Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/32

 against the upper sharp edge of the stigma (but not those in the upper or lower parts of the mass) emit a multitude of tubes, deeply penetrating the stigmatic tissue. After this period the stigma bends a little forward, and the result is that the two friable pillars of pollen almost stand free from their anther-cells, being tied to and supported in front by the penetration of their pollen-tubes into the edge of the stigma. Without this support the pillars would soon fall down.

Differently from Epipactis, the flower stands upright; the lower part of the labellum is turned up parallel to the labellum (Fig. A), and the tips of the lateral petals never become quite separate; so that the pillars of pollen are protected from the wind, and as the flower stands upright they do not tumble down from their weight. These are points of high importance to the plant, as the pollen would otherwise be blown or fall down and be wasted. The labellum is formed of two portions, as in Epipactis; and when the flower is mature, the small triangular distal portion tums down at right angles to the basal portion; thus forming a small landing-place in front of a triangular door, situated half-way up the almost tubular flower, by which insects can enter. I did not observe any nectar; but as the lower cup-formed portion of the labellum presents the same structure as in Epipactis, I presume it is secreted. After a short time, as soon as the flower is fully fertilised, the small distal portion of the labellum rises up, shuts the triangular door, and again perfectly encloses the organs of fructification.

In the early penetration of the stigma by a multitude of pollen-tubes, which I traced far down the stigmatic tissue, we apparently have another case, like that of the Bee Ophrys, of perpetual self-fertilisation. I was much surprised at this fact, and asked myself: Why does the distal portion of the labellum open for a short period? what is the use of the great mass of pollen above and below the layer of grains, the tubes of which alone penetrate the upper edge of the stigma? The stigma has a large flat viscid surface; and during several years I have almost invariably found masses of pollen adhering to its surface, and the friable pillars of pollen by some means broken down. It occurred to me, that although the flowers stand upright, and the pillars are well protected from the wind, yet that the pollen-masses might ultimately topple over from their own weight, and so fall on the stigma, thus completing the act of self-fertilisation. Accordingly, I covered up with a net a plant with four buds, and examined the flowers as soon as they had withered; the broad stigmas of three of these flowers were perfectly free from pollen, but a little had fallen on one comer of the fourth. With the exception, also, of the summit of the pillar of pollen in this latter flower, all the other pillars still stood upright and unbroken. I looked at the flowers of some surrounding plants, and everywhere found, as I had so often done before, broken-down pillars and masses of pollen on the stigmas.

Hence we may safely infer that insects visit the flowers, disturb the pollen, and leave masses of it on the stigmas. We thus see that the reflexion of the distal portion of the labellum, by which a temporary landing-place and door are formed,—the upturned labellum, by which the flower is made tubular and insects are compelled to crawl close by the stigmatic surface,—the pollen readily cohering to any object, and standing in friable pillars protected from the wind,—and, lastly, the large masses of pollen standing above and below that layer of pollen-grains, of which alone the tubes penetrate the edge of the stigma, are all co-ordinated structures, far from useless; and useless they would be if these flowers were capable of perfect self-fertilisation.

To ascertain how far the early and invariable penetration of the upper edge of the stigma by the pollen-grains, which rest on it, is effectual for fertilisation, I covered up a plant, just before the flowers opened, and removed the thin net as soon as they had begun to wither. From long experience I am sure that this temporary covering could not have injured their fertility. The four covered flowers produced as fine seed-capsules as any on the surrounding plants. When ripe, I gathered them, and likewise capsules from several surrounding plants, growing under similar conditions, and weighed the seed in a chemical balance. The seed from the four capsules from the uncovered plants weighed 1.5 grain, from the covered plant the seed of an equal number of capsules weighed under one grain; but this does not give a fair idea of the relative difference of fertility, for I observed that a great number of the seeds from the covered plant were mere minute and shrivelled husks. Accordingly I mixed the seeds well together, and took four little lots from one heap and four little lots from the other heap, and, having soaked them in water, compared them under the compound microscope: out of forty seeds from the uncovered plants there were only four bad, whereas of forty seeds from the covered-up plants there were at least twenty-seven bad; so that there were nearly seven times as many bad seeds in the covered-up plants as in those left to the free accsess of insects.

Hence we have the following curious and complex case. Perpetual self-fertilisation, but in an extremely imperfect degree, by the early penetration of the pollen-tubes; this would be useful to the plants, if insects