PTEROSTYLIS AND MASDEVALLIA 595 



limb (Fig. 246 a, /), and a claw (n) of variable length. Where the 

 limb joins the claw, it bears, on its upper side, an appendage (a), the 

 precise form of which varies in different species. The characteristic 

 movement of the labellum takes place when a small insect alights 

 upon the limb and crawls upwards ; in these circumstances the claw, 

 which is the actual motor organ, curves suddenly inwards, throwing the 

 limb upwards against the column and imprisoning the insect in the 

 flower. The animal can only escape by ascending the column, and 

 forcing its way between the two wing-like outgrowths (/), which crown 

 the distal extremity of that structure. In crawling over the base of 

 the column, the insect transfers any pollinia that it may have brought 

 with it from other Pterostylis-fiowevs to the stigmatic surface (st), while 

 after ernemin^ between the wings of the column it cannot fail to rub 

 against the rostellum, and thus to carry off the pollen-masses, if these 

 have not been previously removed. 



So far as existing observations go, it seems highly probable that, in 

 Pterostylis curta, P. striata, P. coccinea, P. truncata and allied species, the 

 movement of the labellum takes place at the instant when the insect 

 touches the appendage of the limb. In these species the appendage 

 consists of a ribbon-shaped basal portion which is bent upwards, and a 

 distal part composed of numerous lobes and lappets, which are densely 

 clothed with short thin-walled hairs. The position and structure of 

 this remarkable appendage strongly suggests that it is the tactile sense- 

 organ of the labellum ; this hypothesis, however, still stands in need of 

 experimental verification. 290 



Masdevallia ruascosa, an Orchid which grows in New Granada, like- 

 wise possesses an irritable labellum. In this case F. W. Oliver has 

 shown, that the characteristic movement only takes place, when an 

 insect visitor touches a " tactile ridge " situated on the upper surface of 

 the limb. The epidermis of this ridge consists of remarkably small 

 thin-walled cells, with papillose outer walls, and acts as a sensory 

 epithelium. The ecological significance of the movement is the same as 

 in Pterostylis ; insect visitors are imprisoned as a result of the sudden 

 curvature of the labellum, and bring about cross-pollination in making 

 their escape. 



III. GRAVITATIONAL SENSE ORGANS [STATOCYSTS]. 



A. GENERAL CONSIDERATIONS. 



The majority of plants possess the capacity of orientating their 

 several organs in an appropriate manner. The most important 

 means of orientation is furnished by geotropic irritability, that is, the 

 power of perceiving the " direction of gravity," and of placing the various 



