26 GENERAL BOTANY 



One of the principal reasons why such adjusting movements 

 are necessary is the fact that the foods and energy which must 

 be absorbed by green plants for their maintenance are usually 

 quite unequally distributed in the air and soil about an indi- 

 vidual plant. The plant must therefore turn its parts toward 

 the most favorable source of supply to reap the maximum benefit 

 of any given condition. Thus, the two sides of a plant before a 

 window are exposed to very unequal intensities of light, and in 



6 

 FIG. 14. Phototropic response in leaves of Nasturtium tropaeolum 



a, a plant grown under normal illumination in a greenhouse ; 6, the same plant after 

 exposure for six hours to lateral illumination. From Cowles's " Ecology " 



order to receive enough of the sun's energy for effective work 

 in making starchy foods it must turn or swing its leaves in such 

 a manner that their broad green surfaces shall be exposed di- 

 rectly to light coming in from the outside. We are all familiar 

 with the fact that plants under such circumstances are able to 

 turn the entire leaf surface from the normal horizontal position 

 to an oblique position (Fig. 14), so as to face the sources of 

 maximum light supply. Under these circumstances the separa- 

 tion of the leaves by internodes, and their cyclic or spiral 

 arrangement, is still effective in preventing shading, but the 

 nicer adjustment of the leaves to unequal light exposure on the 

 two sides of the organ is made through the power of the leaves 

 to respond to light acting as a stimulus and to turn their 



