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 



films losing water. The water supply 



may not be able to make good such 



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 

 of the vascular cylinder, and 

 then is free to ascend to the 



FIG. 



76. Root-hair of wheat, 

 which is shown to be an out- 

 growth from an epidermal 

 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. 



