EXTERNAL CHARACTERISTICS 



209 



right at the axils; such leaves are called sessile. (See 

 Figure 72.) Sessile leaves often embrace the stems; that 

 is, their blades extend some- 

 what below the point of at- 

 tachment ; such leaves are 

 said to be clasping. This de- 

 velopment of the bases of the 

 leaf blades along the sides of 

 the stem sometimes completely 

 covers the stem. The leaves 

 are then said to be sheathing. 

 Corn is a plant whose leaf 

 bases completely sheathe the 

 stem. 



The attachment of leaves 

 and the length of their petioles 

 often have a good deal to do 

 with the relation of the leaves 

 to light. Leaves are attached 

 to stems singly, or in pairs, or 

 in clusters, but, whatever the 

 method, one principle appears 

 to be observed by all. That 

 principle is the avoidance of 

 getting in each other's light. 

 A leaf which is shaded by 

 another leaf has its photo- 

 synthesis much reduced, and, 

 if the light has to pass through 

 two leaves first, it has been 

 noted that no photosynthesis T 



PIG. 72. Phlox, showing opposite 

 OCCUrS at all. and sessile leaves. 



