The Stem 



79 



ment than in drier seasons, because the roots may then 

 secure the needed food and water from a smaller area. 

 Nursery trees grown on fertile soils have a more com- 

 pact root system than those grown on poorer soils. 



112. Root Tubercles. Plants belonging to the nat- 

 ural order Leguminosae (le-gu-mi-no'-sie) , of which the 

 clover, pea and bean are familiar examples, when grown 

 in ordinary soil have swellings or tubercles on their 

 roots (Fig. 33). These 

 are caused by micro- 

 organisms, of the 

 class known as bec- 

 teria, and are of spe- 

 cial interest, because 

 the organisms pro- 

 ducing them render 

 the nitrogen of the 

 air available as plant 

 food. Plants have no 

 power to utilize di- 

 rectly th& free nitro- 

 gen of the air (259). 



Section VII. 

 Stem. 



The 



Fig. .34. Potato plant. U. St., under- 

 ground stems ; E, roots. The tubers are 

 the thickened distal* ends of the under- 

 ground stems. Much reduced. (After 

 Frank and Tschlrch.) 



113. As the root 

 develops from the 

 base of the hypocotyl, the plumule, or primary shoot 

 (55), develops from the other end and becomes, at least 

 for a time, the main axis or stem of the plant. 



* See foot note on page 80, 



