FOLIAGE LEAVES: THE LIGHT-EELATION 
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- 
