EEL-TRAP HAIRS 



211 



hairs. The flap overhanging the mouth of the pitcher has its 

 adaxial surface which faces the opening covered with numerous 

 unicellular hairs that point obliquely downwards ; the cuticle of each 

 hair is provided with conspicuous longitudinal ridges, which extend, at 

 regular intervals, from the broad base or foot right up to the apex of 

 the hair. The other eel-trap hairs occur on the inner surface of the 

 narrow lower portion of the pitcher. These have thick walls, like the 

 first-mentioned hairs, but are much smaller and devoid of cuticular 

 ridges, while the foot is not broad but elongated. 



The more elaborate types of eel-trap hair are furnished with 

 special locking devices, which prevent the hair from being pushed back 

 so far as to allow the insect to climb over the obstacle. According 

 to the author's observations 115 , the stem of Biophytum prolifcrum bears, 

 just below the insertion of each whorl of leaves, a ring of densely 

 crowded, stiff hairs, arranged in a 

 somewhat irregular manner, but all 

 pointing obliquely downwards. Each 

 hair is about 3 mm. in length 

 and consists of a single cell, with 

 very thick lignified walls showing 

 obvious stratification ; it has a 

 short sunken foot which is likewise 

 thick-walled and provided with nu- 

 merous pits. Just above the foot, 

 the hair is produced, on its upper 

 side, into a blunt process of varying 

 length ; this protuberance is also 



very thick-walled, and is often bent backwards towards the surface of 

 the stem (Fig. 82). When an insect which is trying to ascend the 

 stem pushes one of these hairs upwards, the blunt end of the aforesaid 

 process soon comes into contact with the stem, and so arrests the 

 movement of the hair ; the protuberance thus acts as a catch or stop. 



The eel-trap hairs in the perianth-tube of Aristolochia Clcmatitis, 

 which have been investigated by Hildebrand and by Correns, are 

 provided with a locking arrangement that acts on a similar prin- 

 ciple. 116 As is w r ell known, these hairs serve to prevent the pre- 

 mature egress of small flies which find their way into the perianth- 

 tube : the flower is protogynous, and the flies are only allowed to 

 escape, by the collapse of the hairs, when they have been covered with 

 pollen owing to the dehiscence of the anthers. Each hair (Fig. 83) 

 consists of three parts. The foot is composed of two cells, the 

 upper of which has very thick outer walls. The solitary hinge-cell 

 is narrow below, but expands towards its distal extremity ; its outer 



Fig. 82. 



Basal portion of an eel-trap hair from the stem 

 of Biophytum proliftrum. 



