160 ROOTS 
creases. They retard absorption and may become so great as to 
actually prevent it. The wilting of plants when the soil becomes 
dry is not due to the fact that there is no water in the soil, but to 
the tact that the roots can not pull the water, known as the un- 
available water, away from the soil particles. It has been found 
Fic. 141. — Lengthwise section through a root, showing the way the water 
and mineral substances of the soil reach the vascular bundles. e, epidermis 
with root hairs; c, cortex; a, endodermis; p, pericycle; x, xylem of vascular 
bundle. The arrows indicate the way the water and dissolved substances 
pass to the vascular bundles. After MacDougal. 
that most plants wilt when the soil moisture is reduced to 4.6 per 
cent in medium fertile garden soil, 7.8 per cent in sandy loam, and 
49.7 per cent in peaty soil. From these figures it is seen that the 
amount of unavailable water depends much upon the kind of soil. 
As shown in the table below, it also depends much upon the kind 
of plant, for plants differ widely in their ability to pull water 
away from the soil particles. 
UNAVAILABLE WATER FOR DIFFERENT PLANTS IN LOAM SOIL 
Plant. Unavailable water. 
Per cent. 
Sag Cand in oa Se tue Aes RReUaN Asa 5.8 
Wank Doel yh tise svt 8 i as Uri is soe cad caval sits ie lel 5.9 
sc pales vcs aaa eer ase bees ceo 6.2 
peeinewnteeg ny tau equal Ap aleroats 7.0 
WeMeyiows eabieys MES seeea aoe ese 8.5 
inne haa tao dp eMail s elias ie oa. aes eal 10.8 
Pondweed (a water plant)............... , 24 8 
