STIMULATION AND ITS RESULTS 397 



with other roots growing near them they develop a swell- 

 ing at the point of contact, from which certain cells grow 

 out and penetrate the host, forming haustoria (fig. 157). 

 The parasite Cuscuta, often found growing on clover, is 

 affected in the same way, first twining round the clover 

 stem and then putting out haustoria, which penetrate its 

 tissues (fig. 158). 



Another form of irritability is exhibited by many 



Kiu. 157. Thesium alpinum. PIECE OF A ROOT WITH SUCKEB IN 

 SECTION. X 35. (After Kerner.) 



growing shoots, which is perhaps somewhat akin to sensi- 

 tiveness to contact. It is an appreciation of oscillation 

 or shaking. If a shoot is gently struck laterally several 

 times near its base, its apex curves over towards .the 

 side struck. If the blows are given near the apex, the 

 resulting curvature is in the opposite direction. If a plant 

 of Mimosa pudica is shaken, the leaves fall as- they do when 

 they are violently handled. 



The mechanism whereby the response to the stimulus of 

 contact is brought about "in growing organs we have seen 

 to be an increased turgidity on the convex side, followed 

 by growth. In those cases where the organ is mature it is 



