IRRITABILITY. 



127 



Horizontal leaves and shoots can be shown to be subject to 

 the same influence, and are therefore diageotropic. 



261. Influence of light. Not only is light a very important 

 factor for plants during photosynthesis, it exerts great influ- 

 ence on plant growth and movement. 



262. Growth in the absence of light. Plants grown in the dark 

 are subject to a number of changes. The stems are often longer, 

 more slender and 



weaker since they 

 contain a larger 

 amount of water 

 in proportion to 

 building material 

 which the plant 

 obtains from car- 

 bohydrates manu- 

 factured in the 

 light. On many 

 plants the leaves 

 are very small 

 when grown in the 

 dark. 



263. Influence of light on direction of 

 growth. While we are growing seedlings, 

 the pots or boxes of some of them should be 



rig. no. 



placed SO that the plants will have a One- Radish seedlings grown in 

 . j , .,, . ,. , . . the light, shorter, stouter, 



sided illumination. I his can be done by and green in c 9 ior. Growth 



, .., . , retarded by light. 



placing them near an open window, in a 



room with a one-sided illumination, or they may be placed in a 

 box closed on all sides but one which is facing the window or 

 light. In 12-24 hours, or even in a much shorter time in some 

 cases, the stems of the seedlings will be directed toward the 

 source of light. This influence exerted by the rays of light is 

 heliotropism, a turning influenced by the sun or sunlight. 



264. Diaheliotropism. Horizontal leaves and shoots are 

 diaheliotropic as well as diageotropic. The general direction 



Fig. ii 

 Radish seedlings grown 

 dark, long, slender, not 



