The Stem. 75 
112. Relation of Roots to Food Supply. Root growth 
is relatively less, in the extent of ground occupied, in moist 
and fertile soils, than in poorer and drier soils, but the roots 
are proportionally more branched. In wet seasons, a given 
plant has less extensive root development than in drier sea- 
sons, because the roots may 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. 
113. Root Tubercles. 
Plants belonging to the 
pulse family (natural or- 
der Leguminose (le-gu- 
mi-no’-s)), of which the 
clover, pea and bean are 
familiar examples, when 
grown in ordinary soil, 
have swellings or tuber- 
cles on their roots (Fig. 
31). These are caused by 
parasitic fuogi known as 
bacteroids (bac’-te-roids) 
and are of special interest, 
because the bacteroids 
producing them render 
nitrogen of theair, which 
Fic. 31. Young clover plant showing tu- 
plants have no power to bercles (t) on roots. (From nature). 
directly appropriate, available as plant food (260). 
Section VIII. Tae Stem 
114. As the root develops from the base of the bypo- 
cotyl, the plumule, or primary shoot (56), develops from the 
