ORCHIDS, FORMS OF FLOWERS, POLLINATION
It may seem odd that Darwin followed up on his epochal work On the Origin of Species with a book on orchids, a subject seemingly far narrower in scope. But he saw great significance in the intricacies of orchid flowers, declaring that in studying orchids, “hardly any fact has struck me so much as the endless diversity of structure,—the prodigality of resources,—for gaining the very same end, namely, the fertilisation of one flower by pollen from another plant. This fact is to a large extent intelligible on the principle of natural selection.”107 His book thus had an important subtext: “it was a ‘flank movement’ on the enemy,” he confessed to Asa Gray, “It bears on design—that endless question.”108
The name “orchid” is derived from the Ancient Greek term for testicles, in reference to the paired ovate root tubers so common in the group. Carl Linnaeus published the first monograph on orchids in 1740, a work that served as the basis of his landmark Species Plantarum of 1753, where the type genus Orchis was named and a host of orchid species were first given binomials. Following Linnaeus’s tendency to be more “lumper” than “splitter,” early on, just about all orchids were considered genus Orchis, but later systematic and phylogenetic analyses led to the naming of a host of additional genera. Today about twenty-one Orchis species are recognized, all Old World and ranging from Europe and Eurasia to north Africa. Several native orchids grew near Darwin’s home at a favorite site known to him and his family as Orchis Bank, now preserved by the Kent Wildlife Trust. Orchis mascula was common there, as was Orchis (=Anacamptis) pyramidalis.
The structure of Orchis flowers served as a reference for Darwin to compare and relate pollination mechanisms of other orchid genera. Orchis species were among his favorites. “I declare I think its adaptations in every part of flower quite as beautiful and plain, or even more beautiful, than in Woodpecker,” he wrote to Joseph Hooker, at Kew, “I never saw anything so beautiful.”109 He was especially enthralled with the lock-and-key relationship between orchids and their pollinators, and the various “contrivances” by which the pollen packets (pollinia) with their sticky bases are glued to the insects. In one study, he monitored insects actively pollinating some flowers, covered others with a bell jar to prevent visitation as test cases, and, in another, simulated the visit of a pollinator by using a pencil tip to extract the pollinia, marveling how the stalks bend over within seconds of being removed to position themselves for delivering pollen to the next flower. He first noticed this in 1860, sketching it out in a letter to naturalist (and fellow orchid enthusiast) Alexander More, and describing how the stigmas of this species are held laterally to receive the pollinia: “Is this not a pretty relation to visits of insects?”110
Orchis mascula—The accompanying diagrams show the relative position of the more important organs in the flower of the Early Orchis. The sepals and the petals have been removed, excepting the labellum with its nectary. The nectary is shown only in the side view (n, fig. A); for its enlarged orifice is almost hidden in shade in the front view (B). The stigma (s) is bilobed and consists of two almost confluent stigmas; it lies under the pouch-formed rostellum (r). The anther (a, in B and A) consists of two rather widely separated cells, which are longitudinally open in front: each cell includes a pollen-mass or pollinium.
A pollinium removed out of one of the two anther-cells is represented by fig. C; it consists of a number of wedge-formed packets of pollen-grains (see fig. F, in which the packets are forcibly separated), united together by excessively elastic, thin threads. These threads become confluent at the lower end of each pollen-mass, and compose the straight elastic caudicle (c, C). The end of the caudicle is firmly attached to the viscid disc (d, C), which consists (as may be seen in the section of the pouch-formed rostellum, fig. E) of a minute oval piece of membrane, with a ball of viscid matter on its underside. Each pollinium has its separate disc; and the two balls of viscid matter lie enclosed together (fig. D) within the rostellum.
Orchis mascula. a. anther, consisting of two cells. n. nectary. r. rostellum. p. pollen-mass. s. stigma. c. caudicle of pollinium. l. labellum. d. viscid disc of pollinium.
Now let us see in the case of Orchis mascula how this complex mechanism acts. Suppose an insect to alight on the labellum, which forms a good landing-place, and to push its head into the chamber (see side view, A, or front view, B), at the back of which lies the stigma (s), in order to reach with its proboscis the end of the nectary; or, which does equally well to show the action, push very gently a sharply-pointed common pencil into the nectary. Owing to the pouch-formed rostellum projecting into the gangway of the nectary, it is scarcely possible that any object can be pushed into it without the rostellum being touched. The exterior membrane of the rostellum then ruptures in the proper lines, and the lip or pouch is easily depressed. When this is effected, one or both of the viscid balls will almost infallibly touch the intruding body. So viscid are these balls that whatever they touch they firmly stick to. Moreover, the viscid matter has the peculiar chemical quality of setting, like a cement, hard and dry in a few minutes’ time. As the anther-cells are open in front, when the insect withdraws its head, or when the pencil is withdrawn, one pollinium, or both, will be withdrawn, firmly cemented to the object, projecting up like horns, as shown … by the upper figure, A. The firmness of the attachment of the cement is very necessary, for if the pollinia were to fall sideways or backwards they could never fertilise the flower. From the position in which the two pollinia lie in their cells, they diverge a little when attached to any object. Now suppose that the insect flies to another flower, or let us insert the pencil (A), with the attached pollinium, into the same or into another nectary; by looking at the diagram … it will be evident that the firmly attached pollinium will be simply pushed against or into its old position, namely, into the anther-cell. How then can the flower be fertilised? This is effected by a beautiful contrivance: though the viscid surface remains immovably affixed, the apparently insignificant and minute disc of membrane to which the caudicle adheres is endowed with a remarkable power of contraction … which causes the pollinium to sweep through an angle of about ninety degrees, always in one direction, viz., towards the apex of the proboscis or pencil, in the course of thirty seconds on an average. The position of the pollinium after the movement is shown at B. After this movement, completed in an interval of time which would allow an insect to fly to another plant, it will be seen, by turning to the diagram … that, if the pencil be inserted into the nectary, the thick end of the pollinium now exactly strikes the stigmatic surface.
I have now described the structure of most of the British and of a few foreign species in the genus Orchis and its close allies. All these species require the aid of insects for their fertilisation. This is obvious from the fact that the pollinia are so closely embedded in the anther-cells, and the ball of viscid matter in the pouch-formed rostellum, that they cannot be shaken out by violence. We have also seen that the pollinia do not assume the proper position for striking the stigmatic surface until some time has elapsed; and this indicates that they are adapted to fertilise, not their own flowers, but those on a distinct plant. To prove that insects are necessary for the fertilisation of the flowers, I covered up a plant of Orchis morio under a bell-glass, before any of its pollinia had been removed, leaving three adjoining plants uncovered; I looked at the latter every morning, and daily found some of the pollinia removed, till all were gone with the exception of those in a single flower low down on one spike, and of those in one or two flowers on the summits of all the spikes, which were never removed. But it should be observed that when only a very few flowers remain open on the summits of the spikes, these are no longer conspicuous, and would consequently be rarely visited by insects. I then looked at the perfectly healthy plant under the bell-glass, and it had, of course, all its pollinia in the anther-cells. I tried an analogous experiment with specimens of O. mascula with the same result. It deserves notice that the spikes which had been covered up, when subsequently left uncovered, never had their pollinia carried away by insects, and did not, of course, set any seed, whereas the adjoining plants produced plenty of seed. From this fact it may be inferred that there is a proper season for each kind of Orchis, and that insects cease their visits after the proper season has passed. …
A. Pollen-mass of O. mascula, when first attached. B. Pollen-mass of O. mascula, after the act of depression.
I have examined spikes of O. pyramidalis in which every single expanded flower had its pollinia removed. The forty-nine lower flowers of a spike from Folkestone (sent me by Sir Charles Lyell) actually produced forty-eight fine seed-capsules; and of the sixty-nine lower flowers in three other spikes, seven alone had failed to produce capsules. These facts show how well moths and butterflies perform their office of marriage-priests. …
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.
Oxalis acetosella. Water and bodycolor on vellum by English School artist, Album of Garden Flowers.