The majority of orchids differ from other seed plants (with the exception of the Asclepiads) in having no dust-like pollen. The pollen, or more correctly, the pollen-tetrads, remain fastened together as club-shaped pollinia usually borne on a slender pedicel. At the base of the pedicel is a small viscid disc by which the pollinium is attached to the head or proboscis of one of the insects which visit the flower. Darwin demonstrated that in Orchis and other flowers the pedicel of the pollinium, after its removal from the anther, undergoes a curving movement. If the pollinium was originally vertical, after a time it assumed a horizontal position. In the latter position, if the insect visited another flower, the pollinium would exactly hit the sticky stigmatic surface and thus effect fertilisation. The relation between the behaviour of the viscid disc and the secretion of nectar by the flower is especially remarkable. The flowers possess a spur which in some species (e.g. Gymnadenia conopsea, Platanthera bifolia, etc.) contains honey (nectar), which serves as an attractive bait for insects, but in others (e.g. our native species of Orchis) the spur is empty. Darwin held the opinion, confirmed by later investigations, that in the case of flowers without honey the insects must penetrate the wall of the nectarless spurs in order to obtain a nectar-like substance. The glands behave differently in the nectar-bearing and in the nectarless flowers. In the former they are so sticky that they at once adhere to the body of the insect; in the nectarless flowers firm adherence only occurs after the viscid disc has hardened. It is, therefore, adaptively of value that the insects should be detained longer in the nectarless flowers (by having to bore into the spur),--than in flowers in which the nectar is freely exposed. "If this relation, on the one hand, between the viscid matter requiring some little time to set hard, and the nectar being so lodged that moths are delayed in getting it; and, on the other hand, between the viscid matter being at first as viscid as ever it will become, and the nectar lying all ready for rapid suction, be accidental, it is a fortunate accident for the plant. If not accidental, and I cannot believe it to be accidental, what a singular case of adaptation!" ("Fertilisation of Orchids" (1st edition), page 53.)
Among exotic orchids Catasetum is particularly remarkable. One and the same species bears different forms of flowers. The species known as Catasetum tridentatum has pollinia with very large viscid discs; on touching one of the two filaments (antennae) which occur on the gynostemium of the flower the pollinia are shot out to a fairly long distance (as far as 1 metre) and in such manner that they alight on the back of the insect, where they are held. The antennae have, moreover, acquired an importance, from the point of view of the physiology of stimulation, as stimulus- perceiving organs. Darwin had shown that it is only a touch on the antennae that causes the explosion, while contact, blows, wounding, etc. on other places produce no effect. This form of flower proved to be the male. The second form, formerly regarded as a distinct species and named Monachanthus viridis, is shown to be the female flower. The anthers have only rudimentary pollinia and do not open; there are no antennae, but on the other hand numerous seeds are produced. Another type of flower, known as Myanthus barbatus, was regarded by Darwin as a third form: this was afterwards recognised by Rolfe (Rolfe, R.A. "On the sexual forms of Catasetum with special reference to the researches of Darwin and others," "Journ. Linn. Soc." Vol. XXVII. (Botany), 1891, pages 206-225.) as the male flower of another species, Catasetum barbatum Link, an identification in accordance with the discovery made by Cruger in Trinidad that it always remains sterile.
Darwin had noticed that the flowers of Catasetum do not secrete nectar, and he conjectured that in place of it the insects gnaw a tissue in the cavity of the labellum which has a "slightly sweet, pleasant and nutritious taste." This conjecture as well as other conclusions drawn by Darwin from Catasetum have been confirmed by Cruger--assuredly the best proof of the acumen with which the wonderful floral structure of this "most remarkable of the Orchids" was interpretated far from its native habitat.
As is shown by what we have said about Catasetum, other problems in addition to those concerned with fertilisation are dealt with in the Orchid book. This is especially the case in regard to flower morphology. The scope of flower morphology cannot be more clearly and better expressed than by these words: "He will see how curiously a flower may be moulded out of many separate organs--how perfect the cohesion of primordially distinct parts may become,--how organs may be used for purposes widely different from their proper function,--how other organs may be entirely suppressed, or leave mere useless emblems of their former existence." ("Fertilisation of Orchids", page 289.)
In attempting, from this point of view, to refer the floral structure of orchids to their original form, Darwin employed a much more thorough method than that of Robert Brown and others. The result of this was the production of a considerable literature, especially in France, along the lines suggested by Darwin's work. This is the so-called anatomical method, which seeks to draw conclusions as to the morphology of the flower from the course of the vascular bundles in the several parts. (He wrote in one of his letters, "...the destiny of the whole human race is as nothing to the course of vessels of orchids" ("More Letters", Vol. II. page 275.) Although the interpretation of the orchid flower given by Darwin has not proved satisfactory in one particular point--the composition of the labellum--the general results have received universal assent, namely "that all Orchids owe what they have in common to descent from some monocotyledonous plant, which, like so many other plants of the same division, possessed fifteen organs arranged alternately three within three in five whorls." ("Fertilisation of Orchids" (1st edition), page 307.) The alterations which their original form has undergone have persisted so far as they were found to be of use.
We see also that the remarkable adaptations of which we have given some examples are directed towards cross-fertilisation. In only a few of the orchids investigated by Darwin--other similar cases have since been described--was self-fertilisation found to occur regularly or usually. The former is the case in the Bee Ophrys (Ophrys apifera), the mechanism of which greatly surprised Darwin. He once remarked to a friend that one of the things that made him wish to live a few thousand years was his desire to see the extinction of the Bee Ophrys, an end to which he believed its self-fertilising habit was leading. ("Life and Letters", Vol. III. page 276 (footnote).) But, he wrote, "the safest conclusion, as it seems to me, is, that under certain unknown circumstances, and perhaps at very long intervals of time, one individual of the Bee Ophrys is crossed by another." ("Fertilisation of Orchids" page 71.)
If, on the one hand, we remember how much more sure self-fertilisation would be than cross-fertilisation, and, on the other hand, if we call to mind the numerous contrivances for cross-fertilisation, the conclusion is naturally reached that "it is an astonishing fact that self-fertilisation should not have been an habitual occurrence. It apparently demonstrates to us that there must be something injurious in the process. Nature thus tells us, in the most emphatic manner, that she abhors perpetual self- fertilisation...For may we not further infer as probable, in accordance with the belief of the vast majority of the breeders of our domestic productions, that marriage between near relations is likewise in some way injurious, that some unknown great good is derived from the union of individuals which have been kept distinct for many generations?" (Ibid., page 359.)
This view was supported by observations on plants of other families, e.g. Papilionaceae; it could, however, in the absence of experimental proof, be regarded only as a "working hypothesis."
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