Roofs 137 



soil has dissolved for them, with the help of carbon- 

 dioxide and other gases. 



The water thus taken up — for what is dissolved by 

 the roots and what is dissolved by water and gas are 

 taken up together — the water thus taken up is a very 

 weak solution of various salts — phosphates, and others 

 — so weak that it may fairly be compared with ordinary 

 drinking-water. 



No water in nature is or can be perfectly pure, as 

 has been said, because it is constantly dissolving some- 

 thing wherever it goes. And though, even \\\\\\ what 

 the roots have dissolved, the solution is still so weak as 

 to pass for ordinary water, yet it must be borne in 

 mind that the roots are constantly sucking it in, and 

 that the leaves are as constantly returning the water 

 to the air — only the water, however. The salts remain 

 behind and accumulate day by day. 



The same sort of thing on a vast scale goes on with 

 the rivers and the ocean. River-water is generally 

 tasteless, though it, too, contains various salts dissolved 

 in it. This small proportion of salts is, however, being 

 constantly poured into the ocean, wliile the sun is 

 constantly taking away by evaporation almost pure 

 water. The salts, therefore,, accumulate, and sea- 

 water is salt and bitter in consequence. 



The salts left in a plant do not usually make it salt 

 or bitter, because the quantity is altogether extremely 

 minute in proportion to the plant's size ; and as they 

 are distributed through the whole of its substance, 

 there is a continual demand for them while the plant 

 is growing or putting forth fresh leaves. 



But if a plant is stunted by drought it may become 

 actually bitter A cabbage, for instance, which has 



