PHYSIOLOGY 47 



be said in any way that the plant knows that its leaves 

 require the light, and yet the result in the plant's whole 

 economy is that the tendency to grow towards the light 

 places the leaves so that the necessary light falls on 

 them, and they are thus able to perform their function 

 of food-making for the benefit of the whole individual. 



Another influence which helps to direct growth is 

 the attraction or repulsion of gravitation. The plant, 

 in some way which has not yet been fully explained, is 

 able to perceive whether it is growing in the direction 

 of the force of gravitation or at an angle to it. The 

 minute starch grains in the tips of organs fall to one 

 side or the other of the cells as the position is changed, 

 and it seems probable that they act somehow like the 

 " statoliths " in the invertebrate animals. Not one 

 organ alone, but various parts of the plant, react in- 

 dependently when the position of the whole is changed. 

 This sensitiveness is called geotropism, and is the main 

 cause of the roots growing down into the earth and of 

 the stems growing upright in the air. Such plants as 

 climb or creep are affected by other influences which 

 to a greater or less extent counteract the rectangular 

 response which is normal in most. An illustration of 

 the strength of the effect of gravitation may be well 

 seen in a tall herb which has been " laid " by the 

 wind or broken under foot in an empty flower-bed. 

 It will begin to "raise its head" in a few hours, 

 and the end of the shoot will grow upright. That this 

 return to the upright position is not due to heliotropism 

 or the growth towards light is shown in the case of a 

 plant in an empty flower-bed, for there the prostrate 

 leaves would not be overshadowed by other vegetation. 

 The fact that the different organs respond differently 

 to gravitation, and roots are positively geo tropic, 



