ORCHIDS, FORMS OF FLOWERS, POLLINATION
Catasetum orchids are remarkably complex and elaborate, with 130 species found in Neotropical regions. It is one of the few orchid groups that bear separate male and female flowers, with a dramatic sexual dimorphism that fooled botanists for decades into thinking they belonged to different genera. Even more exciting for Darwin, the large male flowers forcibly discharge their sticky pollen packets (pollinia) in a form of “ballistic” pollen transfer that had never before been seen in any orchid.29
When Darwin received a Catasetum plant in bloom from James Veitch’s Royal Exotic Nursery in 1861, he declared to Joseph Hooker that it was the most wonderful orchid he had ever seen. He was in good company—nearly forty years earlier William Jackson Hooker, Joseph’s father and director of Kew before him, had declared of Catasetum tridentatum, “I know of no individual of [the orchid] family which has flowers so splendid and so curious.”30 Catasetum inflamed Darwin’s growing orchidelirium, and he wrote to Hooker, “I never was more interested in any subject in my life, than in this of Orchids.”31 The “special contrivance” of the amazing pollinia ejection led him to dub Catasetum “the most remarkable of all orchids,”32 and the jokester couldn’t resist firing salvos of Catasetum pollen at unsuspecting guests visiting his greenhouse.
Catasetum was considered a genus wholly distinct from two other unusual genera in the family, Monachanthus and Myanthus—orchids that, incredibly, were found growing on the same individual plant. Naturalist and explorer Robert Hermann Schomburgk first presented specimens of this Franken-orchid to the Linnean Society in 1836. He soon collected another along the banks of the Essequibo River in South America, this one bearing flowers of Catasetum tridentatum and Monachanthus viridis. The keen observer noted that none of the hundreds of C. tridentatum flowers bore seeds, while he was “astonished” by the large M. viridis fruits. That proved to be the clue Darwin used to solve the mystery; he determined that these were, in fact, floral morphs of one and the same species. Catasetum tridentatum turned out to be male, Monachanthus viridis female (complete with vestigial pollinia!), and Myanthus barbatus a hermaphrodite with male and female parts.33
Darwin discussed these and related species at length in The Various Contrivances by Which Orchids are Fertilised by Insects. Hermann Crüger, a German pharmacist and botanist in Trinidad who had initially insisted the plants were distinctive genera, later confirmed Darwin’s results and sent him specimens of the bees that pollinate them. But the perhaps slightly embarrassed Crüger could not resist taking a jab at Darwin too. “Whoever has read Darwin’s remarkable work on the fecundation of orchids,” he wrote in a letter to the Linnean Society of London,34 “must have regretted that the chapters of tropical and other foreign orchids leave a certain amount of uncertainty on the mind of the reader until the observations and suppositions shall have been endorsed by actual facts observed in the native countries of these plants”—conveniently overlooking the fact that Darwin observed more in the preserved plants than those who had seen them live in the field. Darwin’s conclusions were revised several years later, as often happens in science, but his observations corrected much of what other botanists had presented previously.
George Sowerby illustrated the wonderful flowers of Catasetum and other orchids for Darwin’s book, aiding in his observations and descriptions. Sowerby spent ten days with Darwin, painstakingly illustrating flowers sent from Kew and recreating illustrations by botanical artist Franz Bauer. Darwin sighed in one letter that he was “half dead working with Mr. Sowerby at the orchid drawings,”35 but the results were well worth the time.
Catasetum tridentatum. The general appearance of this species, which is very different from that of C. saccatum, callosum and tabulare, is represented [here] with a sepal on each side cut off.
Catasetum tridentatum a. anther. pd. pedicel of pollinium. an. antennae. l. labellum A. Side view of flower in its natural position, with two of the sepals cut off. B. Front view of column, in position reverse of fig. A.
The flower stands with the labellum uppermost, that is, in a reversed position compared with most Orchids. The labellum is helmet-shaped, its distal portion being reduced to three small points. It cannot hold nectar from its position; but the walls are thick, and have, as in the other species, a pleasant nutritious taste. The stigmatic chamber, though functionless as a stigma, is of large size. The summit of the column, and the spike-like anther, are not so much elongated as in C. saccatum. In other respects, there is no important difference. The antennae are of greater length; their tips for about one-twentieth of their length are roughened by cells produced into papillae. …
I need not further describe the present species, except as to the position of the antennae. They occupied exactly the same position in all the many flowers which were examined. Both lie curled within the helmet-like labellum; the left-hand one stands higher up, with its inwardly bowed extremity in the middle; the right-hand antenna lies lower down and crosses the whole base of the labellum, with the tip just projecting beyond the left margin of the base of the column. Both are sensitive, but apparently the one which is coiled within the middle of the labellum is the more sensitive of the two. From the position of the petals and sepals, an insect visiting the flower would almost certainly alight on the crest of the labellum; and it could hardly gnaw any part of the great cavity without touching one of the two antennae, for the left-hand one guards the upper part, and the right-hand one the lower part. When either of these is touched, the pollinium is ejected and the disc will strike the head or thorax of the insect.
The position of the antennae in this Catasetum may be compared with that of a man with his left arm raised and bent so that his hand stands in front of his chest, and with his right arm crossing his body lower down so that the fingers project just beyond his left side. …
Catasetum tridentatum is interesting under another point of view. Botanists were astonished when Sir R. Schomburgk stated that he had seen three forms, believed to constitute three distinct genera, namely, Catasetum tridentatum, Monachanthus viridis, and Myanthus barbatus, all growing on the same plant. Lindley remarked that “such cases shake to the foundation all our ideas of the stability of genera and species.” Sir R. Schomburgk affirms that he has seen hundreds of plants of C. tridentatum in Essequibo without ever finding one specimen with seeds, whereas he was surprised at the gigantic seed-vessels of the Monachanthus; and he correctly remarks that “here we have traces of sexual difference in Orchideous flowers.” Dr. Crüger also informs me that in Trinidad he never saw capsules naturally produced by the flowers of this Catasetum; nor when they were fertilised by him with their own pollen, as was done repeatedly. On the other hand, when he fertilised the flowers of the Monachanthus viridis with pollen from the Catasetum, the operation never failed. The Monachanthus also commonly produces fruit in a state of nature. …
With respect to Monachanthus viridis, and Myanthus barbatus, the President of the Linnean Society has kindly permitted me to examine the spike bearing these two so-called genera, preserved in spirits, which was sent home by Sir R. Schomburgk. The flower of the Monachanthus (A) resembles pretty closely in external appearance that of Catasetum tridentatum. … The labellum, which holds the same relative position to the other parts, is not nearly so deep, especially on the sides, and its edge is crenated. The other petals and sepals are all reflexed and are not so much spotted as in the Catasetum. The bract at the base of the ovarium is much larger. The whole column, especially the filament and the spike-like anther, are much shorter; and the rostellum is much less protuberant. The antennae are entirely absent, and the pollen-masses are rudimentary. These are interesting facts, from corroborating the view taken of the function of the antennae; for as there are no pollinia to eject, an organ adapted to convey the stimulus from the touch of an insect to the rostellum would be useless. I could find no trace of a viscid disc or pedicel, and no doubt they had been lost; for Dr. Crüger says that “the anther of the female flower drops off immediately after the opening of the same, i. e. before the flower has reached perfection as regards colour, size, and smell. The disc does not cohere, or very slightly, to the pollen-masses, but drops off about the same time, with the anther;” leaving behind them the rudimentary pollen-masses.
a. anther. an. antennae. l. labellum. p. pollen-mass, rudimentary. s. stigma cleft. sep. two lower sepals.
From these facts alone, it is almost certain that Monachanthus is a female plant; and as already stated, Sir E. Schomburgk and Dr. Crüger have both seen it seeding abundantly. Altogether the flower differs in a most remarkable manner that that of the male Catasetum tridentatum, and it is no wonder that the two plants were formerly ranked as distinct genera. …
Thus every detail of structure which characterises the male pollen- masses is represented in the female plant in a useless condition. Such cases are familiar to every naturalist but can never be observed without renewed interest. At a period not far distant, naturalists will hear with surprise, perhaps with derision, that grave and learned men formerly maintained that such useless organs were not remnants retained by inheritance but were specially created and arranged in their proper places like dishes on a table (this is the simile of a distinguished botanist) by an Omnipotent hand “to complete the scheme of nature.”…
The genus Catasetum is interesting to an unusual degree in several respects. The separation of the sexes is unknown amongst other Orchids, except perhaps in the allied genus Cycnoches. In Catasetum we have three sexual forms, generally borne on separate plants, but sometimes mingled together on the same plant; and these three forms are wonderfully different from one another, much more different than, for instance, a peacock is from a peahen. …
This genus is still more interesting in its manner of fertilisation. We see a flower patiently waiting with its antennae stretched forth in a well-adapted position, ready to give notice whenever an insect puts its head into the cavity of the labellum. The female Monachanthus, not having true pollinia to eject, is destitute of antennae. In the male and hermaphrodite forms, namely Catasetum tridentatum and Myanthus barbatus, the pollinia lie doubled up, like a spring, ready to be instantly shot forth when the antennae are touched. The disc end is always projected foremost and is coated with viscid matter which quickly sets hard and affixes the hinged pedicel firmly to the insect’s body. The insect flies from flower to flower, till at last it visits a female plant: it then inserts one of the pollen-masses into the stigmatic cavity. As soon as the insect flies away the elastic caudicle, made weak enough to yield to the viscidity of the stigmatic surface, breaks, and leaves behind a pollen-mass; then the pollen-tubes slowly protrude, penetrate the stigmatic canal, and the act of fertilisation is completed. Who would have been bold enough to have surmised that the propagation of a species depended on so complex, so apparently artificial, and yet so admirable an arrangement?
Clematis repens. Water and bodycolor on vellum by Dame Ann Hamilton, Drawings of Plants.