io8 NOT ALTOGETHER ABOUT PLANTS 



36. Causes of Continued Entrance. The water around 

 a root-hair is a busy place. Molecules of solutes are enter- 

 ing and other molecules are coming out. It is like a hurry- 

 ing crowd, and yet all are moving in an orderly manner, 

 in accordance with law. More go in than come out, but 

 the movement is in both directions. (See page 79.) Be- 

 sides these movements of the solutes, there is the movement 

 of the water itself. It is entering the plant. Day after 

 day it keeps on entering and it does not come back. What 

 becomes of it? 



There are three things which permit the entering move- 

 ment of water and of solutes to go on without ceasing. 

 One of these relates to the water. The others relate to 

 the solutes. You shall see that the movement of the 

 water into the plant and the movement of the solutes 

 into the plant are different things. 



That which permits water to keep entering at the roots 

 is the fact that water keeps going out at the leaves. It 

 does not go out as a liquid. It goes out as a gas. It 

 goes out by the process of evaporation and we do not see 

 it. It goes out through the stomates. More water re- 

 places that which evaporates. More comes in at the 

 roots. Thus, as long as water enters the roots and passes out 

 from the leaves, there is a stream which moves constantly 

 through the body of the plant. The evaporation of water 

 from plants is called transpiration; the ascending stream 

 is called the transpiration stream. Water sometimes 

 escapes from leaves in liquid form, but this is not usual. 



That which permits solutes to keep on entering is not so 

 simple as evaporation. The solutes do not evaporate. They 

 have nothing to do with the water which becomes gas. Their 

 movement is limited to the water which remains liquid. 



