138 



THE EVOLUTION THEORY 



'■^^- Stcih 



thence to the tips of the other tentacles, for they curve throughout 

 their whole length. The utility of the contrivance is obvious, but 

 that an arrangement so divergent from the ordinary dispositions ot 

 plants could be brought about points 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 {Dionwa 



muscixmla) is a marsh plant of North 



America, the leaves, of which, like 



those of Pinguicula and Drosera, are 



arranged in a rosette on the ground. 



The individual leaf has a spatula-like 



stalk and a blade in two halves (Fig. 



a8. A), each edged with long and 



Fig. 28. Leaf of Venus Fly-trap strong spinous processes, directed ob- 



{eS^Zr{SToi:^'^sfV^-^t.± liquely inwards. The halves of the 



stch, sensitive hairs. B, vertical sec- blade, when the nccessary stimulus 



tion through the closed leaf-blade. . , . , , , 1 j? i 



IS applied to the surrace, 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 light 

 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 



Fig. 29. Aldrovandia vesiculosa, a branch with the about On the leaf and 

 traps FA. . - . , , 



m doing 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 unaware of danger and does not 

 try to escape. Then numerous purple mucous glands begin to 

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

 which gradually dissolves it. 



