The Stem. 



79 



the roots may then secure the needed food and water 

 from a smaller area. Nursery trees grown on fertile 

 soils have a more compact root system than those grown 

 on poorer soils. 



112. Root Tubercles. Plants belonging to the nat- 

 ural order Leguminosae (le-gu-mi-no'-see), 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, 

 probably of the class known as bacteria, and are of spe- 

 cial interest, because 

 the organisms pro- 

 ducing them render 

 nitrogen of the air 

 available as plant 

 food. Plants have no 

 power to utilize di- 

 r eetly the free nitro- 

 gen of the air (259). 

 SECTION VII. THE 



STEM. 



113. As the root 

 develops from the 

 base of the hypo- 

 cotyl, the plumule, 



FIG. 34. Potato plant. U. st., under- or primary shoot 

 ground stems; R, roots. The tubers are 



the thickened distal* ends of the under- (55), develops from 

 ground stems. Much reduced. (After 



Frank and Tschirch.) the other end and 



becomes, at least for a time, the main axis or stem of 

 the plant. 



* See foot note on page 80. 



