Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/42

 rostellum, the rostellum curves a little forwards, and perhaps the anther also moves a little backwards. This movement is of much importance, as the tips of the anther would otherwise be caught by the exploded viscid matter, and the pollinia would for ever be locked up and rendered useless. I once found an injured flower which had been pressed and had exploded before fully expanding, and the anther, with the enclosed pollen-masses, was permanently glued down to the crest of the rostellum. At the moment of the explosion the rostellum, which already stands arched over the stigma, quickly bends forwards and downwards as as to stand (Fig. B) at right angles to its surface. The pollinia, if not removed by the touching object that causes the explosion, become fixed to the rostellum, and by its movement are drawn a little forward. If their lower ends are now freed by a needle from the anther-cells, they spring up; but they are not by this movement placed on the stigma. In the course of some hours, or of a day, the rostellum not only slowly recovers its original slightly-arched position, but becomes quite straight and parallel to the stigmatic surface. This backward movement of the rostellum is of importance; for if after the explosion it permanently remained projecting at right angles close over the stigma, pollen could with difficultly have been left on its viscid surface. When the rostellum is touched so quickly that the pollinia are not removed, they are, as I have said, drawn at the moment of explosion a little forward by the movement of th rostellum; but by its subsequent backward movement the pollinia are pushed back into their original position.

From the account here given we may safely infer how the fertilisation of this Orchid is effected. Small insects alight on the broad lower end of the labellum for the sake of the nectar copiously secreted by it; as they lick up the nectar they slowly crawl up its narrowed surface until their heads stand directly under the over-arched crest of the rostellum; as they raise their heads they touch the crest, which explodes, and the pollinia become firmly cemented to them. The insect in flying away withdraws the pollinia, carries them to another flower, and leaves masses of the friable pollen on its viscid stigma.

In order to witness what I felt sure would take place, I watched a group of plants on two or three occasions for an hour; each day I saw numerous specimens of two small Hymenopterous insects, namely, a Hamiteles and a Cryptus, flying about the plants and licking up the nectar; most of the flowers, which were visited over and over again, had already had their pollinia removed, but at last I saw both these insect-species crawl into younger flowers, and suddenly retreat with a pair of bright yellow pollinia sticking to their foreheads; I caught them, and found the point of attachment was to the inner edge of the eye; on the other eye of one specimen there was a ball of the hardened viscid matter, showing that it had previously removed another pair of pollinia, and had subsequently in all probability left them on the stigma of one of the flowers. As I caught these insects, I did not witness the act of fertilisation; but C. K. Sprengel actually saw a Hymenopterous insect leave its pollen-mass on the stigma. My son watched another bed of this Orchid at some miles' distance, and brought me home the same Hymenopterous insects with attached pollinia, and he saw Diptera also visiting the flowers. He was struck with the number of spider-webs spread over these plants, as if the spiders were aware how attractive the Listera was to insects, and how necessary they were to its fertilisation.

To show how delicate a touch suffices to cause the rostellum to explode, I may mention that I found an extremely minute Hymenopterous insect vainly struggling with its whole head burrled in the hardened viscid matter, thus cemented to the crest of the rostellum and to the tips of the pollinia; the insect was not so large as one of the pollinia, and after causing the explosion it had not force to remove them, and was thus punished for attempting a work beyond its strength, and perished miserably.

In Spiranthes the young flowers, which have their pollinia in the best state for removal, cannot possibly be fertilised; they must remain in a virgin condition until they are a little older and the labellum has moved from the column. Here the same thing apparently occurs, for the stigmas of the older flowers were more viscid than those of the younger flowers. These latter have their pollinia quite ready for removal; but immediately after the explosion the rostellum, as we have seen, curls forwards and downwards, thus protecting the stigma for a time, until the rostellum slowly becomes quite straight, when the stigma in its more mature condition is left freely exposed, and is ready for fertilisation.

I was curious to ascertain whether the rostellum would ultimately explode, if never touched; but I have found it difficult to ascertain this fact, as the flowers are so attractive to insects, and the touch of such minute insects suffices to cause the explosion, that it was scarcely possible to exclude them.

I have covered up plants several times, and left them till long after all uncovered plants had set their pods; and without going into unnecessary details, I may positively state that the rostella in several flowers had not exploded, though the stigma was withered, the pollen quite mouldy and incapable of removal. Some few of the very old flowers, however, when roughly touched, were still capable of feeble explosion. Other flowers under the nets had exploded, and the tips of their pollinia were fixed to the crest of the rostellum; but whether these had been touched by some excessively minute insect, or had exploded spontaneously, it is impossible