128 THE NATURE AND WORK OF PLANTS 



curvature which will place the leaf in the most 

 advantageous position for the performance of its 

 work. The stem is sensitive to light itself, as may 

 be shown if a specimen stripped of leaves is placed 

 near a window, when it can be seen to bend toward 

 the light. 



176. Sensitiveness to gravity. — Select a vigor- 

 ously growing specimen of a tomato planted in a 

 pot, and place it with the stem in a horizontal posi- 

 tion. Observe the stem a day later. In what 

 region has the curvature taken place ? Repeat this 

 experiment with a grass. Note the stems of plants 

 in the woods and meadows which have been thrown 

 down by the wind or other causes, and the tips have 

 curved in response to gravity. This form of geotro- 

 pism is exactly the reverse of that exhibited by roots. 

 (See § 43.) Still another response to gravity is to be 

 seen in the lateral branches of coniferous trees and 

 trailing stems. These place their axes at right 

 angles to the action of gravity. Sometimes the 

 trailing habit is the result of the action of the plant 

 in placing its axis at right angles to the rays of light 

 which strike it. If the plant is placed in darkness, 

 it may be found whether the horizontal position is 



