IRRirABILITY. 



127 



Horizontal leaves and shoots can be shown to be subject to 

 the same influence, and are therefore diageolropic. 



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

 factor for plants during pholo.synthesis, 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. — WTiile we are growing seedlings, 

 the pots or boxes of some of them should be 

 placed so that the plants will have a one- 

 sided illumination. This can be done bv 

 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 ofie 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 hght. This inliuence exerted by the rays of hght is 

 Jieliotropism, a turning influenced by the sun or sunHght. 



264. Diaheliotropism. — Horizontal leaves and shoots are 

 diahdiotropic as well as diageolropic. The general direction 



Fig. IT 6. 



Radish seedlings grown in 

 tlie light, shorter, stouter, 

 and green in col<->r. Growth 

 retarded by light. 



