STIMULATION AND ITS EESULTS 



391 



explained similarly. The closing of the leaf of Dioncea 

 (fig. 161) is due also to a redistribution of the water in 

 the cells, brought about by a rapid change in the proto- 

 plasm, perhaps akin to contraction. In Drosera the in- 

 flexion of the tentacles has been found to be preceded by a 

 peculiar churning movement of the protoplasm in the cells 

 upon the side which becomes concave. This movement, 

 which Darwin, who discovered it, called aggregation, is 

 attended by a loss of turgidity. 



FIG. 161. LEAF OF Dioncea muscipula. 



1, open ; 2, closed ; 3, one of the sensitive spines ( x 50) ; 4, glands on 

 the surface of the leaf ( x 100). 



MOISTUEE. Sensibility to variations in the moisture of 

 the environment is not so widely distributed as are the 

 forms of irritability hitherto discussed. It is exhibited 

 among green plants chiefly by young roots and by the 

 rhizoids of the Hepaticce ; it also occurs in the hyphae of 

 certain Fungi. These organs tend to curve in the direc- 

 tion of a moist surface if they are growing near one. 

 When young seedlings are cultivated in a vessel which 



