Darwin and Modern Science [281]
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
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