Chap. VII. OATASETUM. 179 



How then does Nature act ? She has endowed these 

 plants with, what must be called for want of a better 

 term, sensitiyeness, and with the remarkable power of 

 forcibly ejecting their poUinia even to a considerable 

 distance. Hence, when certain definite points of the 

 flower are touched by an insect, the poUinia are shot 

 forth like an arrow, not barbed howeyer, but having 

 a blunt and excessively adhesive point. The insect, 

 disturbed by so sharp a blow, or after having eaten its 

 fill, flies sooner or later away to a female plant, and, 

 whilst standing in the same position as before, the 

 pollen-bearing end of the arrow is inserted into the 

 stigmatic cavity, and a mass of pollen is left on its 

 viscid surface. Thus, and thus alone, can the five 

 species of Catasetum which I have examined be 

 fertilised. 



In many Orchidese, as in Listera, Spiranthes, and 

 Orchis, the surface of the rostellum is so far sensitive, 

 that, when touched or when exposed to the vapour of 

 chloroform, it ruptures in certain defined lines. So it 

 is in the tribe of the Catasetidse, but with this re- 

 markable difi'erence, that in Catasetum the rostellum is 

 prolonged into two curved tapering horns, or, as I shall 

 call them, antennse, which stand over the labellum 

 where insects alight. If these are touched even very 

 lightly, they convey some stimulus to the membrane 

 which surrounds and connects the disc of the pol- 

 linium with the adjoining surface, causing it instantly 

 to rupture; and as soon as this happens the disc is 

 suddenly set free. We have also seen in several 

 Vandeae that the pedicels of the pollinia are fastened 

 flat down in a state of tension, and are highly elastic, 

 so that, when freed, they immediately spring up, appa- 

 rently for the sake of detaching the pollen-masses from 

 the anther-cells. In the genus Catasetum, on the 



