FOLIAGE LEAVES: THE LIGHT-RELATION. 13 



Fig. 6. These plants are growing near a window. It will be noticed that the stems 

 bend strongly towards the light, and that the leaves face the light. 



of the most obvious relations of foliage leaves to light, and 

 that the important part which heliotropism plays, not only 

 in connection with foliage leaves, but also in connection 

 with other plant organs, is one of the most important and 

 extensive subjects of plant physiology. 



RELATION OF LEAVES TO ONE ANOTHER. 



A. On erect stems. 



In view of what has been said, it would seem that the 

 position of foliage leaves on the stem, and their relation to 

 one another, must be determined to some extent by the 

 necessity of a favorable light-relation. It is apparent that 

 the conditions of the problem are not the same for an erect 

 as for a horizontal stem. 



17. Relation of breadth to number of vertical rows. — 

 Upon an erect stem it is observed that the leaves are usu- 



