390 



VEGETABLE PHYSIOLOGY 



green in consequence of the replacement of the air there 

 by water. If a leaf is cut off just above the pulvinus and 

 the plant allowed to recover from the effects of the injury, 

 subsequent stimulation of an adjacent leaf causes water to 

 exude from the cut surface of the pulvinus. The cases 

 of the irritable stamens and stigmas are probably to be 

 explained similarly. The closing of the leaf of Dioncea 

 (fig. 159) is due also to a redistribution of the water in 



FIG. 159. 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). 



the cells of a band of tissue lying along the mid-rib, 

 brought about by a rapid change in the protoplasm, 

 perhaps akin to contraction. In Drosera the inflexion 

 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. 



MOISTURE, Sensitiveness to variations in the moisture 



