A TEXT-BOOK OF BOTANY 



FIG. 41. A scarlet runner bean, showing leaf- 



ilternodes ' and 



Branches as well as leaves appear at the nodes; and there is 

 usually a very definite relation between them, the branch 



appearing in the up- 

 per angle between leaf 

 and stem, called the 

 axil of the leaf (Fig. 

 41). Most branches 

 are thus axillary in 

 position. The inter- 

 nodes give length to 

 the stem, separating 

 the nodes from each 

 other, and so display- 



ing the leaves more 



freely to the air and 

 the sunlight. 



23. Direction of stems. The directions in which stems 

 grow are due to a variety of causes, some of which will be 

 considered later; but for the present only certain positions 

 will be noted. 



(1) Erect stems. The upright stem is the most common; 

 and it seems altogether the best adapted for the proper 

 display of leaves, for they can be spread out on all sides 

 and carried upward toward the light. To maintain the 

 erect position is not a simple mechanical problem, and in 

 large woody stems it involves an extensive development 

 and arrangement of supporting tissues. That some special 

 organization is necessary to maintain the erect position in 

 the air is evident when aerial erect stems are contrasted 

 with submerged erect stems. In small lakes and slow- 

 moving streams submerged plants are commonly seen, as 

 the pickerel-weed and numerous others. In the water 

 the stems are erect; but when taken out they collapse, 

 having been sustained in position by the water. 



Among aerial seems the tree is the most impressive, and 



