30 GROWTH AND WORK OF PLANTS 



by such delicate plants as mushrooms is seen where they burst 

 through hard dry ground. One should improve the opportuni- 

 ties for observing all such phenomena when occasion offers. 



48. 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 the stimulus of gravity 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. In response to the stimulus of light the stem 

 turns toward the light. 



49. Heliotropism. The turning of plant parts in response to 

 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 is a 

 turning toward the light; negative heliotropism is a turning away 

 from the light; and transverse heliotropism is a lateral turning 

 under the influence of light. 



