8 4 



PHYSIOLOGY. 



Horizontal leaves and shoots can be shown to be subject to 

 the same influence, and are therefore diageoiropic . 



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

 factor for plants during carbon conversion, it exerts great influ- 

 ence on plant growth and movement. 



199. Retarding influence of light on growth. — We have 

 only to return to the experiments performed in growing plants in 

 the dark to see 

 one of the influ- 

 ences which light 

 exerts on plants. 

 The plants grown 

 in the dark were 

 longer and more 

 slender than those 

 grown in the light. 

 Light then has a 

 retarding influ- 

 ence on the elong- 

 ation of the stem. 



200. Influence Fi s ?'- 



_ _ . , _ . Radish seedlings grown in the 



Of light On ttireC- dark, long, slender, not green. 



tion of growth. — While 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 Radish seedlings grown in 



the light, 'shorter, stouter, and 



be done by placing them near an open green in color. Growth re- 



. tarded hy light. 



window, m a room with a one-sided illu- 

 mination, 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. 



201. Diaheliotropism. — Horizontal leaves and shoots are 

 diahelio tropic as well as diageoiropic. The general direction 



