38 PLANT-ANIMALS [CH. 



selves with reference to the directions in which light 

 and gravity act upon them. A geranium in a cottage 

 window so disposes its leaves that they receive the 

 maximum of such light as may reach them. Each 

 leaf places itself at right angles to the direction of 

 the incident light. The stem of the plant behaves 

 differently, bending till its tip is parallel with 

 the rays, it grows toward the source of light. Light 

 is the agent, or stimulus, which induces these orien- 

 tations. The mode of orientation is determined by 

 the plant itself and has, in each case, a purposeful 

 significance. The leaves in the window are none too 

 well illuminated ; the work which they have to per- 

 form depends on ample light and thus, by their 

 orientations, they secure for themselves the most 

 favoured light-treatment. 



The root of a plant grows vertically downward 

 through the soil. When, from one cause or another, 

 the tip is displaced from the vertical line, the rate of 

 elongation of the growing region of the root becomes 

 faster on the upper than on the lower side. A growth- 

 curvature results, and the tip is carried by the bending 

 root once more into the vertical line. Here gravity is 

 the stimulus, and the result of the stimulus is a motor 

 response a purposeful growth-curvature. Cut away 

 the root-tip, and the root, although it be displaced from 

 the vertical, grows indifferently in any direction till a 

 new root-tip is regenerated. From such and similar 



