STIMULATION AND ITS EESULTS 



389 



grow out and penetrate the host, forming haustoria (fig. 159). 



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 IMhii^i m in 7, 



penetrate its tissues 



(fig. 160). 



Another form of 

 irritability is exhi- 

 bited by many grow- 

 ing shoots, which is 

 perhaps somewhat 

 akin to sensitiveness 

 to contact. If a shoot 

 is gently struck late- 

 rally 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. 



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 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 on the lower sides of its 

 pulvini, 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 the air there 

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



FIG. 159 Thesium alpinum. PIECE OF A 



BOOT WITH SUCKER IN SECTION. X 35. 



(After Kerner.) 



