Roots 



103 



kept in water for any length of time, and are, as a rule, 

 in better health when allowed to take up the moisture they 

 need from that which is contained, invisibly, in the pores 

 of the soil, when it does not look actually wet at all, and 

 no moisture could be squeezed out of it. 



But our main point now is to show that soil may, and 

 does, become damp without rain. It may be dried by sun 

 and wind by day, but it makes up for this by drawing 

 moisture from the air by night, and it is this, partly, 

 which enables plants at least to hve through a time of 

 drought, though their very stunted growth shows that the 

 supply has been insufficient for their needs. 



The moisture which the soil thus draws from the air 

 does not remain on the surface, but like the rain and dew, 

 sinks into the ground, penetrating deeper and deeper, and 

 moistening the soil until it is used up, or stopped by meet- 

 ing either with damp soil, or with soil or rock through 

 which it cannot pass. Rain falling upon a porous soil, 

 such as sand, or even soil containing much sand, passes 

 quickly through it until it meets with a bed of stiff clay or 

 rock, which prevents its going further; and then what is 

 left, over and above what the soil has taken up, accumu- 

 lates, and may in time form a spring, or even a sheet of 

 water. In some places there are known to be very exten- 

 sive 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 water, which enables the plants to 

 live through the long, terrible drought. 



But it may be asked, What is the use to the plants of 



