CHAPTER IV. 



SENSITIVENESS. 



Movements dependent on external conditions. Gravitation, light, heat, 

 moisture. Action of gravity on roots. Geotropism. Influence of 

 light, heat, moisture, and contact on roots. Passage of roots through 

 the soil. Action of gravitation on leaves. Heliotropism. Sleep of 

 leaves. Action of heat and moisture on leaves. Defensive arrange- 

 ments. Selection of hardy varieties. Influence of contact on leaves. 

 Action of gravity, light, heat, moisture, and contact on stems. 

 After-effects. Climbing plants. Combined effect of external and 

 internal agencies. 



CLOSELY analogous to the growth-movements are a series 

 of alterations of position dependent upon various circum- 

 stances, such as gravity, the influence of heat and light or 

 their absence, the result of contact or irritation, and so 

 on. They are probably essentially of the same nature as 

 the growth movements, but, unlike them, they are not con- 

 fined to structures still in a growing state ; moreover, in 

 some cases they exhibit a sort of reflex action, contact or 

 irritation of one part bringing about a movement of some 

 other part at a distance. It is often difficult to dissociate 

 the effects of these several movements ; for a living plant 

 and its parts are subjected at the same time to the com- 

 bined influence of several of these agencies, and the force 

 and direction of growth are necessarily essentially modified 

 by them. 



It may be well in this place to indicate very generally in 



