74 THE NATURS AND WORK OF PLANTS 



ment with a geranium, and note the position of the 

 leaves five or six hours and a day later. 



Repeat the experiment by bringing a plant from 

 the open air and placing it in a box with one small 

 opening. Sketch the position of the stems and 

 leaves and do the same a day later. In what region 

 has the movement taken place, the leaf stalk or the 

 stem to which it is attached? The movements of 

 roots away from the light, § 46, are to be recalled 

 in this connection. 



92. Movements of leaves and stems in I'esponse to 

 gravity. — Many leaves and stems are seen to curve 

 upward without regard to the direction from which 

 the light comes. This power of response to gravity 

 is termed geotropisni. It is this property of the 

 plant which enables it to hold its stems outright. A 

 different kind of geotropism makes the stems of the 

 trailing species lie along the surface of the soil. 



93. Movements to avoid injury. — The rays of 

 light from the sun exert the greatest effect when 

 they strike the surface of a leaf squarely or perpen- 

 dicularly, as any one may know when he recalls that 

 the sun shines the hottest when it is directly over- 



