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. 
Cg 
113. Root Tubercles. “ad Bas 
Plants belonging to the 5 al 
pulse family (natural or- 2 
der Leguminose (le-gu- 
mi-no’-sz)), of which the ae 
. | SF 
clover, pea and bean are \\ CE 
eG Re 
familiar examples, when a ay oe 
—— Em ae 
grown in ordinary soil, “@r@ eP iY = -=----b 
have swellings or tuber- At oe rt 
; Gry oe yt 
cles on their roots (Fig. sis 
31). These are caused by ez 
parasitic fungi known as rs ‘ 
bacteroids (bac’-te-roids) 
and are of special interest, te 
because the bacteroids oe 
producing them render of 
nitrogen of theair, which Fig. 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. Tue Stem 
114. As the root develops from the base of the hypo- 
cotyl, the plumule, or primary shoot (56), develops from the 
