The Boot and the Soil. 



77 



must have been produced at the rate of at least one 

 thousand feet per daj- during the latter part of the growth 

 period. 



112. 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 proportion- 

 ally more branched. In 

 wet seasons, a given plant 

 has less extensive root 

 development than in 

 drier seasons, because 

 the roots may then se- 

 cure 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. 



fl3. Root Tubercles. 

 Plants belonging to the 

 natural order Legumi- 

 nosce (le-gu-mi-no'-sse), 

 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 special interest, be- 

 cause the organisms producing them render nitrogen of 

 the air available as plant food. Plants have no power 

 to utilize directly the free nitrogen of the air (2{)0). 



Fig. 33. Younj^ clover plant showing 

 tubercles on roots (t). (From nature). 



