Charles Darwin

“ But when we reflect on the incalculable number of plants which have lived during a great length of time, all requiring that insects should carry the pollen-masses from flower to flower in each generation; and as we further know from the number of the pollen-masses attached to their proboscides, that the same insects visit a large number of flowers, we can hardly believe in so gigantic an imposture. He who believes in Sprengel's doctrine must rank the sense or instinctive knowledge of many kinds of insects, even bees, very low in the scale. ”

“ During most years the pollen-masses of the many hundred flowers which were examined, adhered with the rarest exceptions to the stigma, with their discs still enclosed within the pouches. But in the year 1868, from some cause the nature of which I cannot conjecture, out of 116 flowers gathered in two localities in Kent, seventy-five retained both pollinia in their cells; ten had one pollinium, and only thirty-one had both adhering to the stigma. Long and often as I have watched plants of the Bee Ophrys, I have never seen one visited by any insect. ”

“ In Habenaria the stigmatic surface lies beneath and between the two widely-separated anther-cells, and the pollinia here converge, instead of diverging as in Orchis pyramidalis, and likewise move downwards. A poet might imagine that whilst the pollinia were borne through the air from flower to flower, adhering to an insect's body, they voluntarily and eagerly placed themselves in that exact position, in which alone they could hope to gain their wish and perpetuate their race. ”

“ It has been shown how numerous and beautiful are the contrivances for the fertilisation of Orchids. We know that it is of the highest importance that the pollinia, when attached to the head or proboscis of an insect, should be fixed symmetrically, so as not to fall either sideways or backwards. We know that in the species as yet described the viscid matter of the disc sets hard in a few minutes when exposed to the air, so that it would be a great advantage to the plant if insects were delayed in sucking the nectar, time being thus allowed for the disc to become immovably affixed. ”

“ Unless we bear in mind the good effects which have been proved to follow in most cases from cross-fertilisation, it is an astonishing fact that the flowers of the Orchideæ should not have been regularly self-fertilised. It apparently demonstrates that there must be something injurious in this latter process, of which fact I have elsewhere given direct proof. It is hardly an exaggeration to say that Nature tells us, in the most emphatic manner, that she abhors perpetual self-fertilisation. ”

“ When an insect inserts its proboscis or head into the narrow space between the upright labellum and the rostellum, it will infallibly touch the little projecting viscid mass, and as soon as it flies away it will withdraw the pollinia. I easily imitated this action by inserting any small object into the tubular flower between the labellum and rostellum. When the insect visits another flower, the very thin pollen-leaves attached parallel to the proboscis, or head, will be forced into the pocket-like stigma with their broad ends foremost. ”

“ But nature suffers no such waste. The bee goes first to the lowest flower, and, crawling spirally up the spike, effects nothing on the first spike which she visits till she reaches the upper flowers, and then she withdraws the pollinia. She soon flies to another plant, and, alighting on the lowest and oldest flower, into which a wide passage will have been formed from the greater reflexion of the column, the pollinia strike the protuberant stigma. If the stigma of the lowest flower has already been fully fertilised, little or no pollen will be left on its dried surface ”

“ When flowers having more than a single nectary are visited by many bees, so that the nectar is exhausted in most of them, the bees which afterwards visit such flowers insert, their proboscides only into one of the nectaries, and if they find this exhausted, they instantly pass on to another flower. Can it be believed that bees which show this much intelligence, should persevere in visiting flower after flower of the above-named Orchids, and in keeping their proboscides in constant movement for some time within the nectaries, in the hope of obtaining nectar which is never present? ”

“ The importance of the science of Homology rests on its giving us the key-note of the possible amount of difference in plan within any group; it allows us to class under proper heads the most diversified organs; it shows us gradations which would otherwise have been overlooked, and thus aids us in classification; it explains many monstrosities; it leads to the detection of obscure and hidden parts, or mere vestiges of parts, and shows us the meaning of rudiments. ”

“ The cause of the flowers of various kinds of Orchids being constructed so that the pollinia are always affixed to the eyes or proboscides of Lepidoptera, and to the naked foreheads or proboscides of Hymenoptera, no doubt is that the viscid discs cannot adhere to a scaly or very hairy surface; the scales themselves being easily detached. Variations in the structure of the flower of an Orchid, unless they led to the viscid discs touching some part of the body of an insect where they would remain firmly attached, would be of no service, but an injury to the plant ”

“ A living-bee or other large insect alighting on the fringed edge of the labellum, and scrambling into the flower, would depress the labellum and would be less likely to disturb the rostellum, until it had sucked the nectar and began to retreat. When a dead bee, with the four waxy balls of pollen dangling by their caudicles from its back, is forced into another flower, some or all of them are caught with certainty by the broad, shallow, and highly viscid stigmatic surface, which likewise tears off the grains of pollen from the threads of the caudicles. ”

“ As all orchids, with the exception of a few in the present small sub-family, as well as all the members of several allied groups of plants, are hermaphrodites, there can be no doubt that the common progenitor of the Orchideæ was an hermaphrodite. We may therefore attribute the hermaphrodite condition and the general appearance of Myanthus to reversion to a former state; and if so, the ancestors of all the species of Catasetum must have resembled the males of C. saccatum and callosum, for as we have just seen, it is to these two plants that Myanthus presents so many striking resemblances. ”

“ We thus see how beautifully everything is contrived, that the pollinia should be withdrawn by insects visiting the flowers. They are already attached to the disc by their threads, and, from the early withering of the anther-cells, they hang loosely suspended but protected within the clinandrum. The touch of the proboscis causes the rostellum to split in front and behind, and frees the long, narrow, boat-formed disc, which is filled with extremely viscid matter, and is sure to adhere longitudinally to the proboscis. When the bee flies away, so surely will it carry away the pollinia. ”

“ Delpino [4] with much sagacity foresaw that some insect would be discovered to act in this manner; for he argued that if an insect were to insert its proboscis, as I had supposed, from the outside through one of the small orifices close to one of the anthers, the stigma would be liable to be fertilised by the plant's own pollen: and in this he did not believe, from having confidence in what I have often insisted on—namely, that all the contrivances for fertilisation are arranged so that the stigma shall receive pollen from a distinct flower or plant. ”

“ When an insect with the pollinia attached to its back or head enters another flower, the easy depression of the distal portion of the labellum probably plays an important part; for the pollen-masses are extremely friable, and if they were struck against the tips of the petals much of the pollen would be lost; but as it is, an open gangway is offered, and the viscid stigma, with its lower protuberant part lying in front, is the first object against which the pollen-masses projecting forwards from the insect's head or back would naturally strike. ”

“ Bearing in mind these facts and others before given, it appears to me incredible that the same insect should go on visiting flower after flower of these Orchids, although it never obtains any nectar. Insects, or at least bees, are by no means destitute of intelligence. They recognise from a distance the flowers of the same species, and keep to them as long as they can. When humble-bees have bitten holes through the corolla, as they often do, so as to reach the nectar more easily, hive-bees immediately perceive what has been done and take advantage of the perforations. ”

“ The anterior face of the rostellum (r, C, D) projects a little beyond the surface of the upper part of the stigma, and this is of importance. In the early bud the rostellum consists of a friable mass of cells, with the exterior surface rough: these superficial cells undergo a great change during development, and become converted into a soft, smooth, highly elastic membrane or tissue, so excessively tender that it can be penetrated by a human hair; when thus penetrated, or when slightly rubbed, the surface becomes milky and in some degree viscid, so that the pollen-grains adhere to it. ”

“ Finally, if we consider how precious a substance pollen is, and what care has been bestowed on its elaboration and on the accessory parts in the Orchideæ,—considering how large an amount is necessary for the impregnation of the almost innumerable seeds produced by these plants,—considering that the anther stands close behind or above the stigma, self-fertilisation would have been an incomparably safer and easier process than the transportal of pollen from flower to flower. ”

“ The object of the following work is to show that the contrivances by which Orchids are fertilised, are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom; and, secondly, to show that these contrivances have for their main object the fertilisation of the flowers with pollen brought by insects from a distinct plant. In my volume 'On the Origin of Species' I gave only general reasons for the belief that it is an almost universal law of nature that the higher organic beings require an occasional cross with another individual ”

“ Hence, when certain definite points of the flower are touched by an insect, the pollinia are shot forth like an arrow, not barbed however, but having a blunt and excessively adhesive point. The insect, disturbed by so sharp a blow, or after having eaten its fill, flies sooner or later away to a female plant, and, whilst standing in the same position as before, the pollen-bearing end of the arrow is inserted into the stigmatic cavity, and a mass of pollen is left on its viscid surface. Thus, and thus alone, can the five species of Catasetum which I have examined be fertilised. ”

“ It is also certain in a majority of the cases, that insects withdraw the pollinia only when retreating from the flower, and by carrying them away, effect a union between two flowers, generally on distinct plants. This can hardly fail to occur in all the many cases in which the pollinia slowly change their position, when removed from the rostellum, in order to assume a proper direction for striking the stigma; for the insects during this interval will have had time to fly from the flowers on one plant which will serve as the male, to those on another plant which will serve as the female. ”

“ In many Vandeæ the caudicles are easily ruptured, and the fertilisation of the flower, as far as this point is concerned, is a simple affair; but in other cases their strength, and the length to which they can be stretched before they break, are surprising. I was at first perplexed to understand what purpose these qualities could serve. The explanation probably is that the pollen-masses in this tribe are very precious objects; in most of the genera a flower produces only two, and judging from the size of the stigma both are generally left adhering to it. ”

“ Fitzgerald could not detect even in the bud any trace of a rostellum or of viscid matter. He states that the pollinia touch the upper edge of the stigma, and fertilise it at an early age. Protecting a plant from the access of insects by a bell-glass made no difference in its fertility; and Mr. Fitzgerald, though he examined many flowers, never noticed the slightest derangement of the pollinia, or any pollen on the surfaces of the stigmas. Here then we have a species which fertilises itself as regularly as does Ophrys apifera. ”

“ In all the other species of Epidendrum which I examined, and in all the above-mentioned genera, it is evident that the viscid matter has to be forced upwards into the lip of the anther by a retreating insect, which would thus necessarily carry the pollinia from one flower to the stigma of another.
Nevertheless, self-fertilisation takes place in some Epidendreæ. Dr. Crüger says [6] that "we have in Trinidad three plants belonging to this family (a Schomburgkia, Cattleya, and Epidendron) which rarely open their flowers, and they are invariably found to be impregnated when they do open them. ”

“ But if in our diagram we suppose, for instance, the stigma to be seated at the bottom of a deep cavity, low down in the column, or the anther to be seated higher up, or the pedicel of the rostellum to slope more upwards, &c.—all of which contingencies occur in various species,—in such cases, an insect with a pollinium attached to its head, if it flew to another flower, would not place the pollen-masses on the stigma, unless their position had become greatly changed after attachment. ”

“ We shall, moreover, presently see that Catasetum is exclusively a male form; so that the pollen-masses must be transported to the female plant, in order that seed should be produced. The pollinium is furnished with a viscid disc of huge size; but this, instead of being placed in a position likely to touch and adhere to an insect visiting the flower, is turned inwards and lies close to the upper and back surface of a chamber, which must be called the stigmatic chamber, though functionless as a stigma. There is nothing in this chamber to attract insects ”

“ If an insect failed to remove the pollinium by means of the viscid matter from the rostellum, the pollinium would first be jerked downwards on to the protuberant surface of the labellum, placed immediately beneath the stigma. But it must be remembered that the labellum is elastic, and that at the same instant that the insect, in the act of leaving the flower, lifts up the lip of the anther, and so causes the pollinium to be shot out, the labellum will rebound back, and striking the pollinium will pitch it upwards, so as to hit the adhesive stigma. ”