138 



THE EVOLUTION THEORY 



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thence to the ti]-»s of tlie otlier tentacles, for tliey cnrve throuj^-hout 

 tlieir whole Icnu'th. The ntility of the contrivance is obvious, but 

 that an arranfjonient so divergent from the ordinary dispositions of 

 plants conltl be brought about jioints to the length of time that the 



processes of natural selection must 

 have gone on, preserving every new 

 little variation, and adding it to the 

 rest. 



Two plants remain to be noticed 

 in conclusion, both possessing movable, 

 closing traps for catching animals. 

 The so-called Venus fly-trap [Dioncea 

 muscipula) is a marsh plant of North 

 America, the leaves of wdiich, like 

 those of Pinguicula and Dirjsera, are 

 arranged in a rosette on the ground. 

 The individual leaf has a spatula-like 

 stalk and a blade in two halves (Fig. 

 28, A), each edged with long and 

 strong spinous processes, directed ob- 

 liquely inwards. The halves of the 

 blade, when the necessary stimulus 

 is applied to the surface, can close 

 together in a very short time, from 10 to 30 seconds. The two 

 rows of marginal spines then cross, as the interlocking fingers of 

 the hands do, and thus form a cage out of which the imprisoned 

 insect cannot escape. The appropriate stimulus to set the mechanism 



in motion is a licjlit 

 iA w/f>^^^ touch, while a more 



violent shock, or strong 

 pressure, or a current 

 of air, does not cause 

 the trap to close. But 

 if a fly comes to creep 

 about on the leaf, and 

 in doine: so touches one 

 of six short jointed hairs rising erect from a minute cushion of cells, 

 then the leaf closes, quickly indeed, but at the same time so gently 

 and imperceptibly that the fly is unaw-are of danger and does not 

 try to escape. Then numerous purple mucous glands begin to 

 surround the victim with pepsin-containing, acid, digestive juice 

 wdiich gradually dissolves it. 



Fig. 28. Leaf of Yenus Fly-trap 

 (Dmuea muscipula), after Kerner. A, 

 leaf-blade {S})r) open. St, leaf-stalk. 

 StcJi, sensitive hairs. B, vertical sec- 

 tion through the closed leaf-blade. 



Fie. 29. 

 traps FA. 



Aklrovandia vesiculosa, a branch with the 



