70 Principles of Plant Culture. 
102. Root-Hairs Absorb Water with considerable 
Force. It is the absorptive power of the root-hairs that 
causes water (sap) to flow so freely from injured stems of 
grape vines* and some other plants in spring, and from 
wounds in the trunks of some trees in summer. This force 
is probably due to the absorptive power of the protoplasm 
in the very active young root cells. It is affected by the 
temperature of the soil, within certain limits, lessening as 
the temperature falls, and increasing as it rises. Sachs 
found that the foliage of plants of tobacco and pumpkin 
drooped when the temperature of the soil in which they 
were growing was reduced much below 55° F., showing that 
the roots, at that temperature, did not absorb sufficient 
water to compensate for the loss by transpiration (75). 
When the soil is warm, on the other hand, the absorptive 
power of roots may be sufficient to force water from the tips 
of leaves during cool nights when transpiration is slight (63). 
103. Only the Youngest Parts of Roots are Active 
in Absorption. The part from which the root-hairs have 
perished absorbs little water, but is chiefly useful in giving 
strength and in conductiog the plant fluids. 
The absorbing part of any given rootlet is, therefore, com- 
paratively short. It follows that the amount of nourish- 
ment a given plant can receive will depend upon the number 
of its root-tips. Our treatment of the plant should, there- 
fore, be aimed at promoting the formation of root-tips. In 
other words, we should encourage root branching.t How may 
we do this? 
* Hales found the absorbing force of the roots of a grape vine equal to the 
weight of a column of mercury thirty-two and one-half inches high. 
+ Root branches must not be confounded with root-hairs. In Fig. 26, branches 
of the roots appear at e.e.e. The branches bear root-hairs when of sufficient 
length, but root-hairs never develop into branches. 
