THE STEM AND ITS FUNCTIONS 



91 



two leaves may be exactly half way around the stem from each 

 other, but it is much more common for their angle of divergence 

 to be less than 180° and for the points of attachment of a series of 

 successive leaves thus to form a loose spiral around the stem. 

 The closeness of this spiral and the position of the leaves thereon 

 show great diversity, but are generally constant within any par- 



FiG. 42.— A tree. 



The shagbark hickory (Carya ovata). 

 States Forest Service) . 



{Courtesy United 



ticular species. If two leaves arise from^the same node they are 

 always directly across the stem from each other and are said to 

 be opposite in arrangement (Fig. 43). When there are more 

 than two leaves at a node, they are disposed about the stem in a 

 circle or whorl. 



Surface. — The surface of a young stem is protected only by an 

 epidermis, but later this is replaced in wood}- plants bj^ a 

 characteristic layer of corky cells, the bark. The necessary 

 exchange of gases between the air and the living tissues of the 



