SECT. II 



PHYSIOLOGY 



237 



\\iPinguicula it is the leaf-margins which fold over any small insects that may 

 be held by the minute epidermal glands. In species of Utricularia (Fig. 47), 

 growing frequently in stagnant water,small green bladders (metamorphosed leaf-seg- 

 ments) are found on the dissected leaves. In .each bladder there is a small opening 

 closed by an elastic valve which only opens inwards. Small snails and crustaceans 

 can readily pass through this opening, guided to it by special outgrowths ; but 

 their egress is prevented by the trap-like action of the valve, so that in one 



FIG. 207. Myrmecoitia n-hhu'tn. Tulirnms ^ti-m cut through longitudinally. Another epiphyte 

 is seen below. (J nat. size ; from .SCHIMI-ER'S Plant-Geography.) 



bladder as many as ten or twelve crustaceans will often be found imprisoned at 

 the same time. The absorption of the disorganised animal remains seems to be 

 performed by forked hairs which spring from the walls of the bladder. 



More remarkable still, and even better adapted for its purpose, is the mechan- 

 ism exhibited by some exotic insectivorous plants. In the case of Venus' Fly-trap 

 (Dionaea], growing in the peat-bogs of North Carolina, the capture of insects is 

 effected by the sudden closing together of the two halves of the leaves (Fig. 209). 

 This action is especially due to the irritability of three bristles on the upper side 

 of each half-leaf (the leaf surfaces themselves are much less sensitive). Upon the 



