Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/9

 Description of Fig. III.

a. anther.

s.s. stigma.

r. rostellum.

l. labellum.

l'. guiding plate on the labellum.

n. nectary.

A. Front view, with all sepals and petals removed, except the labellum.

B. Side view, with all the sepals and petals removed, with the labellum longitudinally bisected, and with the near side of the upper part of the nectary cut away.

C. The two pollinia attached to the saddle-shaped viscid disc.

D. The disc after the first act of contraction, with no object seized.

E. The disc seen from above, and flattened by force, with one pollinium removed; showing the depression, by which the second act of contraction is effected.

F. The pollinium removed by the insertion of a needle into the nectary, after it has clasped the needle by the first act of contraction.

G. The same pollinium after the second act of contraction and depression.

We now come to Orchis pyramidalis, one of the most highly organised species which I have examined, and which is ranked by several botanists in a distinct genus. The relative position of the parts (Fig. III.) is here considerably different from what it is in O. mascula and its allies. There are two quite distinct rounded stigmatic surfaces (s, s, A) placed on each side of the pouch-formed rostellum. This latter organ, instead of standing some height above the nectary, is brought down (see side view B) so as to overhang and partially to close its orifice. The ante-chamber to the nectary formed by the union of the edges of the labellum to the column, which is large in O. mascula and its allies, is here small. The pouch-formed rostellum is hollowed out on the under side in the middle: it is filled with fluid. The viscid disc is single, of the shape of a saddle (Figs. C and E), carrying on its nearly flat top or seat the two caudicles of the pollinia; of which the two truncated ends firmly adhere to its upper surface. Before the membrane of the rostellum ruptures, it can be clearly seen that the saddle-formed disc forms part of the continuous surface of the rostellum. The disc is partially hidden and kept damp (which is of great importance) by the largely over-folded basal membranes of the two anther-cells. The upper membrane of the disc consists of several layers of minute cells, and is therefore rather thick; it is lined beneath with a layer of highly adhesive matter, which is formed within the rostellum. The single saddle-formed disc strictly answers to the two minute, oval, separate discs of membrane to which the two caudicles of O. mascula and its allies are attached: two separate discs have here become completely confluent.

When the flower opens and the rostellum has become symmetrically ruptured, either from a touch or spontaneously (I know which), the slightest touch depresses the lip, that is, the lower and bilobed portion of the exterior membrane of the rostellum, which projects into the mouth of the nectary. When the lip is depressed, the under and viscid surface of the disc, still remaining in its proper place, is uncovered, and is almost certain to adhere to the touching object. Even a human hair, when pushed into the nectary, is stiff enough to depress the lip or pouch; and the viscid surface of the saddle adheres to it. If, however, the lip be touched too slightly, it springs back and re-covers the under side of the saddle.

The perfect adaptation of the parts is well shown by cutting off the end of the nectary and inserting a bristle at that end; consequently in a reversed direction to that in which nature intended moths to insert their probosces, and it will be found that the rostellum may easily be torn or penetrated, but that the saddle is rarely or never caught. When the saddle sticking to a bristle together with its pollinia is removed, the under lip instantly curls closely inwards, and leaves the orifice of the nectary more open than it was before; but whether this is of any real use to >moths which so frequently visit the flowers, and consequently to the plant, I will not pretend to decide.

Lastly, the labellum is furnished with two prominent ridges (l', Fig. A, B), sloping down to the middle and expanding outwards like the mouth of a decoy; these ridges perfectly serve to guide any flexible body, like a