114 THE LIFE OF A TREE. 
kinds of manures to promote the growth of plants. 
How do these act? It is possible that many who 
employ, and some who even manufacture them, do 
not clearly know. But it is hoped the reader of 
this little book could, at this stage of our pro- 
ceedings, give something like a correct answer. 
They promote the growth of plants because they 
supply to them these very substances about which 
we have just been speaking—the alkaline and 
other saline and mineral food of plants.* In or- 
dinary circumstances, plants obtain this part of 
their food from the earth; the rain, as it soaks 
into the mould, dissolving out various salts, 
which are then drunk in by the tender rootlets 
of the tree—and so form part of its food. 
The great chemist Baron Liebig, who first 
explained this very remarkable fact about plants, 
proposes a very ingenious and useful application 
of it. He says we have only got to burn a plant, 
and then get a chemist to analyse or find out the 
different substances contained in its ashes; and 
we thus have a sure guide as to what sort of 
manure we ought to give it, supposing we wish 
it to become very fruitful and supply us with a 
* Some manures, such as Guano, supply plants with ammonia also. 
