Popular Science Monthly/Volume 33/October 1888/A Living Mystery
By GRANT ALLEN.
I HOLD in my hand here a key to one of the greatest mysteries of life—the perennial mystery of birth and reproduction.
And yet you needn't be in the least afraid that the mystery or its solution involves any technical scientific language, or possesses any tinge of occult abstruseness. It is only a pea that I hold here before me, an ordinary small, round, yellow marrowfat, the seed of the commonest of garden annuals. Nevertheless, that familiar little object, which all of us have known all the days of our life, incloses in itself the entire solution of the riddle of birth. If we understand the pea clearly, we understand the whole science of biology. Let us ask ourselves first, exactly what it is, and then see how it helps us to comprehend the coming into existence of all the higher plants and animals.
The pea is, in fact, here as it stands, a whole embryo plant in a dormant condition, the product, so to speak, of a distinct marriage. More than that, it is a totally new individual, produced by the interaction of separate cells from two pre-existing individual pea-plants. And it is that fact—which it owns in common with every other seed—that gives it illustrative importance as an example of the mode of production of all higher organisms, animal or vegetable. We may use it to explain this fundamental mystery of advanced life, because the principles which govern its origin and growth are the same as the principles which govern the beginning of all other conspicuous plants or animals in the world around us.
If you bend down a branch of a rose-tree, and cover it with earth, it will take root—make a layer, as we say, and grow up apparently into a separate rose-bush. After it has rooted itself firmly in the damp soil, you can cut it off with safety from the mother-plant, and remove it or transplant it to another part of the garden, where it will form, to all outward show, a distinct individual. Similarly, if you take cuttings from a scarlet geranium, and plant them in pots, you can multiply your original specimens in different places to almost any desired extent. In many cases. Nature has even provided beforehand, as it were, for such purely vegetative propagation of a particular species. In the tiger-lily, for example, the tiny bulbels, that spring from the axil of every leaf, fall off when mature, and form distinct or separate plants on the ground beneath. In other instances, suckers, offshoots, or scions are produced, sometimes underground, as in the Jerusalem artichoke, sometimes above, as in the potentillas and hawkweeds, all of which grow out, to all appearance, into plants like the one from which they originally separated themselves. Many plants produce long, creeping branches, which regularly and systematically root at the nodes. The runners of strawberries are a familiar example of this mode of growth; so, in a somewhat different way, are the eyes of potatoes, the small side-bulbs in certain forms of onion, and the long, underground suckers or scions of the twitch or couch-grass.
When we come to look a little closer, however, at the nature of such seeming reproduction, we can see at once that in none of these cases is a new individual—in the truest sense of the word—really produced: all that has been done is to split up the original single organism into a number of colonies, as it were, or component parts, all still retaining the primitive individuality in shape, color, and every other particular. The branch is a branch while it remains on the tree; it is still none the less a branch in all essentials after it has been severed as a cutting, and made to root afresh like a distinct plant, apart from the remainder of the primitive individual to which it belongs. Gardeners and agriculturists are perfectly aware of the truth of this principle, at least as regards its practical aspect, for they take advantage of it freely in the case of varieties which, as they say, "will not come true from seed." A particular potato-plant, let us say, or a particular rose-tree, possesses certain individual points, which render it desirable in cultivation; and, instead of seeding it, by crossing with another individual, and taking their chance among the seedlings (in which the special peculiarities seldom reappear), gardeners prefer to divide and multiply the original individual to the utmost possible extent, so as to make sure of retaining all the strong points of the plant in question, undiluted by crossing. All the Marshal Niels in existence, for example, are, in the last resort, cuttings from a particular, individual French rose-bush; all the British-Queen strawberry-plants are offsets by runners from a single, exceptionally fine-fruiting seedling.
Take an instance which I see before my eyes this very moment as I raise my head from my temporary study-table on a North African hill-side. The date-palms, which form the wealth of the Arabs of the desert, and one of which now waves its long boughs in the breeze before us, are all female; the male or pollen-bearing flowers of the date kind always grow on a separate tree; and as pollen is produced by them in vast quantities, it is not necessary in palm-groves to have more than a single male stem to some forty or fifty fruit-bearing individuals. The Arabs, therefore, never raise their palms from seed, as they can not make sure of the sex of seedlings; they take suckers from the root of a female tree, already known to be a good bearer of fine fruit; and these suckers not only follow the sex of the so-called mother, but also reproduce its special peculiarities of flower and seed in every respect. They can not fail to do so, indeed, seeing that they are part and parcel of the original palm, actual members of the self-same plant; just as the various branches of an apple-tree all bear the same kind of apples, or the boughs of a currant-bush all produce the self-same currants.
And now let us hark back, by way of contrast, to the case of the pea, which is a true, distinct, individual plant, the product of a veritable marriage union. Whence came it? Was it born from a pea-blossom? So, indeed, we mostly imagine; though very incorrectly. As well say that a child is the son of his mother, but not of his father, as that a pea is the seed produced by a pea flower. It is nothing of the sort. The whole secret of sex and reproduction is bound up in this simple illustrative instance. The pea is the product of two different pea-blossoms.
The mere accidental fact that each pea-blossom had stamens and pistil in its own flower must not blind us to the truth of this underlying principle of cross-fertilization, which every pea exemplifies for us as truly as every date or every melon. In the date and the melon the flowers on one plant are all male or all female; on the pea-vine they are all hermaphrodite. But, none the less, they intermarry. What happened when the pea was first launched into life was briefly this: A row of peas grew in the garden of the Moorish villa that gleams in the sun on the hill-side opposite; and on one of these vines hung a particular white-winged blossom, which supplied the pollen for the production of this individual pea. On another vine hung a second flower, from whose midst protruded the pistil which was finally to grow out into the particular pod that contained my pea. A wandering bumble-bee, on dinner intent, poked his long proboscis into pea-flower number one, and, after rifling it of its honey, covered his hairy legs and thighs, half accidentally, with abundant pollen from the stamens, which formed a sheath or tube round its twisted style. Then he flew away to pea-flower number two, and, in his clumsy attempts to thrust his long sucker down its nectar-bearing throat, he brushed a lot of number one's pollen from his legs and breast on to the ripe stigma or sensitive surface of number two's undeveloped pod. So much alone we can see for ourselves with the unaided eye of outer observation. How much more of the history of this matter will dissection and the microscope finally tell us?
Inside the keel or lower petal of the pea the young pod pushes out its style and brush-like stigma to meet the advances of the fertilizing bee. On the end of the style, at the inner surface, a group of delicate hairs protrudes from the stigma; and it is on these hairs that the bee casually and almost accidentally (so far as he is concerned) deposits the pollen-grains he has carried off from the brother-blossom. Forthwith, each pollen-grain, meeting with the sensitive surface of a sister-style, and recognizing its position, begins to emit a tube of highly vital matter, which bursts out from its side and seeks a vent to penetrate the pod in the exact center of the neighboring flower. Now the hairs, on whose tip the poll engrain has been deposited, are tubular and hollow; and the pollen tubes, running down the style along these pre-established routes, soon reach the little ovules, or undeveloped peas, that lie concealed in the pod within. There it is that the actual, intimate work of fertilization itself really takes place. The vital material of plant number one, laid by in the pollen, enters and mixes with the vital material of plant number two, laid by in the ovule; and from their intermixture and union, in the most physical sense, there springs at last the wonderful little object I see before me—the pea itself, a dormant plantlet, waiting only for heat and moisture to wake it into life, that it may grow into a new and separate individual pea-vine.
Now, note the importance of this act of fertilization. Unless the pollen had reached the ovules in the undeveloped pod, the tiny peas therein contained would never have swollen or developed into perfect seeds at all. The flower in that case would have withered on its stalk, and the pod would have dried up to an abortive and shriveled mass of empty membrane. It was the union of the pollen of one plant with the ovules of another that produced this entirely new individual, a compound and outgrowth, not of one but of two distinct pre-existing organisms. The vital material inside the bee is the vital material of the one, re-enforced and vivified by the diverse vital material of the other.
In order to understand the use and object of this peculiar provision of Nature, whereby every higher plant or animal is the product of two prior individuals whom we call its parents, we must look first more closely at the phenomena of ordinary vegetative growth, and thus see wherein this higher mode of reproduction differs essentially from that simpler and lower function.
All plants (roughly speaking) can produce from certain parts 01 themselves new leaves and branches; and each such leaf, from the extreme theoretical and biological point of view, must be regarded to some extent as a distinct individual. Nevertheless, the entire colony of leaves, the herb, shrub, or tree, as we generally call it, has also a sort of complex individuality of its own; it is an organism in itself, containing various parts or members, such as roots, stems, leaves, and so forth, each performing distinct functions for the good of the entire complex body. Now, the plant goes on for a certain length of time producing leaves from its surplus material one after another; and, as long as fresh material is supplied, this production or growth seems in some cases to have hardly any distinct limit. Strawberries, for example, will go on sending out runners (which are merely branches with tufts of leaves at the end that root from time to time) almost endlessly. We have here an example of continuous non-sexual reproduction. There are in nature innumerable variations in the manner of such purely vegetative growth. Sometimes, as in deciduous trees, the leaves all fall off in autumn, and totally new ones are brought forth from buds in the succeeding season; sometimes, as in the potato, new shoots spring from swollen underground branches; sometimes, as in the crocus, small bulbs are developed as stocks on the top of the old one. But, whatever the variation, the central fact still remains the same: the leaves, stems, or branches thus put forth are, strictly speaking, parts of the same compound organism, asexually produced, not entirely new and separate individuals.
A plant, however, no matter how vivacious, can hardly go on living forever. Sooner or later, there is reason to believe, this purely vegetative growth fails. The original vigor of its constitution gets used up; the life and go of the plant become hopelessly weakened. This seems to be the case at the present day, for instance, with the cultivated potato, which has been propagated from the tubers almost exclusively for many years, so that the existing plants must be of immense age, and have grown effete and feeble for want of proper sexual renewal. How are plants which have thus reached their dotage to restore their youth? How are they to carry on to future years the life of the species?
Nature has answered this problem of life by the wonderful device of intercrossing. The organism, like every other machine, tends in time to wear out and decay. But, unlike other machines, it contains in itself (through the action of natural selection) the means for manufacturing its own successors.
The leaf, we saw, grows out from the leaf. If you cut a piece of the common cactus or prickly-pear, and drop it on the ground, it roots at once and grows up afresh into a full-grown cactus plant. There are some leaves which, if hung up, produce other leaves and little plants from their edges; and everybody must have noticed how the common stone-crops will grow and root from any little scrap or fragment or bit that falls by accident upon damp soil. If we go down to the very bottom of the matter, it is clear that the plant tends to reproduce itself, whole and complete, from every part of itself—tends to increase in its own shape, and repeat itself anew in fresh leaves and branches. Why is this? Well, such a tendency results necessarily from the fundamental principle of cell-growth. Every living vegetable cell containing chlorophyl is always producing within itself fresh vital matter of its own kind; and this vital matter, at last outgrowing the capacity of the mother-cell, pushes itself out through the cell-wall, and grows into a new cell like the one it left. And it does so in the very last resort in virtue of that curious chemical property of the stuff we call chlorophyl, whereby such chlorophyl, under the influence of sunlight, separates the carbon and oxygen of carbonic acid, and builds them up once more into living matter of the particular sort composing the plant in which it exists.
Given a chemical body which can so increase the sum-total of living matter, and there must needs result the phenomenon of growth. Living matter is always being made anew from the non-living. But observe that in each plant the material thus assimilated from the air (or rather the carbonic acid floating in it), and more remotely from the earth and water, is built up into the forms of the particular plant itself—becomes distinctively, not mere living matter in the abstract, but strawberry matter, or stone-crop matter, or cactus matter, or whatever else the individual plant may happen to be. In this we get the real secret of like reproducing like. It results as a corollary from the principle of assimilation. Most people see a mystery in the particular fact that offspring resemble parents, but they see no mystery in the general fact that the parent reproduces or renews the parts of itself from alien material. In reality, the final explanation lies on this deeper and more essential level. It is just as strange that a rose should put out fresh leaves and shoots as that its seed should grow up into a fresh rose-bush.
The true explanation seems to be, as Mr. Herbert Spencer long ago suggested, that each organism has an inherent physical tendency (of the nature of polarity) to complete its own organic form, in somewhat the same way as a broken crystal, placed in a solution of its own material, has a tendency to replace its lost portions. The organic type, in other words, resembles the crystalline in this—that the material of which it is composed, when left to its own internal forces, tends, under the free play of those forces alone, to arrange itself in a certain definite specific shape.
In time, however, every organism or colony of organisms seems to lose this primitive plastic power of producing fresh parts out of its own material. Old age, as we say, comes on. And this is specially true of the higher and more complex organisms. The tree no longer puts forth new leaves; the plant no longer sends out fresh branches. Its individual vigor appears to be used up. Unless, then, some fresh stimulus can be supplied it from without, the plant must die, and the species thus must suffer extinction.
At this point, therefore, Nature steps in with a special remedy—the special remedy of cross-fertilization. The earliest and simplest form of this device is seen in certain algæ or pond-weeds, mere long green hairs that wave about like tresses in the water, and consist each of endless rows of cells growing out in single file like the beads of a necklace one from the other. But every now and then two of these algæ "conjugate," as biologists put it—that is to say, a cell of one bends over and unites with a cell of the other, the cell-contents (or protoplasm and chlorophyl) of one cell breaking through to join the cell-contents of its neighbor. The union thus effected seems to supply a fresh stimulus to growth: the two matters coalesce and combine, and a new and more vigorous alga springs up as the final result of this combination.
Now, in the higher plants we get exactly the same sort of combination, only far more complex in its mechanism and results. If we take any annual plant, like the pea, and look when and where the flowers are produced, we shall see that they come as soon as the plant has attained its full growth, and when the purely vegetative reproductive impulse is beginning to fail. As a rule, too, the flowers come at the end of the branches, and in many—indeed, in most—plants they form a terminal spike or bunch at the summit of the stem, as in the familiar instances of the hyacinth, the buttercup, the sunflower, or the grasses. In other words, as soon as the vegetative growth is beginning to slacken, the need is felt for "fresh blood," for the special stimulus or fillip to further exertion given by union with another individual.
For the purpose of bringing about the desired union, all the higher plants are supplied with special organs known as stamens and pistils. The pistils produce the embryo seed, which is, in fact, a tiny separate plant, whose development is arrested at a very early stage, unless fresh material from a neighboring stamen is supplied to supplement it. The stamens produce the pollen-grains, which are, in fact, free cells containing a large quantity of very vitalized matter capable of fertilizing and vivifying the embryo seed. When a grain of pollen is placed by any agency whatsoever—wind, an insect, or a camel's-hair brush, as the case may be—on a neighboring stigma, it sends out a pollen-tube which penetrates the ovary and at last enters into and coalesces with the embryo seed itself. The fresh material thus added to the embryo seems to upset the dormant condition—allows the failing growth to continue. The seed swells, the fruit ripens, and a new plant is shed forth upon the earth, the product of two distinct prior individuals.
But if the embryo is not thus quickened, growth in it ceases altogether. The seed shrivels up, the pod does not swell, and no new plant is produced at all. It does not contain within itself the needful energy for further development. Supposing all the flowers on a pea-plant were thus to fail—supposing no pollen were ever to be carried from blossom to blossom—then that particular plant would wither and die out altogether, leaving no offspring at all behind to represent it.
In the case we have supposed, however, the flower did get fertilized, and the pea before me—a dormant but still a living plant—is the irrefragable proof that it actually did so. Now, in some instances, perhaps in this one, a flower gets fertilized with its own pollen. In such cases, as a rule, the fruit nevertheless swells out properly and the seed produces a young plant. How, then, are we to reconcile this apparent discrepancy with the general principles of sexual growth laid down above? Well, we must recollect that in a certain sense each leaf is a distinct individual. Again, from the biological point of view the flower consists of modified leaves, some of them specialized to do duty as sepals, some as petals, some as stamens, and some as ovaries. Each of these is therefore in some sense an individual. In the entire community or compound organism, in other words, we may regard the stamens and ovaries as particular members, told off, like the queen-bees and drones in the hive, to fulfill the part of fathers and mothers, while the true leaves, like the workers, provide the food or material for growth. Thus, even in the same flower the stamens and ovaries are properly to be regarded as distinct individuals, capable of producing healthy offspring with one another, like the queen bees and drones of the same hive.
Nature, however, does not stop here. The fundamental fact at the bottom of all fertilization whatsoever seems to be this, that where individual formative power fails it can be supplemented and set on foot again by an access of fresh formative power from without. Union is strength: what one can not do, two can. But the fresh fillip seems to be most distinctly felt when it comes not from another member of the same original colony—that is to say, from a stamen of the same blossom or of another blossom on the same plant—but from a totally distinct and separate colony, or, in other words and in more familiar language, from the flower on another neighboring plant. Where the parents are too closely related, it would seem, both are apt to have the same weak points, which therefore reappear in the offspring and vitiate it. But deeper down even than that, since both belong to the same colony at the same period of failing growth, the impulse to fresh effort afforded by such a union would appear to be less; indeed, in some cases it is quite inoperative; whereas, when each comes from a separate plant, not only are the chances of diversity in constitution greater, but the constitutional fillip or stimulus to growth is more distinctly marked. Birth is a result of the union of unlikenesses.
Hence, while among the lowest and least developed flowers self-fertilization (or, to speak more correctly, fertilization of each ovary by its brother-stamens) is very common, among the higher and more specially adapted plants devices for promoting crossfertilization, either by wind or insects, are almost universal. In some instances, indeed, the ovary can not be impregnated by pollen coming from the same flower—the fillip does not seem sufficient to promote growth, and the ovary touched only with pollen of a neighboring stamen remains to the end perfectly sterile. Truly distinct pollen is needed to quicken it. In other cases, though such incapacity does not exist, special arrangements have been made to prevent self-fertilization—the stamens and pistils do not mature together, or else they are so arranged in the blossom that contact of the pollen with the stigma is almost impossible. And in some of the very highest plants of all, the stamen-bearing and ovary-bearing flowers are distributed on totally distinct trees or bushes, thus affording the most perfect known development of the sexual principle—a sort of automatic compulsory exogamy, whereby each blossom must needs intermarry with a member of an entirely different colony.
For the same reason it will now, I hope, at once be clear why the offspring in every case resembles on the whole both parents equally. The various leaves which each rose-tree puts forth are exactly alike, and we don't expect them to be at all otherwise, because they are all similar products of the self-same active and formative energy. However much we may subdivide the parts of a plant, we look forward to finding its manifestations remain unchanged, as in the familiar case of cuttings, grafts, layers, suckers, bulbs, and runners. The different leaves, made of the same ultimate stuff, the new material of the species, resemble one another exactly as two parts of the same lump of clay or putty have similar characters; or exactly as the two halves of the same crystal rebuild their lost parts and renew their original shape alike when immersed in a mass of the same mother-liquid. So, too, we may well believe the undeveloped embryo or unfertilized seed potentially resembles in all things (as far as it goes) the mother-plant; but, as soon as it is fertilized by the pollen from its neighbor, it becomes in every portion of itself part and parcel of two previous plants; or rather, the resulting new organism is the outcome of a compromise, perhaps even of a struggle for mastery, between all the parts or component elements of the two parent plants. Hence, in all species, animal or vegetable alike, the young on the whole tend to resemble both parents equally, but in different modes of combination, which give them each what we call individuality, and so make them really and truly new plants, not mere reissues of either parent form.
When I had written thus far on this present article, I laid down my pen for a little rest, and strolled out alone upon the dry African hill-side, a lower shoulder of the Atlas range, that stands opposite the villa whence I date these words. By a curious coincidence, as I rambled through the lentisk scrub, I happened to light upon a little bed of natural hybrid orchids, which so admirably illustrate the nature of this peculiar intermixture that I joyfully accepted them to point the moral with which I must close this long lay sermon. Numbers of a large and handsome yellow orchid grow on the slopes of that particular hill, and in and out among them spring members of another yet closely related species, dingier brown, and different in shape, disposition of parts, and general appearance. Some wandering bee, visiting a flower of the yellow orchid at this spot where I stood, had carried away on his head its gummy pollen-masses, and then, contrary to the common habit of bees (who generally visit only one particular species of plant at a time), had deposited them on the stigma of a neighboring brown specimen. I suppose he was a young and inexperienced insect, who had not yet learned to avoid the bad practice of mixing his honeys. From this chance fertilization any number of hybrids had taken their rise, all of them more or less resembling in certain respects both parents. In most cases they had, to a great extent, the distinctive shapes of the brown kind, with a preponderating amount of yellow color. But among them all they presented every possible intermediate type between the two parent forms. It seemed to me that this accidental find exactly fitted in with the subject of my paper. We see here how each embryo seed, separately impregnated by a pollen-grain from another plant, grows out with a tendency to reproduce both ancestral forms equally, and how the conflict between the two tendencies, both of which can not fully be realized, produces in the end an individual compromise—a something which is not quite either, but which combines in varying and incalculable degrees the strongest points of both.
Unless I mistake, we have here the solution (suggested in the main by Mr. Herbert Spencer) for one of the deepest and most fundamental problems of all life, animal or vegetable—the problem of reproduction, heredity, and individual variation.