68 



PLANT EELATIONS. 



48. Relation to light— As stems bearing foliage leaves 

 hold a special relation to light, it is necessary to speak of 

 the influence of light upon their direction, the response to 



Fig. 59. 



An elm in foliage, showing the breaking up of the trunk into branches and 

 the spreading top. 



which is known as heliotropism, already referred to under 

 foliage leaves. In the case of an erect stem the tendency 

 is to grow towards the source of light (see Figs. 1, 64). 



