ROOTS 79 
as osmosis. As water is absorbed from the films they 
become thinner, and this loss is supplied from neighboring 
films. In this way a flow from regions of the soil deeper 
and more distant than those to which 
the root reaches is set up toward the sper, 
films losing water. The water supply "aay Roe : 
may not be able to make good such <i o 
loss indefinitely; and if so, the films 
gradually become thinner, until a 
point is reached when the root-hair 
can obtain no more water, the film 
holding tenaciously to its particle of 
soil. After the roots have obtained 
all the water they can from the soil, 
and it seems perfectly dry, it still 
contains two to twelve per cent of 
water in the form of films. 
The water thus obtained by the 
root-hairs passes inward through 
the cortex and enters the wood we 
of the vascular cylinder, and fre. 76.—Root-hair of wheat, 
. which is shown to be an out- 
then is free to ascend to the lec agree ge ea 
wood of the stem, and so to the cell, in close contact with soil 
leaves particles. 
It should be understood that the water does not carry 
into the plant the soil substances dissolved in it; but each 
dissolved substance, although it must be in solution in 
order to enter the plant, is turned back or enters upon con- 
ditions that belong to itself alone. Certain dissolved 
substances may not be able to enter at all, and in con- 
sequence of this the root has been said to possess a selective 
power; while other substances may enter with greater or less 
rapidity at different times, or may even be turned back at 
certain times. All this diversity of behavior is dependent 
upon definite laws of physics. 
