224 THE ORCHIDACEiS OF THE 



separate, and the balls of viscid 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. 6, or front view Fig. 9), 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 infallibly 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 insect withdraws its head, or when the pencil is withdrawn, one 

 pollinium, or both, will be withdrawn, firmly cemented to the 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 (Figs. 3 and 5). 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 with the 

 attached pollinium into the same or another nectary. By looking 

 at the diagram (Fig. 3) it^vill be evident that the firmly attached 

 pollinium will be simply pushed against or into its old position, 

 namely, into its anther-cell. How, then (continues Mr. Darwin), 

 can the flower be fertilised ? This is effected by a beautiful con- 

 trivance. Though the viscid surface remains immovably affixed, 

 the apparently insignificant minute disc of membrane to which the 

 caudicle adheres is endowed with a remarkable power of contrac- 

 tion, which causes the pollinium to sweep through about 90 

 degrees, always in one direction — viz., towards the apex of the 



