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Out in the desert regions of the southwestern United States there grows a very remarkable plant called the yucca, or Spanish bayonet. Doubtless you have noticed it from the car window, or you may have seen it growing in a garden. Its bristling array of bayonet-like leaves gives it a very individual appearance, and the cluster of creamy white flowers that it puts forth on its tall central stalk gives it added distinction. But even if you are familiar with the appearance of the plant, you perhaps have never heard the wonderful story of its alliance with a particular species of insect, upon which alliance the lives of both plant and insect absolutely depend. The story is one of the most curious ones in the entire range of plant and animal life. Cases of so-called "symbiosis," in which an insect and a plant are mutually modified for mutual aid, are common enough in nature. But so extreme a case as that of the yucca and the insect that is its inseparable associate is seldom duplicated. The insect in question is a little yellowish white moth, so unfamiliar that it probably has no colloquial name, but known to the entomologist as the Pronuba yuccasella. If you were to watch closely you might see these moths visiting the flowers of the yucca in the twilight. You would require exceptional opportunities for observation if you were to discover precisely what takes place during this visit. But entomologists have kept watch to good purpose, and the terms of the extraordinary coalition between the yucca and the pronuba moth are now an open secret. It appears that the female moth that visits the yucca blossoms has developed a long ovipositor with which she can pierce the tissues of the ovary of the plant and so lay her eggs within it. Her prime object in visiting the yucca flower is thus to deposit her eggs. In due course the eggs hatch and the growing seeds of the yucca will furnish them a supply of food. So there is nothing very remarkable about this part of the procedure. The surprise comes when we learn of certain maneuvers preliminary to the deposit of the eggs. If you could watch the little moth on her visit to the first flower, you would see her begin at once industriously to gather the adhesive pollen grains with the aid of a curious pair of tentacles growing about her mouth; tentacles unlike those of any other moth. As the pollen grains are gathered they are rolled into a small pellet, and when this is of a satisfactory size, the moth leaves the flower and flies to another. But here instead of continuing her task of pollen-gathering, the insect makes her way to the center of the flower and, piercing the basis of the pistil with her ovipositor, lays her eggs among the embryo seeds of the ovary. Then she crawls carefully up the style and, poising at the tip, pushes the little ball of pollen down into the cavity of the stigma. By this seemingly preconceived and carefully perfected plan, then, the little moth has obviously done precisely the thing necessary to insure fertilization of the flower in which her eggs are deposited, with pollen from another flower. No plant experimenter, whatever his skill, could have done the thing better. Cross-fertilization is assured; the ovules among which the eggs of the moth were deposited are sure to develop, giving an abundant supply of food for the larvae when in due course they are hatched. The little grubs will grow and thrive, and presently will eat their way out of the ovary. A fall to the ground, where they will bury themselves for a season; coming forth as adult moths in the succeeding summer, just at the time when the yucca is flowering.


At first glance it is not obvious how the yucca profits by this curious arrangement. But observation shows that the progeny of the moth seldom or never consume all the yucca seeds that are so conveniently stored about them. After they have eaten their fill and have sought a new shelter, enough yucca seeds remain to insure perpetuation of the species. The progeny of the moth have indeed taken toll of part of the crop of yucca seeds in recompense for the services performed by their mother. But, on the other hand, had not the moth paid its visit, the flower would by no chance have been fertilized at all. Here, then, is a case in which there is absolute mutual dependence between a particular species of insect and a particular species of plant. In the desert regions it inhabits, the moth could find no other place to deposit her eggs where food would be assured her offspring; and in the burning desert air, the stigma of the yucca, if not placed deep within the tissues, could hardly endure exposure and still perform its function. The arrangement between stamens and pistil of the yucca is such that no other insect is likely to pollenize it, even were there other insects at hand. All in all, as I said, this is one of the most curious and thought-provoking instances in all nature of mutual dependence between an animate creature and a plant. One can scarcely leave the yucca and its strange visitor without inquiring how so extraordinary a coalition could have been brought about. Unfortunately no very precise answer can be supplied. We can only assume that the complex and intricate relationship now manifested is the final result of a long series of slight adaptations through which insect and plant were mutually specialized in such a way as to conform to each other's needs. It is impossible to conceive that any sudden mutation of form on the part of the plant or of habit on the part of the insect could have led to so complicated an alliance. The change must have been very slow and gradual. First, we may suppose a condition in which the ancestors of the yucca were sometimes visited by the ancestors of the moth, but were not dependent on them for any very complicated method of pollenation. Then successive ages in which the moth gradually developed its special pair of pollen-gathering jaws, while the plant correspondingly shortened its pistil and became more and more dependent upon the peculiar process of fertilization to which the moth was becoming adapted. To any one who has not thought long and carefully, with the examination of many examples, along the lines of the evolution of organic forms through natural selection, as explained by Darwin, all this will probably seem rather vague and unsatisfactory. And, indeed, it must be admitted that among all the extraordinary cases of adaptation through which insects and plants have come to be mutually helpful, this is at least as difficult to understand as any other. The seeming intelligence of the act of gathering and depositing the ball of pollen is emphasized by the fact that this pollen is never of direct use to the progeny of the moth, yet is vitally important to them indirectly because it fertilizes the seed embryos of the plant that are to serve them as food. At first glance, then, one can scarcely escape the thought that the moth must have had some such comprehension of the plant's needs as that which leads the human plant-experimenter to cross-pollenize his flowers. One might even be excused a momentary half-conviction that the insect must be endowed with intelligence almost of the human order.


Such a thought is dispelled, however, when we reflect on the seeming intelligence of plants themselves and the apparently well-reasoned schemes by which certain flowers ensure the taking of effective toll of the insects attracted by their nectar. Even in the case of the yucca, it will be observed that the plant was not quite a passive partner in the arrangement through which the perpetuation of its kind was assured. The pistil of the flower had gradually been depressed below the pollen-bearing anthers, in full confidence that the moth would carry out its share of the mutual compact. And when we reflect that this conformation of stamens and pistil was doubtless modified from an earlier arrangement less advantageous to the plant, we are confronted with evidence of a seemingly intelligent capacity to adapt its structure to its needs on the part of the plant that to some extent matches the apparent intelligence of the insect. Similar evidence of seeming design on the part of flowers in the arrangement for guarding against self-pollenation meets us on every side. Consider, for example, the way in which the lilies project the receptive stigmas far beyond the stamens; or the way in which the amaryllis, the carnation, the balloon-flower, the geranium, and numerous others effect the same purpose by careful provision that the stamens and pistils of any given flower shall not come to maturity at the same time. Then there are plants like the sage, the stamens of which seem to lie in wait for the visitor; being observed to bend quickly over, under stimulus of contact, and rub their pollen on either side of the insect's back. Again there is the milk-weed (Asclepias cornuca), which stores its pollen in tiers of hand-bags connected with a strap that entangles the feet of the bees-and which, in its over eagerness to make sure of the transfer of its precious wares, sometimes defeats its own purpose by so overloading the insect that it cannot fly away. There are water plants, too, that adopt methods to secure cross-fertilization that are ingenious and wonderful almost beyond belief. Thus the little water plant called Villarsia nymphoides sends out its flowers from its submerged haunts as little detached balloons that float to the surface of the water and then burst open to offer their pollen to the insect messengers. And the eel grass (Vallesneria spiralis), by an even more wonderful arrangement, projects its pistillate flower up to the surface of the water on a long spiral stem grown solely for that purpose; while its staminate flower strains at the short stalk on which it is tethered until it breaks away and rises detached to the surface. The pistillate flower, once pollen has been brought to it by the insects from its floating mate, is drawn again beneath the water by the recoiling stem, never to reappear. In the pre-evolutionary days, such instances as these were cited as giving incontrovertible evidence of design in nature. But no one nowadays regards them in that light, if we use the word in the old theological sense. Since Darwin taught us the way, we are able to explain these marvelous adaptations; but as evidences of the operation of the great principle of natural selection they are no less wonderful. And most remarkable of all, as viewed from the present standpoint, are the orchids, the extraordinary pollenizing devices of which were first made generally known through the studies of Darwin. A familiar illustration of the methods adopted by this curious tribe is furnished by the species known as Orchis mnascula, which bears its pollen in small bundles at the end of a slender stalk, at the other end of which is a disc covered with a sticky secretion. An insect cannot secure nectar from the flower without carrying away at least one of these pollen stalks. But the most remarkable part of the operation is that, so soon as the insect withdraws from the flower, the pollen stalk attached horn-like to its head, bends over and curls itself into precisely the position that will inevitably cause it to strike the pistil of the next orchid that the insect visits. Another species of orchid, known as Orchis pyramidulis, grows two pollen bundles held together by a sort of collar, with which it decorates its insect visitor, clasping it, for example, about the proboscis of a butterfly. Here as in the other case the pollen-carriers adjust themselves in precisely the right position for the deposit of their important burden; and in this case the arrangement is such that a portion of the fructifying powder is deposited on each of the two pistils with which this species is equipped.


It is needless to multiply instances of the wonderful adaptations of form through which the various species of plants have made sure that the insects for which nectar is provided shall carry out their part of the bargain. Some flowers have long tubes which only the coiled proboscis of a moth or the slender bill of a humming-bird can fathom. These are sure to provide pollen-carriers of a bulky character which only humming-birds or large insects like the moth could transport. Mechanisms may even be provided to exclude from the nectar chamber bees and other small insects that could be of no service to the flower. But such cases, while in the aggregate numerous, are on the whole very exceptional. In general the plants with which the horticulturist deals, and particularly the plants of the temperate zone, have contented themselves with a much more simple arrangement, whereby the pollen-bearers are so arranged that any small insect that visits the flower is sure to go away laden with pollen. In particular, provision has been made by the vast majority of flowers of the orchard and garden to attract a single species of insect, the bee. This familiar insect, the one member of its vast tribe that is directly helpful to man as a producer of food, is the indispensable coadjutor of the most important varieties of cultivated plants. Bees of one species or another are the universal distributors of pollen in orchard and garden. The beautiful flowers that the apple and plum and cherry put forth, and the perfumes they exhale, are primarily designed as advertisements for the bee and the bee alone. Whoever realizes this truth will not be likely to doubt that the bee, in common with other insects, has good olfactory organs and an eye for the discernment of color. Yet there have been entomologists, even in recent times, who have questioned whether insects really have the sense of smell, and, others who have challenged their color sense. As to the latter point, whoever has taken the trouble to observe the maneuverings of an individual bee in the flower garden, and has seen it pass from one red flower to another, confining its visits exclusively to blossoms of one hue, will have gained sufficient evidence that the bee is by no means color blind. As to the sense of smell, if further evidence than that supplied by every-day observation of the visits of insects to perfumed flowers were required, it is furnished by an interesting and remarkable experiment made by Professor Jacques Loeb, formerly of California University, now of the Rockefeller Institute in New York. Professor Loeb placed a female butterfly in a cigar box. Closing the box he suspended it in mid-air between the ceiling and floor of a room, and opened the window. "At first," says Professor Loeb, "no butterfly of this species was visible far or near. In less than an hour a male butterfly of the same species appeared on the street. When it reached the high window its flight was retarded and it came gradually toward the window. It flew into the room and went up to the cigar box upon which it perched. During the afternoon two other males of the same species came to the box." A commentator observed that the experiment makes it unequivocally clear that insects possess an olfactory sense of almost inconceivable delicacy. But the question as to what is the real character of the stimulus that produces the sense of smell, is one of the mysteries of science. "A substance like musk," he says, "may give out a characteristic odor continuously for an indefinite period, while the substance itself appears to lose no weight. If particles of the odoriferous substance are really thrown off, these particles must be almost infinitely tenuous. If, on the other hand, the stimulus is due to the giving out of waves or vibrations comparable to the waves of light or of sound, the nature and other characteristics of these manifestations of energy are absolutely unknown." Another experimenter has shown that ants will follow a trail that has been made by other ants bearing honey or sugar. The inference seems obvious that the ants are following a very delicate trail by the sense of smell. But perhaps it is well, considering the unrevealed nature of the stimulus associated with odors, to adopt Professor Loeb's cautious phrasing and speak of the sense through which insects are guided to odoriferous objects as "chemical irritability." The fact that a bee is able to travel in a straight line backward and forward between its distant hive and the flower bed or the apple tree from which it is harvesting, even though the distance be a matter of miles, suggests the possession of organs of sense of a far more delicate character than our olfactory nerves. It is hardly probable that vision is an important aid in these long-distance flights; for Professor Loeb's experiments have led him to infer that the dioptric apparatus of insects is very inferior to the human eye. Moreover the flowers would scarcely find it necessary to put out expansive corollas and deck themselves in gaudy colors if their signals were meant for creatures having very acute vision. In point of fact, the complex multiple eye of the insect, devoid of any such adaptive apparatus for focusing as the lens of the mammalian eye, does not suggest acuteness of vision, but rather a more or less vague appreciation of large masses of color. The recent experiments of Dr. Charles A. Turner, of the St. Louis Academy of Science, have, indeed, demonstrated that bees can distinguish between color patterns as well as between different colors. But, although the tests of the naturalist Plateau, which seemed to show that insects are attracted solely by odor, are thus controverted, it doubtless remains true that the sense of smell-or some equivalent sense of a kind as yet unanalyzed-is the chief guide in bringing insects from a distance to the vicinity of flower bed or fruit tree. Professor Loeb declares that the "chemical irritability" of the insect, as excited by odoriferous objects, is immeasurably superior to the sense of smell of human beings, and possibly even finer than that of the best bloodhound. Observation of the honey-gatherer making his "bee line" from hive to orchard and back again prepares us to accept this statement at its full valuation. There must even arise a question as to whether the insect's equipment of "chemical irritability," or whatever it may be called, does not amount to the possession of a sixth sense.


We have instanced over and over the vital importance of the process of cross-fertilization which the bee accomplishes for the flower. It may be of interest to cite a few familiar illustrative instances of devices adopted by certain familiar flowers to make the services of the bee surer and more effective. Inasmuch as the bee has no conscious share in the plant's solicitude to effect cross-fertilization, it has been found expedient on the part of many flowers to adjust the arrangement of stamens and pistils in such a way that the visiting insect shall surely receive a modicum of pollen, yet cannot rub this pollen against the stigma of the same flower. Some illustrations of what might be called extreme measures to prevent such inadvertent self-fertilization, were given earlier in the present chapter. Let us note a few additional instances, with reference in particular to flowers that are largely pollenated by the bee. A simple and effective method of guarding against self-pollenation we have seen illustrated in the common geranium (Pelargonium). When the geranium flower first opens, a little cluster of anthers may be seen on the tip of the erect filament in the center of the bright, showy flower. At this stage the undeveloped stigma lies closely folded up and wholly unreceptive among the stamens. But soon after the pollen has been shed or gathered, the anthers drop off; then the stigma spreads out its five receptive lobes from the tips of the connecting filaments, and is ready to receive pollen from another flower. In the snap-dragon flower, and in many other related plants, the anthers lie along the roof of the corolla tube, where they are brushed by insects that pass down the tube in search of nectar. The stigma holds a similar position, but is farther out toward the mouth of the tube. The stigma is a very interesting structure; it is composed of two flattened lips, which respond to the slightest touch. When a bee, after visits to other flowers, enters the tube, the hair-like appendages on its back brush against the lower lip of the stigma, and the irritation causes the lips to close tightly together, coming thus in contact with and scraping the pollen-dusted back of the bee. Whether or not the receptive lips have secured any pollen, they remain closed for four or five minutes, so there is no danger that they will encounter the bee as it leaves the flower laden with a fresh supply of pollen from the companion anthers. But a few minutes later the stigma lobes open again, like a trap set for the next visitor. Human ingenuity could not well devise a mechanism better adapted than this to secure cross-pollenation and ensure against the possibility of self-fertilization. The foxglove (Digitalis) also has stamens and pistils lying along the roof of the corolla tube. Its device to prevent self-fertilization is the less ingenious but equally effective one of ripening the stigma only after the pollen has been discharged-an expedient which, as we have seen, is very commonly resorted to by other species of plants, including the lilies. The Spanish broom (Spartium junceumr) is a typical butterfly-like flower, that, in common with others of the same family, has developed a peculiar mechanism to bring about cross-pollenation. The two lower petals are joined together into a keel-shaped structure that connects the stamens and pistils. The other three petals are more enlarged, and are spread to make a more effective advertisement, challenging the attention of insects. The visiting bee naturally alights upon the projecting keel. The weight of its body presses this downward and the stamens and pistils, by a spring-like action, are thrust out against the body of the insect, scattering the pollen freely. Thus the stigma may become covered with pollen that the bee has received from some other flower while the anthers supply a new coat of pollen for future distribution. Still a different arrangement is that of the common iris. Here the anthers lie in a fold of the large petal-like branches of the style. The stigmatic surface is confined to a little crescent-shaped patch near the tip of the style-branches, and is protected by a thin, sack-like shield. The structure of the flower is such that an insect as it passes down the petals on its way to the nectary, brushes against the anthers, and dusts off the pollen. As the insect passes out, the stigma-shield protects the stigmatic surface completely. But as the insect visits another flower, its pollen-covered back comes in contact with the edge of the stigmatic shield and the pollen is scraped off against the receptive surface. These, then, are familiar illustrations of the really wonderful adaptations through which it comes to pass that the bees carry out their part of the ancestral compact that ensures the plant such interchange of pollen as is essential to racial progress. Perhaps the most alluring feature of the entire coalition is that the bee performs its all-important function unwittingly in the course of the quest of sweets that appeal to its appetite. There is no compulsion in the matter; the plant depends upon the more powerful influence of persuasion. And to add to the satisfactoriness of the entire arrangement, from a human standpoint, it must be recalled that the efforts of the industrious insect, which thus make possible the work of the plant experimenter, result at the same time in storing the nectar gathered from the flowers to form one of the most delectable of foods.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 6