THE HOOT AND THE SOIL 



71 



sand feet a day during the latter part of the growth 

 period. 



111. Relation of roots to food supply. In the extent 

 of ground occupied, root growth is relatively less in moist 

 and fertile soils than in poorer and drier ones, but the 

 roots are proportionately more branched. In wet seasons, 

 a given plant has 



less extensive root 

 development 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 

 compact root sys- 

 tem than those 

 grown on poorer 

 soils. 



112. Root tuber- 

 cles. Plants be- 

 longing to the natural order Leguminosae, 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-organ- 

 isms, of the class known as bacteria, and are of special 

 interest, because the organisms producing them render 

 the nitrogen of the air available as plant food. Plants 

 have no power to utilize directly the free nitrogen of 

 the air (259). 



FIG. 34. Part of a growing potato plant. 

 The whole plant has been developed from the 

 dark-colored tuber in the center. 



