ORCHIDS, FORMS OF FLOWERS, POLLINATION
Coryanthes is a genus of nearly fifty Neotropical species, pollinated by jewel-like orchid bees that have coevolved with them. Orchid bees are in the subfamily Euglossini, closely related to Bombini, the bumblebees or, in Darwin’s day, humble-bees. They are spectacular metallic-iridescent insects, and the males have an enlarged, hollow tibia on each hind leg. These serve as storage vessels for aromatic oils produced by the plants, which the males use to court female bees.40 The inverted bucket-shaped lip of the flower, the labellum, gives the group its common name and stores the liquid which contains the oils. When a bee falls into the bucket, the liquid (mostly water) wets its wings, preventing it from flying away and compelling it to climb out of the bucket through a narrow spout. As it squeezes through, the orchid’s pollinia become attached to its thorax, ready to be transferred to another flower.
Darwin’s research on Coryanthes was facilitated by his correspondent in Trinidad, Hermann Crüger, then director of the Royal Botanic Gardens in Port-of-Spain. Excited as always by the intricate structure and function of orchid flower parts, he enthused to Asa Gray, “Cruger’s account of Coryanthes and the use of the bucket-like labellum full of water beats everything. I suspect the bees being well wetted flattens hairs and allows viscid disc to adhere.”41 Aware of the “inexhaustible number of contrivances” of orchids, he considered Coryanthes a case of extraordinary adaption. Crüger sent him bees and flowers preserved in alcohol in 1864,42 and wrote his own treatise on the genus in 1864. Darwin first discussed Coryanthes in the fourth edition of Origin, where he marveled at this “acme of perfect adaptation,”43 then went into more detail in his orchid book.
[Coryanthes] has its labellum or lower lip hollowed out into a great bucket, into which drops of almost pure water, not nectar, continually fall from two secreting horns which stand above it; and when the bucket is half full, the water overflows by a spout on one side. The basal part of the labellum curves over the bucket and is itself hollowed out into a sort of chamber with two lateral entrances, within which and outside there are some curious fleshy ridges.
The most ingenious man, if he had not witnessed what takes place, could never have imagined what purpose all these parts served. But Dr. Crüger saw crowds of large humble-bees visiting the gigantic flowers of this orchid in the early morning, and they came, not to suck nectar, but to gnaw off the ridges above the bucket; in doing this they frequently pushed each other into the bucket, and thus their wings were wetted, so that they could not fly out, but had to crawl out through the passage formed by the spout or overflow. Dr. Crüger has seen a “continual procession” of bees thus crawling out of their involuntary bath. The passage is narrow, and is roofed over by the column, so that a bee, in forcing its way out, first rubs its back against the viscid stigma and then against the viscid glands of the pollen-masses. The pollen-masses are thus glued to the back of the bee which first happens to crawl through the passage of a lately expanded flower and are thus carried away. Dr. Crüger sent me a flower in spirits of wine, with a bee which he had killed before it had quite crawled out of the passage with a pollen-mass fastened to its back. When the bee, thus provided, flies to another flower, or to the same flower a second time, and is pushed by its comrades into the bucket and then crawls out by the passage, the pollen-mass necessarily comes first into contact with the viscid stigma and adheres to it, and the flower is fertilised. Now at last we see the full use of the water-secreting horns, of the bucket with its spout, and of the shape of every part of the flower!
Coryanthes speciosa. Copied from Lindley’s Vegetable Kingdom (1846–1853) L. labellum. B. bucket of the labellum. H. fluid-secreting appendages. P. spout of bucket, over-arched by the end of the column, bearing the anther and stigma.
Coryanthes.— The flowers are very large and hang downwards. The distal portion of the labellum (L) in the woodcut is converted into a large bucket (B). Two appendages (H), arising from the narrowed base of the labellum, stand directly over the bucket and secrete so much fluid that drops may be seen falling into it. This fluid is limpid and so slightly sweet that it does not deserve to be called nectar, though evidently of the same nature; nor does it serve to attract insects. When the bucket is full the fluid overflows by the spout. This spout is closely over-arched by the end of the column, which bears the stigma and pollen-masses in such a position that an insect forcing its way out of the bucket through this passage would first brush with its back against the stigma and afterwards against the viscid discs of the pollinia, and thus remove them. We are now prepared to hear what Dr. Crüger says about the fertilisation of an allied species, the C. macrantha, the labellum of which is provided with crests. I may premise that he sent me specimens of the bees which he saw gnawing these crests, and they belong to the genus Euglossa. Dr. Crüger states that these bees may be “seen in great numbers disputing with each other for a place on the edge of the hypochil (i.e. the basal part of the labellum). Partly by this contest, partly perhaps intoxicated by the matter they are indulging in, they tumble down into the ‘bucket,’ half-full of a fluid secreted by organs situated at the base of the column. They then crawl along in the water towards the anterior side of the bucket, where there is a passage for them between the opening of this and the column. If one is early on the look-out, as these Hymenopterae are early risers, one can see in every flower how fecundation is performed. The humble-bee, in forcing its way out of its involuntary bath, has to exert itself considerably, as the mouth of the epichil (i.e. the distal part of the labellum) and the face of the column fit together exactly, and are very stiff and elastic. The first bee, then, which is immersed will have the gland of the pollen-mass glued to its back. The insect then generally gets through the passage, and comes out with this peculiar appendage, to return nearly immediately to its feast, when it is generally precipitated a second time into the bucket, passing out through the same opening, and so inserting the pollen-masses into the stigma while it forces its way out and thereby impregnating either the same or some other flower. I have often seen this; and sometimes there are so many of these humble-bees assembled that there is a continual procession of them through the passage specified.”
There cannot be the least doubt that the fertilisation of the flower absolutely depends on insects crawling out through the passage formed by the extremity of the labellum and the over-arching column. If the large distal portion of the labellum or bucket had been dry, the bees could easily have escaped by flying away. Therefore we must believe that the fluid is secreted by the appendages in such extraordinary quantity and is collected in the bucket, not as a palatable attraction for the bees, as these are known to gnaw the labellum, but for the sake of wetting their wings and thus compelling them to crawl out through the passage.
Cyclamen europaeum (= purpurascens). Watercolor by Elizabeth Pieth Schmitz, Botanical Manuscript.