Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/6

 shall hereafter meet with other cases, in which the slightest touch or the vapour of chloroform causes the exterior membrane of the rostellum to rupture along certain defined lines.

At the same time that the rostellum becomes transversely ruptured in front, it probably (for it was impossible to ascertain this fact from the position of the parts) ruptures behind in two oval lines, thus separating and freeing from the rest of the exterior surface of the rostellum the two little discs of membrane, to which externally the two caudicles are attached, and to which internally the two balls of viscid matter adhere. The line of rupture is thus very complex, but strictly defined.

As the two anther-cells open longtudinally in front from top to bottom, even before the flower expands, as soon as the rostellum is properly ruptured from the effects of a slight touch, its lip can be easily depressed, and, the two little discs of membrane being already separate, the two pollinia now lie absolutely free, but are still embedded in their proper places. So that the packets of pollen and the caudicles lie within the anther-cells; the discs still form part of the posterior surface of the rostellum, but are separate: and the balls of visvid matter still lie concealed within the rostellum.

Now let us see how this complex mechanism acts. Let us 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 Fig. I., 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 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 most easily depressed. When this is effected, one or both of the viscid balls will almost infallible 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 (Fig. II.) by the upper figure.

Fig. II.

A. Pollen-mass of O. Mascula, when first attached.

B. do. do. after the act of depression.

The firmness of the attachment of the cement is very necessary, as we shall immediately see; 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 let us suppose our insect to fly to another flower, or insert the pencil (A, Fig. II.), with the attached pollinium, into the same or into another nectary: by looking at the diagram (Fig. I., A) it will be evident that the firmly attached pollinium will be simply pushed against or into its old position, namely, into its anther-cell. How then can the flower be fertilised? This is effected by a beautiful contrivance: though the viscid surface remains immoveably affixed, the apparently insignificant and minute disc of membrane to which the caudicle adheres is endowed with a remarkable power of contraction (as will hereafter be more minutely described), which causes the pollinium to sweep through about 90 degrees, always in one direction, viz., towards the apex of the proboscis or pencil, in the course, on an average, of thirty seconds. The position of the pollinium after the movement is shown in B in Fig. II. Now after this movement and interval of time (which would allow the insect to fly to another flower), it will be seen, by turning to the diagram (Fig. I., A), that, if the pencil is inserted into the nectary, the thick end of the pollinium will exactly strike the stigmatic surface.