Popular Science Monthly/Volume 25/June 1884/Modes of Reproduction in Plants
THE sexual generation of a plant is that stage in its life-history which bears the male and female organs, while the asexual generations are those having no sexuality manifest. The two kinds of generations frequently follow each other in alternate order, when there is what is known as an alternation of generations.
Growing plants continue to increase in size in a well-defined manner for a time, and then a single cell, or a small group of cells, begins on a new line of development. This new growth finally becomes detached from the parent-plant. The offspring may, or may not, be like the parent. Plants which result from similar cells developed in the same manner belong to the same generation. If the original reproductive cells grow into plants without any union with other cells, these plants are asexually produced. If the union of the contents of two or more cells is necessary for further development, there results a sexual product.
It is the purpose of this paper to trace the relative size of the two generations above described, in a number of the higher orders of plants. In doing this, the fact of the alternation will be developed. The series of orders will begin with the humble Hepaticæ, and end with the most highly developed of flowering plants. The Hepaticæ, or liverworts, are small, flowerless plants of very simple structure, which grow for the most part in moist places upon the bark of trees, surface of long-exposed rocks, earth, etc. One of the leading genera is Marchantia species of which abound on the earth of flower-pots in greenhouses and elsewhere. The leaf-like expansion or thallus is the sexual generation, and bears the male and female organs in depressions of the surface. The male parts, called antheridia, produce spermatozoids, which are spiral, slender bodies, provided with two motile hairs or cilia, as locomotive appendages. The female organs (archegonia) are at first single cells, which by division form flask-like structures, the lower cell of which is the female germ-cell. When this germ is fertilized by the antherozoids, which enter at the neck of the "flask," it undergoes a development, varying somewhat in the different orders, but essentially a sporangium or spore-case is produced, in which are very many spores and slender spiral threads arranged in rows. This sporangium is the second and asexual generation of the liver-wort. The complexity of the structure of
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the first or leaf-like generation and of the sexual organs and sporangia increases in the Hepaticæ group in passing from the lower to the higher forms. In the highest group there are stems with leaves arranged in rows, and the sporangia are raised on long stalks. Fig. 1 shows at t a portion of Marchantia polymorphia, with an upright receptacle, h, bearing the male organs. In Fig. 2 is seen a stem of Plagiochila asplenioides; a is a ripe sporangium, and b one that has opened.
In the mosses, the next class in the upward scale of plant-life, the spore germinates by producing a fine green thread, which branches and forms a plant much resembling many of the filamentous fresh-water algæ. In the order Sphagnaceæ this "alga form" is a flat expansion similar to the sexual generation of many liverworts. Fig. 3 shows
the germinating spores of Funaria hygrometrica, at A, and the fine branching, green threads (B) that are afterward produced. The true moss-plant, with its small stems, and fine, regularly arranged leaves, originates from specialized cells in the protonema, or alga form. K, in Fig. 3, shows the rudiment of a leaf-bearing axis. On the conspicuous moss-plant, arising from such small beginnings, the sexual organs are borne. They are usually produced in clusters at the ends of the leafy axes. Most mosses have only one sex represented in a single tip, and some species have the separation of the sexes so complete that a plant bears only one kind. Mosses, as well as trees and shrubs, are sometimes monœcious or diœcious. Fig. 4 shows the male and female organs very much magnified. The antheridium. A, is a stalked, club-shaped structure, inclosing a large number of sperm-cells, b, each of which produces a spiral spermatozoid, c. These minute bodies move rapidly by means of two cilia, and find their way to the neck of the female organ, B. The germ-cell, b, to be fertilized, is at the base of the long structure, with a mucilaginous channel. h, leading down to it. After the spermatozoids have united with the germ-cell, the latter soon begins a new growth, and a young sporangium results. Fig. 5 shows different stages in the development of the sperm-case; and in Fig. 6 is seen a vertical section of one fully grown, showing the various parts of theca, calyptera, operculum, etc. In this complicated sporangium, small spores in great numbers are produced, and with their perfection ends the last chapter in the life-history of the moss.
The spore produces a fine filamentous growth, from which the true moss-plant develops. This is the sexual generation, and from fertilized germ-cells, which it bears, the asexual generation is produced, consisting of the spore-case, its stalk, and, most important of all, the many spores.
We now come to a more exalted group of plants, and the first of the cryptogams with spiral vessels and other ducts in the wood. The ferns are so familiar to all that any description of their general appearance is unnecessary. The first generation proceeds directly from the spore, and consists of a simple green expansion which is short-lived and very small, not usually exceeding half the size of a small finger-nail. This prothallus, as it is termed, has small, root-like hairs which fix it to the earth or elsewhere. The prothallia are to be seen in large numbers on the sides of flower-pots in neglected greenhouses. Each little green scale is a young fern-plant during its sexual generation. The male and female organs are much the same shape as those of mosses. Fig. 7 shows a prothallus seen from the under side and much magnified; h, are the root-hairs; an, antheridia; and ar, archegonia. The antheridia produce cork-screw coiled antherozoids which pass to the archegonia and fertilize their germ-cells. The second generation develops
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from the germ-cell, as shown in Fig. 8. By a further growth of stem and fronds, the well-known state of the fern is produced. The spores are borne on the under side or edges of the fronds. In some species the spores are formed only on a portion of the fronds, the others being sterile. The plant commonly known as the fern does not have any male or female parts, and may live for many years, producing countless spores. The sexes are confined to the minute scale, which is so small as to pass unnoticed, and if seen would not suggest its origin or destiny. Dr. Farlow has discovered instances where the prothallia produced fern-plants without the usual process of fertilization. These are only the exceptions which prove the rule.
There is a little group of ferns to which the "adder-tongue" belongs, that has the prothallus underground, consisting of an irregular mass of colorless cells, which may not exceed one twenty-fifth of an inch in diameter. This group forms another step toward a greaterFig. 8. simplicity of the sexual generation in plants. The spores of the Ophioglossaceæ are developed less superficially on the fronds than in the lower orders of ferns. This is a morphological point which is worthy of mention here. The whole structure of the asexual generation is more highly developed than in other ferns, while the sexual generation is much reduced and simplified.
The Equisetaceæ, or horse-tails, form a small group of flowerless plants, with hollow, jointed stems and cone-like spore-heads (Fig. 9). The scouring-rush, with its rough, grooved stem, is a leading member of this family. The prothallia are small and irregularly branched, and in most species the male and female parts are on separate plants (diœcious). The antheridia-bearing prothallia are much smaller than the female, the latter being sometimes half an inch in length. The structure of the male and female organs is
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much the same as in ferns. The antherozoids are larger, and the archegonia are more deeply situated in the prothallus. The conspicuous horse-tail develops from the fertilized germ-cell, and constitutes the spore-bearing asexual generation. In some species a colorless or brownish stem comes up in early spring, which bears the spores on whorls of modified leaves, and afterward perishes. Later in the spring the green stems arise. This shows a greater differentiation in the asexual generation.
Next above the horse-tails come the Rhizocarpeœ, a small cryptogamic group of water-plants, inhabiting ditches, streams, etc. Thus far, in our upward course, we have found only one kind of spore. Here there are two sorts, the large and the small. The former produce archegonia, and are therefore essentially female, while the smaller spores are male, and produce antherozoids. These spores are formed in spore-cases, termed sporocarps. Fig. 10 shows a plant of Marsilia salvatrix, reduced one half; K is the terminal bud; b b, leaves; f f, sporocarps. In these last the spores of both sizes are produced. The contents of the small or male spores divide and develop into a number of antherozoids, which afterward escape through a rupture in the spore-wall. A small portion of the spore does not take part in this formation of antherozoids, and may be considered the prothallus. In the large or female spore the prothallus is larger, and only one end of the spore bears a single archegonium. In Fig. 11, at A, is shown
a vertical section of the archegonium end of a large spore; w w are parts of the ruptured spore-wall; p p is the prothallus, and g the germ-cell. At B is a male spore of the same species, with its wall ruptured, and the corkscrew-like antherozoids, 5, escaping. The second generation soon develops from the fertilized germ-cells, and produces the mature plant. It is seen that the sexual generation in the Marsilia group is reduced to two kinds of spores, with their rudimentary prothallia. In another branch of the Rhizocarpeœ, while in most features the life-history is as just described, there is a further differentiation in the sporocarps. The male and female spores are produced in separate sporocarps. Fig. 12 shows a section through three spore-cases, two of them bearing the small males pores, and the third, a, the large female spores.
In the next higher group, as arranged by botanists, we find the club-mosses; these are common plants with trailing or upright very leafy stems. Fig. 13, A, shows the tip of a spore-bearing branch, natural size, and B a longitudinal section much enlarged. The large spores are borne in sporangia on one side, while the small ones are on the left. The differentiation has now reached the place where there is a definite arrangement of the sporangia on the plant bearing them. In the development of the male spores the cells in which the antherozoids form are not produced directly from the spore-contents. This is a valuable link in the chain of relationship which binds this group with higher plants—in fact, helps to bridge what gulf Fig. 13. there may have been thought existing between the flowerless and flowering plants. The ripe male spore-contents are changed into a few cells, one of which remains sterile and is considered the prothallus, while from the other cells—which taken as a whole constitute the antheridium—the cells which afterward bear the antherozoids are formed. In the genus Selaginella the female spore produces a small prothallus, as shown at 1, Fig. 14. The portion above d d, in this cross-section of the spore, is the prothallium, and at e e are two embryo plants. At 2 is a young archegonium not opened; 3 shows one further advanced, with the fertilized germ-cell divided. A is a male spore, showing the cell-division; D is a later state of the same, with the large antheridium filled with sperm-cells. The rudimentary prothallus is at V. The female is still more simple in Isoetes, shown in Fig. 15: 1 is the longitudinal section of the female spore, with an archegonium, a r, at the top; 2 shows the early differentiation of cells into archegonia, a r, a r, with their germ-cells, g g; 3, 4, and 5 show successive stages in the development of the germ-cell. There is a close resemblance to the embryo sac of Gymnosperms (pines, spruces, etc.). The prothallus is very much reduced, and projects
through the slit in the spore-covering. In the development of the female germ-cell after fertilization, there is an elongation of the upper part (see e, Fig. 14), forming the part called the suspensor, a
body which is not found in other cryptogams, but is present in embryos of flowering plants.
We now pass to phenogamous or flowering plants, in which the small male spore of the higher cryptogams takes the name of pollen-grains, and the larger female spore is known as the embryo sac. The latter does not sever its connection with the mother-plant until after an embryo plant has formed. On this account the prothallium—which we have seen as an independent structure in ferns and diminishing gradually as we ascended in the scale of flowerless plants—is here but feebly developed. The flower-bearing plant, whether herb, shrub, or tree, is the asexual generation producing two kinds of structures, which, by their development and union of parts, produce a plant like the one from which the sexual generation sprang. The pollen-grain is usually a small spherical or oval body that, when mature, separates from the case (anther) in which it was formed. Figs. 16 shows the form of some simple pollen-grains. Grain A shows the rudimentary Fig. 16. prothallium as a small cell, y; B is a pollen-grain forming the tube. Much the same is seen at C and D, excepting that the prothallium is made up of three small cells. In structure and function these pollen-grains are almost identical with the male spores of higher cryptogams. The embryo sac is more or less surrounded by the substance of the parent-plant, and develops within itself a prothallus of small size which is known as the endosperm, and is a store-house of nourishing matter for the young embryo. One or more cells form the homologue of the archegonia in higher cryptogams with its female germ. The male cell or pollen-grain no longer develops a number of mobile, fertilizing antherozoids or spermatozoids; but, instead, the whole pollen-grain passes to a receptive surface (stigma) situated somewhere near the female organ, from which it sends out a tube that penetrates the tissue provided for its passage (style) until it reaches its destination and mingles its contents with those of the female cell. Circumstances obtain in the flowering plants which render mobile bodies like spermatozoids worthless as a means of fertilization. In many cases the male element needs to pass from one tree to another, and even from one country to another. The first observed result of fertilization is the formation of the suspensors, mentioned under Selaginella. At the lower end of the suspensor the young plantlet is formed with its one or more small seed-leaves and a short root and stem. In this growth the food-material in the endosperm is frequently entirely exhausted. The ovule, as the female cell with its immediate surrounding tissue is termed, becomes inclosed with one or more coats of varying thickness, and, when the whole structure has reached maturity and is ready to separate from the parent-plant, we have the familiar body known as a seed. The seed is an independent plant-structure designed to develop into a mature herb, shrub, or tree, when the conditions are favorable for its germination and growth.
The lowest class of flowering plants is the Gymnosperms—which includes the cycads and cone-bearing plants. The ovules are naked, and the embryo develops considerable endosperm. This corresponds to the prothallus of higher cryptogams, as in it the corpuscula, which correspond with archegonia, are formed. The pollen-grains are several-celled, thus suggesting a prothallus, especially as only a part of the grain takes part in the formation of the tube and the male fertilizing fluid. The gymnosperms evidently occupy an intermediate place between the higher cryptogams and the angiosperms or flowering plants with their ovules inclosed in an ovary. This last class contains the great mass of plants with evident floral organs, and is divided into the exogens, like the oak, apple, and rose, and the endogens, illustrated by the grasses and cereal grains.
The points that interest us most in the present consideration are, that, unlike the gymnosperms, the ovules are inclosed in an ovary; the endosperm forms in the embryo sac after fertilization, and the pollen-grain sends out its tube without previous cell-division. The pollen-grains Fig. 17. often have no rudimentary prothallus. The first result of fertilization is the formation of a cellulose wall around the germ-cell. This cell soon divides, forming the suspensor at the lower end, from which the embryo plantlet is developed. The endosperm-cells form at the same time at the opposite end of the embryo sac. Fig. 17, A, shows a longitudinal section of a young ovule shortly after fertilization. The embryo sac is at e, with a small embryo at the left end, and free endosperm-cells formed at the other. The embryo is shown more magnified at B, and at C is seen the same more advanced. The endosperm is rich in food-materials for the growing embryo, and may be entirely absorbed and the space occupied by the latter.
It now remains for us to determine the extent of the sexual generation in the flowering plants. Among gymnosperms it is not difficult to see that it consists of the pollen-grain and the embryo sac with its endosperm. The male prothallus is reduced and rudimentary, and one cell of the pollen-grain, representing the antheridium, produces a tube instead of antherozoids. The endosperm is the female prothallus, and in it the germ-cell develops. The asexual generation is the plant that grows from the fertilized germ. In the angiosperms the sexual generation is reduced to its simplest form, namely, a single cell for the male part, and one or a few cells for the female.
We thus see that the alternation of generation, viewed in the light of its presentation among mosses and ferns, practically disappears in the higher flowering plants. The sexual generation is so reduced and merged with the asexual that the two seem to become one, and, were it not that the gradual simplifying of this generation may be traced, it would not be thought to exist.
If we recapitulate, in the reverse order, it is easy to evolve a conspicuous, independent plant from the single-celled pollen-grain, and a similar self-supporting plant from the simple embryo sac. The first step back is to the gymnosperms, where the ovules are not in ovaries, and the embryo sac has a rudimentary prothallus in the endosperm. The pollen-grain is made up of more then one cell. From this group we pass to the Selaginellæ, in which we have the female spore, with its rudimentary prothallus, and the smaller male spore, having one cell for the prothallus, the remaining ones forming the antherozoids. Descending to the Rhizocarpeæ both the spores increase in size and complexity. Further back we come to the higher orders of ferns, with but one kind of spore, the small prothallus, bearing both sexual organs. The lower orders have this sex-bearing generation much more developed. In the mosses and Hepaticæ the sexual generation surpasses the asexual in size and complexity.
The relative size of the two generations might be represented to the eye by drawing a rectangle with a diagonal. Fig. 18. One triangle
would indicate the size and complexity of the sexual generation, while the other represented the asexual generation. The sexual generation diminishes from the Hepaticæ to the angiosperms, while the asexual generation increases.
The engravings here employed are from treatises on botany by Sach, Prantl, and Bessey, to whom the writer is also indebted for many of the facts brought together.