GROWTH OF ROOT AND STEM 2$ 



through hard dry ground. One should improve the opportuni- 

 ties for observing all such phenomena when occasion offers. 



41 . Influence of light on the direction of growth of stems 

 and roots. Besides the stimulus of gravity there is the stimu- 

 lus of light which influences the direction of growth of stem and 

 root. Roots are mostly in the dark and therefore are rarely 

 influenced by light. When seedlings of certain plants are grown 

 in water cultures and have a one-sided illumination the roots 

 turn away from the light, but since geotropism is also acting on 

 them they are turned obliquely to one side. In the same way 

 when the stems have come above ground they are still under 

 the influence of the stimulus of gravity. But the stimulus of 

 light is usually more powerful and has more influence in deter- 

 mining*the direction of growth of the stem and its branches. Light 

 thus has a very important influence in determining the form of 

 the stem. The stimulus of light causes the stem to turn toward 

 the light. 



42. Heliotropism. The turning of plant parts under the 

 influence of light is called heliotropism, a turning toward the sun, 

 or light, or more properly speaking, a turning influenced by the 

 stimulus of light. Heliotropism then is the name -given to the 

 phenomenon in its broadest sense. Positive heliotropism or pro- 

 heliotropism is a turning toward the light; negative heliotropism 

 or apoheliotropism is a turning away from the light; and trans- 

 verse heliotropism or diaheliotropism is a lateral turning under 

 the influence of light. 



