388 



VEGETABLE PHYSIOLOGY 



wall or other support to which it is clinging. The roots of 

 Thesium show a similar property. When they come into 

 contact with other roots growing near them they develop a 



swelling at the point 

 of contact, from which 

 certain cells grow out 

 and penetrate the 

 host, forming haust- 

 oria (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 

 irritabilit y is exhi- 

 bited by many growing shoots, which is perhaps somewhat 

 akin to sensitiveness 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 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 



FIG. 157. Thesium alpinum. PIECE OF A 



BOOT WITH SUCKER IN SECTION. X 35. 



(After Kerner.) 



