Marriage Among the Clovers
( Originally Published Early 1900's )
PLANTS marry and give in marriage just as truly as animals. They have their loves and their hatreds, their friendships and their enmities. The marriage customs of many among them are vastly interesting; and yet, in spite of all the attention that has been given to the subject of recent years, comparatively few people are even now aware how quaintly they pair, how varied and curious are their matrimonial arrangements. Most of us, it is true, have heard by this time the bare facts of the case—that flowers are mainly fertilized by the visits of insects ; many of us even know that in the majority of instances the little golden dust which we call pollen must be transferred from the hanging bags on one blossom to the sensitive surface of an-other, or else seed will never be set; but not all of us are aware how intricate and how numerous are the minor devices by which each kind of plant effects this important object in its own fashion. I am going, therefore, in the present paper, to describe briefly the marriage customs of two alone among our commonest clovers, which I shall adduce as specimens of the strange variety to be found within the limits of a single type.
To begin with, however, I propose to examine, as a mere introduction, a couple of flowers of a well-known and dainty hot-house begonia, which may help us to the comprehension of the more plebeian clover-heads. Proverbial philosophy has long since taught us that "the longest way round is the shortest way home"; and when I drag in the begonia, which has apparently so little connection with the clover, and which is really about as unrelated to it by descent as two flowering plants can well be to one another, you may suspect that I do so for some sufficient reason. The fact is, begonias happen to be plants in which the differences of the sexes are exceptionally well marked, so that they may be apprehended with ease by the naked eye, and by every observer, even the most casual. I advise those who have conservatories of their own to verify my statements in this matter on the specimens in their possession.
Most cultivated begonias have the flowers on their branches arranged in groups or clusters of three, the central one of which is often a female, while the two outer blossoms are usually males. This is the ordinary plan, but it does not hold good of all the species, some of which, on the contrary, have only one male to each pair of females. Now, these male and female flowers are so very unlike in form and structure, when you come to look into them, that you would hardly believe they belonged to the same plant if you did not find them growing on one branch together. They differ quite as markedly as the peacock differs from the pea-hen, much more markedly than man differs from woman.
The first point of difference which you will note in the two is that the female begonia has five petals, while the male has four only. But what is more important than the number of the petals is the fact that the female flower has wedged at its back a large, triangular winged ovary, or seed-capsule. It is the possession of this ovary, indeed, that marks it out at once as a female: for by a female plant or animal we mean, of course, the one which lays the eggs, produces the seeds, or becomes the mother of the young individuals. If you compare the back of the female flower with the back of the male flower, you will recognize at once the importance of this distinction. The female blossom has a seed-bag, while the male is barren.
But this is not all: the other parts of the two flowers differ almost equally. The center of the female blossom is occupied by several twisted and wriggling arms, the upper surface of which is more or less sticky. This surface forms the receptive portion, or mouth of the flower, on which grains of pollen must be duly deposited before the embryo seeds in the capsule below can begin to swell and develop. On the other hand, the center of the male flower is occupied by a very different set of organs, the stamens, or pollen-bags, whose business it is to produce and shed the fertilizing powder. Without pollen to start them, the seeds are useless. In the wild state, any winged insect which visits the plant is likely to alight first on the lip or platform of one or the other of the outer male flowers. In his search for honey, which is secreted by the plant at the base of the petals on purpose to allure him, the flying visitor dusts himself over abundantly, though unconsciously, with grains of pollen from the very numerous little sacs which are placed there in a convenient situation with that precise object. He then flies away to the female flower, in which he alights, as a rule, on the central sticky portion (called by botanists the stigma) : and, as he walks over it in search of the honey at the base of each petal, he turns himself round and round in five directions, and thus unwittingly rubs off the pollen which clings to his legs and hairs, transferring it to the sticky and receptive surface. After visiting and fertilizing the female flower in the center in this manner, he then usually proceeds to visit the second brother beside it, from which he carries away pollen in turn to the next plant he visits. The object of this curious arrangement is that each flower may be fertilized by pollen from an-other blossom, and, as far as possible, at least by pollen from a distinct neighboring plant. But you will gather at once from what I have said already that each plant must be regarded in strictness not as an individual, but rather as a community or commonwealth, of which the leaves and flowers are the separate members told off to perform different duties. You may compare it, indeed, to a hive of bees, the leaves representing the workers, while the five-petaled flowers are analogous to the queen bees, and the four-petaled blossoms to the husbands or drones. Nay, more; those of my readers who have begonia plants of their own may observe for themselves another singular resemblance to the habits and manners of honey bees. For after the drones have done their work in life by fertilizing the queen bee, the prudent workers sting them to death as being useless mouths of no further benefit to the community; but the queen bee necessarily survives to become the mother of young swarms or future generations. If she were killed it would be all up with the community. Just so with the begonias ; as soon as the male flowers have performed their whole duty in life by producing and disseminating the grains of pollen which the insects carry away and smear upon the sister blossoms, they break off at the joint and fall to the ground; the plant refuses to feed them any longer be-cause it has now no use for them : but the fertilized female flowers remain fixed on their stems to produce the seeds from which will spring in time the future generations.
What, however, do I mean by fertilization? Well, each pollen grain, when closely examined under the micro-scope, looks like a tiny egg, with a very thin shell and a very sticky, active contents. As soon as the pollen grains are rubbed all over the curly branches in the center of the female flower, they empty their contents down long tubes, which reach at last to the seeds; and under this vivifying influence, the seeds begin to swell and become capable of producing young plants. The pollen, in short, has quickening power. It is for the sake of this final result alone that the flowers exist; they are provided with bright colored petals as advertisements to let the insects know where honey may be expected; they secrete the sweet liquid itself in order to induce their winged allies to become common carriers of pollen for the benefit of the begonia; and as soon as each flower has served its purpose in this respect, it drops off or is retained by the plant according as it is or is not wanted in future for its seed producing properties.
To the clovers, then, which are our proper subject, I will next proceed. And I began with the begonia only by way of introduction, only because that afforded a case in which the husbands and wives of the community were so distinct from one another that nobody with a pair of eyes in his head could fail to distinguish them when they were once pointed out to him. In the clovers, on the other hand, we have a much more complicated arrangement, and one much less like the ordinary cases with which we are familiar in the animal world. Here the flowers are collected in heads or clusters, and each flower is in itself at once both male and female. This method, indeed, is common amongst plants ; it occurs in by far the greater number of species; the reason why I started with the begonia is just because in that type the sexes are so well and clearly separated in distinct blossoms. In the clovers, however, each separate flower resembles a small pea blossom in shape, having four petals, which botanists name, respectively, from below upwards, the keel, the two wings, and the standard. They are en-closed beneath a small greenish calyx, or cup, and they contain within ten stamens, or pollen bags, as well as a tiny capsule like a miniature pea pod. At the tip of this capsule is a small hook—the sensitive surface on which the pollen has to be deposited. You would say at first sight that under such circumstances, male and female being mixed up in one, cross-fertilization must be impossible—that each flower must surely be fertilized by its own pollen. But the clever clovers have invented an ingenious device of their own for overcoming this difficulty; the pollen bags and the sensitive surface of the capsule do not arrive at maturity together. In this way each flower or plant gets fertilized itself at one time by pollen from another plant, and at another time dusts the bee that visits it with its own pollen, which the bee transfers in due course to the next plant it visits.
The Dutch clover is a rather smooth specimen of its type, not nearly so hairy or silky as most other clovers, for a reason which I will explain a little later on; it has prostrate stems which creep along the ground, and root every now and again as they proceed, somewhat after the fashion of strawberry runners. Like all other clovers, it has trefoil leaves, each of the three leaflets in which is usually marked with a curved spot in the center resembling a horseshoe. But it is the flower heads with which I am here particularly concerned. These are raised on long, erect, leafless stems, each of which bears on its summit a globular head of little white pea flowers, often delicately tinged with pink or salmon. The flowers are thus lifted to a considerable height, because this clover grows, as a rule, among rather tall grasses, and so tries to push up its blossoms to a height where they may receive the polite attentions of passing insects.
The visitors for which Dutch clover specially lays it-self out are for the most part bees. It disdains small pilferers. Each blossom has a long tube enclosing its honey, and only insects with a correspondingly long proboscis can reach its deep store of delicious nectar. It thus saves itself from being rifled uselessly by small insect riff-raff, such as flies and midges which might visit the flower, as we botanists call it, "illegitimately"—that is to say, might rob the honey without conveying the pollen from the pollen bags of one head to the sensitive surface or stigma of the next. The parts of the flower, in fact, are specially arranged with a definite relation to the head and honey sucking tube of hive bees and wild bees, which cannot visit it without dusting themselves over with pollen on one blossom which they unconsciously rub off on the receptive surface of the next. In one word, Dutch clover encourages bees for its own purposes, because they are useful to it, while it places obstacles in the way of smaller and useless insects by burying its honey in a deep tube.
The florets or blossoms which make up the head begin opening from without and below, inward and upward. Thus the outer and lower florets have opened, while the inner and upper ones are still in bud. When a bee visits such a head of clover, he comes to it first from another head of the same kind, for bees do not usually mix their liquors; on one round of visits they confine themselves, as a rule, to a single species of flower only, and they probably store the honey of each kind in separate cells, just as we ourselves in our wine-cellars keep one bin for champagne, another for claret, and a third for Burgundy. The bee thus begins with the outer flower of the head, which he fertilizes with pollen from the last plant he visited; he then goes on to the second row, where he dusts himself over with pollen for another flower head; and the buds in the center he leaves severely unnoticed.
As soon as he flies away a very curious thing begins to happen. The flowers which he has unconsciously fertilized close over their seed vessel, and grow gradually brown or withered. At the same time they turn down out of the way of the bees by bending the separate little stalks on which they are raised in the head and tucking themselves tight against the common flower stem. This they do partly in order not to confuse and worry their allies the bees, but partly also to avoid certain other dangers to which I will recur later. Plants often try in such ways to save bees or butterflies time and trouble, because the easier they make matters for the bee or butterfly, the more likely he is to visit and fertilize them. He is a useful customer whom they desire to conciliate. If a bee on his rounds finds that any particular species of plant gives him unnecessary trouble in getting at his honey, he is apt to neglect it and pass it by in order to devote himself to other kinds which he sees are more business-like and obliging. The moment he comes to a head of Dutch clover, then he knows at once that he may safely ignore the dry brown flowers tucked away against the stem, because they are already fertilized and honeyless; he therefore directs all his attention to the nature and open flowers which are now producing honey and ready for fertilization. These form practically, as you will see, at each moment the outer row of the flower head, and are the ones which naturally first engage his notice as he alights on the cluster. It sometimes takes four or five days for a single head to pass through all its stages.
You must bear in mind that none of this is true of the common purple clover, nor yet of the brilliant crimson kind (known to our farmers as "carnation trifolium"), both of which are distinct species with totally different marriage customs. The ingenious habit of turning the fertilized flowers downward, out of the way of the insects, is confined to a few species of white, pink and yellow clovers. It is a little dodge on which they happen to have hit, but which has not occurred to their larger and more conspicuous red and purple cousins. So if you try to follow out these hints in nature, you must be careful to hunt for the white kinds only.
Every kind of clover, owing to the richness of the seeds, is much exposed to the attacks of insect enemies. To baffle these wary foes the clovers have invented an extraordinary variety of protective devices, two of which I mean to examine in this essay. Dutch clover meets the difficulty by tucking down the flowers after fertilization out of the way of the bee, and then retaining the withered corolla or set of petals which completely enclose and hide the pod in the center. It is thus a distinct advantage to the clovers in the struggle for life that they have invented devices which enable them to guard their embryo young from the assaults of insects.
Every species of clover—and there are many has some dodge of its own for thus protecting its growing pods and seeds from the grubs which would destroy them. I only propose, however, to examine in detail here one more of these dodges. We have another kind of clover, a good deal like the Dutch clover at a casual glance and commonly confounded with it by unobservant people, though, as we shall see, the habits and manners of the two kinds are in reality very different. The strawberry clover, as it is called, is a somewhat lower and smaller species than the Dutch clover, which it resembles in its creeping stems and in its rich foliage. But the flowers are not separately stalked in the head, so that they can-not turn down after fertilization like those we have just been considering. Moreover, the stems and the flower heads are much hairier; and this difference is due to the two facts that the strawberry clover is smaller, and has a shorter tube than its Dutch relation. It would thus be easy for ants and other crawling insects to creep up the stem and steal the honey, which is intended for the use of fertilizing visitors. To prevent this misfortune and to keep its nectar for the regular customers, the strawberry clover produces a number of hairs on the stem, which baffle the ants, to whom such hairs are an impenetrable thicket. But you may ask, "Why are not ants just as good as bees for the clover?" For this reason : flying insects are mainly guided by sight and color; they flit straight from one flower to another of the same species; and their heads are exactly adapted to the shape of the flowers, which in turn have modeled their tubes and organs on purpose to fit them. Ants and creeping insects, on the contrary, are attracted merely by the sense of smell: they notice the scent of honey; they climb up all stems indiscriminately in search of it; they are bare-faced thieves with no organs adapted for carrying pollen; and as they go about in the most reckless fashion from one kind of plant to 'another, if they did ever by chance succeed in fertilizing a casual flower they would produce, not true species, but monstrous and meaningless hybrids. Therefore many plants protect themselves by endless devices against the crawling ants, just as obviously as they endeavor to allure the winged bees, beetles and butterflies. I may add that the head of strawberry clover is further protected against climbing insects by a number of lobed bracts at its base which effectually disperse these thieving marauders.
While the strawberry clover is young and but recently opened, you might easily mistake it for a small and pinky section of Dutch clover. If you look closely, however, you will see that the petals are not so large, the tube not so deep, and the calyx much hairier. Nevertheless, the hairs do not seriously get in the way of the bee during the stage when the flowers are just fit for fertilization. As soon as the bee has left the plant, however, some-thing happens which is quite different to the turning down of the florets in the Dutch clover. The calyx or little cup which encloses each separate flower begins to swell and inflate itself like a balloon or bladder. The whole flower head then becomes very compact and assumes a pink tint, so that it somewhat resembles a straw berry, whence its ordinary name, though, as a matter of fact, it is much more like a raspberry. The beautiful network on the bladder-like head is closely covered with numerous hairs, which further help to protect the pods from the attacks of insects.
The truth is, Dutch clover is a denizen of rich and lush meadows where it can take care of itself, and for which alone it is perfectly adapted. Strawberry clover, on the other hand, has chosen its home in close cropped pastures, where its creeping habit and low stature help to save it from destruction. The dry and hairy heads are not relished by sheep, and you will often see them left un-cropped where the neighboring foliage has been closely nibbled. The swollen calyx with its hairs also keeps off egg-laying enemies.
Now, what is oddest of all, every one of twenty-five species of clover has some dodge of its own for protecting its seeds after fertilization. This shows how much these rich grains are sought after, and how care-fully the plant is compelled to guard them. In some kinds, the calyx is a loose fluff of silky hair, enclosing the pod; in others it is hard like a nut, or has stiff and pointed lobes which are sharp and prickly. One species closes its hardened lips over the growing seeds and pretends to be empty; a second develops a starry, thistle-like head, with tufts of thick hair which conceal the swelling pod from observation. But the subterranean clover has hit upon a still stranger and more ingenious device. It is a little creeping annual, much addicted to dry pastures or close-cropped hillsides, and particularly common on low knolls or barrows, nibbled over by numerous sheep and donkeys. Under these circumstances it has a hard fight to protect its nutritious seeds and seedlings. It has taken, therefore, to producing small heads of loose white flowers, which look at first sight like poor specimens of Dutch clover. But if you gaze closer you will see that each tiny head consists of two or three properly developed flowers, with four or five undeveloped or abortive blossoms in the center of the group. These undeveloped blossoms form a sort of living corkscrew. After fertilization the stems bend down toward the ground; the corkscrew-like abortive flowers worm their way by pushing into the sod; the pods are pressed down or buried in the loose mould; and the plant thus sows its own seed for itself quite as effectually as a gardener could sow it. This is, perhaps, the furthest point maternal solicitude has ever reached in the vegetable kingdom.