50 OUR WOODLAND TREES. 
surround them and in which they grow, for the 
reason that such temperature is communicated to 
them from surrounding objects by the natural 
process of conduction. 
We have already seen that the principal sources 
of the food of plants are air and water; and it is 
from these apparently simple substances that they 
obtain what they need to build up and consolidate 
their marvellous and beautiful structures. Water 
and air are words which convey very indefinite 
meanings except to the students of natural science. 
Water itself—by which we mean pure water, a 
substance which can only be obtained by distilla- 
tion—is a chemical compound of hydrogen and 
oxygen gas. But natural water, as the term is 
popularly understood—water such ag is found in 
the earth—is never pure, for the reason that it 
holds in solution—that is to say, in a liquid form, 
mixed but not chemically combined with the 
elemental particles of which pure water consists— 
certain chemical substances varying with varying 
circumstances both in quantity and quality. 
These substances often include calcium, iron, 
nitrogen, phosphorus, potash, soda, and sulphur ; 
