498 MANUAL OF BOTANY 



by tendrils, is possessed also, though to a much smaller extent, Ly 

 most climbing stems, whose behaviour has already been described. 

 It is shown also hj the parasite Cuscuta, whose stem twines round 

 those of other plants growing near its germinating seeds, and 

 whose haustoria ultimately penetrate them. Twining petioles 

 of similar behaviour are also met with. 



Another form of irritability is exhibited by growing shoots, 

 which is perhaps somewhat akin to sensitiveness to contact. If 

 a shoot be struck laterally several times near its base, its apex 

 curves over towards the side struck. If the blows be given near 

 the apex, the curvature is in the opposite direction. 



The mechanism whereby the response to the stimulus of 

 contact is brought about in growing organs, we have seen to be 

 an increased turgidit^^ on the convex side, followed by growth. 

 In those cases in which the organ is mature, it is evident 

 that growth can have nothing to do with the movement. In 

 these instances we have rather to do with a modification of 

 turgescence, involving a redistribution of the water contained in 

 the organ. The falling of the leaflets and leaves of Mimosa is 

 due to a sudden change in the protoplasm of the cells in the 

 lower side of its pulvinus, in consequence of which water 

 escapes from them into the intercellular spaces between them. 

 It is attended by a change of colour, the pulvinus becoming of 

 a deeper green in consequence of the replacement of air there bj' 

 water. If a leaf be cut off, and after the plant has recovered from 

 the effects of the injury an adjacent leaf be sharply stimulated, 

 water is seen 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 Dionaea is 

 due also to a redistribution of the water in the cells, brought 

 about by a rapid change in the protoplasm somewhat 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. — Sensibility to variations in the moisture of the 

 environment is not so widely distributed as are the forms of irrita- 

 bility' hitherto discussed. It is exhibited chiefly by roots and by 

 the rhizoids of the Hepaticese among green plants, and by the 

 hyphae of certain Fungi. These tend to curve in the direction of a 

 moist surface if they are growing near one. When young seed- 

 lings are cultivated in a vessel which contains moist sawdust or 



