plants which have been investigated (e.g. Marsilia, Eu-Alchemilla) it has been observed that the microspores are so imperfectly developed as to be incapable of germinating, so that fertilization is impossible; and it is perhaps to this that the occurrence of apogamy is to be attributed. This abnormal development of the spores may be regarded as a variation; and in most cases it occurs in plants that are highly variable and often have a high 2x number of chromosomes.
It will be observed that such physiological explanation as can be given of the phenomena of reproduction is based upon the results of the minute investigation of the changes in nuclear structure associated with them. The explanation is often rather suggested than proved, and some fundamental facts still remain altogether unexplained. But it may be anticipated that a method of research which has already so successfully justified itself will not fail in the future to elucidate what still remains obscure.
Bibliography.—This article should be read in connexion with the following: Algae, Angiosperms, Bryophyta, Cytology, Fungi, Gymnosperms, Heredity, Hybridism, Mendelism, Plants, Pteridophyta.
As the bibliographies to these articles include all the publications containing the facts and theories mentioned here, it will suffice to append only a few papers of general importance: Blackman and Fraser, “Further Studies on the Sexuality of the Uredineae,” Ann. Bot. (1906) vol. xx.; Farmer, “On the Structural Constituents of the Nucleus, and their Relation to the Organization of the Individual” (Croonian Lecture), Proc. Roy. Soc. (1907) vol. 79, series B; Farmer and Digby, “Studies in Apospory and Apogamy in Ferns,” Ann. Bot. (1907) vol. xxi.; Strasburger, Die stofflichen Grundlagen der Vererbung (1905); “Apogamie bei Marsilia,” Flora (1907), vol. 97; D. M. Mottier, Fecundation in Plants (1904), Carnegie Institution, Washington.