Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/5

 Description of Fig. I.

a. anther.

r. rostellum.

s.. stigma.

l. labellum.

n. nectary.

p. pollinium or pollen-mass.

c. caudicle of pollinium.

d. viscid disc of pollinium.

A. Side view of flower, with all the petals and sepals cut off except the labellum, of which the near half is cut away, as well as the upper portion of the near side of the nectary.

B. Front view of flower, with all the sepals and petals removed, except the labellum.

C. One pollinium or pollen-mass, showing the packets of pollen-grains, the caudicle, and viscid disc.

D. Front view of the discs and caudicles of both polinia within the rostellum, with its lip depressed.

E. Section through one side of the rostellum, with the included disc and caudicle of one pollinium.

F. Packets of pollen-grains, tied together by elastic threads, here extended. (Copied from Bauer.)

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 (c C) straight elastic caudicle. The end of the caudicle is firmly attached to the viscid disc (d C), which consists (as may have been seen in the section, Fig. E) of a minute oval piece of membrane, with a ball of viscid matter on its under side. Each pollinium has its separate disc; and the two balls of viscid matter lie enclosed together (Fig. D) within the rostellum.

The rostellum is a nearly spherical, somewhat pointed projection (r Figs. A and B) overhanging the two almost confluent stigmas, and must be fully described, as every detail of its structure is full of signification. A section through one of the discs and balls of viscid matter is given (Fig. E); and a front view of both viscid discs within the rostellum (Fig. D) is likewise given. This latter figure (D) probably best serves to explain the structure of the rostellum; but it must be understood that the front lip is here considerably depressed. The lowest part of the anther is united to the back of the rostellum, as may be seen in Fig. B. At an early period of growth the rostellum consists of a mass of polygonal cells, full of brownish matter, which cells soon resolve themselves into two balls of an extremely viscid semi-fluid substance, void of structure. These viscid masses are slightly elongated, almost flat on the top, and convex below. They lie quite free within the rostellum (being surrounded by fluid), except at the back, where each viscid ball firmly adheres to a small portion or disc of the exterior membrane of the rostellum. The ends of the two caudicles are strongly attached to these two little discs of membrane.

The membrane forming the whole exterior surface of the rostellum is at first continuous; but as soon as the flower opens the slightest touch causes it to rupture transversely in a sinuous line, in front of the anther-cells and of the little crest or fold of membrane (see Fig. D) between them. This act of rupturing makes no difference in the shape of the rostellum, but converts the front part into a lip, which can easily be depressed. This lip is represented considerably depressed in Fig. D, and its edge is seen, Fig. B, in the front view. When the lip is thoroughly depressed, the two balls of viscid matter are exposed. Owing to the elasticity of the hinder part or hinge, the lip or pouch, when not pressed down, springs up and again encloses the two viscid balls.

I will not affirm that the rupturing of the exterior membrane of the rostellum never takes place spontaneously; and no doubt the membrane is prepared for the rupture by having become very weak along defined lines; but several times I saw the act ensue from an excessively slight touch—so slight that I conclude that the action is not simply mechanical, but, for the want of a better term, may be called vital. We