WORK OF THE ROOT 



39 



plumule in opposite ways. The radicle grows in the same 

 direction as that of the force, and the plumule grows in the 

 contrary direction. Such experiments establish the fact that 

 growing organs are sensitive to a physical force in nature, 

 and the force acting on plants in this manner is gravity, the 

 radicle growing along the line of action of gravity towards 

 the earth while the plumule grows in the opposite direction. 

 The response of growing organs to the attraction of 



7 



Fig. 18. Klinostat. c, case containing a clock, by means of 

 which the glass cylinder (ra.c) is slowly revolved; in this are seed- 

 lings (p) growing on moist turf. 



gravity is known as geotropism (Gr. ge = earth, tropos 

 = turning), the radicle being positively geotropic and the 

 plumule negatively geotropic. Any influence which acts 

 upon the living organs of a plant, and induces in them 

 a change of behaviour, is called a stimulus. In addition 

 to gravity, plant organs respond to many other stimuli, 

 e. g. light, heat, contact, electrical currents, also to water 

 and other chemical substances. These stimuli are impor- 

 tant factors in the environment of a plant, and as they vary 

 frequently, it is necessary for the plant to respond and 



