56 PLAixT LIFE Otf THE FARM. 



CHAPTER IV. 



SENSITIVENESS. 



Movements dependent on external conditions. Gravitation, light, 

 heat, moisture. Action of gravity on roots. Geotropism. Influ- 

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

 roots through the soil. Action of gravitation on leaves. Helio- 

 tropism. Sleep of leaves. Action of heat and moisture on leaves. 

 Defensive arrangements. 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 confined to structures still in a growing state more- 

 over, in some cases they exhibit a sort of reflex action, 

 contact or irritation of one part bringing about a move- 

 ment of some other part at a distance. It is often diffi- 

 cult to dissociate the effects of these several movements ; 

 for a living plant and its parts are subjected at the same 

 time to the combined influence of several of these 

 agencies, and the force and direction of growth are neces- 

 sarily essentially modified by them. 



It may be well in this place to indicate very generally 

 in what manner roots, stems, and leaves are sensitive to 

 the effects of gravity, light, moisture, and actual contact 

 or irritation, and then to specify equally briefly what is 

 the general character of the results produced by these 

 several causes acting singly, or in combination. 



The Action of Gravity on Roots. Oeotropism, The 



downward tendency of the main root is one of its most 



