The Profound Effect on the Structure of Plants 53 



consider more fully under light-adjustment, or phototropism. 

 Finally, some leaves exhibit, just where the petiole joins the stem, 

 a pair of little leaf-like bodies called stipules, whose most remark- 

 able feature is the diversity of their somewhat insignificant 

 functions and forms. All of the parts of a typical leaf, blade, 

 petiole and stipules, are well shown and in typical form, in the 

 accompanying picture (figure 10). 



FIG. 9. A fragment of the vein system of a leaf, highly magnified, showing the typical 

 mode of ultimate branching and ending of the veinlets. (From Sachs' Lectures, 

 reduced.) 



FIG. 10. A typical leaf, the Quince. (From Gray's Text-books). 



The most striking of the features of leaves is perhaps the re- 

 markable variety of their shapes, which seem in their multiform- 

 ity to defy explanation or classification. Yet in reality the matter 

 is simple, for there exist only three primary forms of which all 

 the others are modifications and combinations, as the following 

 analysis will show. 



First, the ideal condition for the best working of a leaf is ob- 

 viously that in which it can have full exposure to all the light that 



