128 Roots 



soil has taken up, accumulates, and may in time form a 

 spring, or even a sheet of water- In some places there 

 are known to be very extensive underground lakes, and 

 these must do much to keep the soil above them moist 

 in the absence of rain. The springs, or underground 

 streams, too, do the same, and in some cases they flow 

 such long distances that it seems not unlikely the 

 Kalahari Desert may have some such subterranean 

 supply of waier, which enables the plants to live 

 through the long terrible drought. 



But, it may be asked, what is the use to the plants 

 of water so far beneath as to be quite beyond the reach 

 of their roots ? The answer to which is that it does 

 not remain beyond their reach, but is brought up to 

 them. 



The water in the soil, visible or invisible, is like a 

 stream which is never at rest ; it is in constant motion, 

 always either rising or sinking according as the surface 

 of the soil is damp or dry. 



When the surface is wet from rain, the rain goes 

 on sinking down and down till it is either absorbed 

 by the soil through which it passes, or accumulates 

 at some greater or less depth below. But when the 

 sun shines out again, or a dry wind blows,' the 

 moisture of the surface is evaporated, the upper soil 

 grows dry again, and, the moment it is drier than the 

 soil below, the damp soil begins to give up its moisture 

 to this drier bed immediately above it. Thus the 

 stream of moisture at once begins to ascend, and will 

 continue to ascend until the surface is wetted again ; 

 it moves, in fact, either up or down, to the drier parts 

 of the soil, whichever these may be, the movement 

 .being exactly like that of the oil in the wick of a lamp ; 



