HOW BIG TREES GROW 39 
Evergreen trees are far less exacting regarding soil 
and moisture requirements than most deciduous vari- 
eties.. As a rule conifers demand only one-sixth to 
one-tenth as much moisture and one-quarter to one- 
half as much fertility, so that pines, spruces, firs, etc., 
will grow well on soils that would afford poor accom- 
modations to oaks, maples, beech, ete. 
How a Tree Develops.—So much for the general fac- 
tors required for plants. Now in order that we may 
clearly understand how a forest develops and its rela- 
tion to its surroundings, the growth of the individual 
tree must be studied. 
A tree is composed of roots, stem, branches, and 
leaves, and for reproducing the species, to the above 
flower, fruit and seed must be added. 
Roots really serve two purposes, they anchor the 
tree firmly in position, defying the effects of wind 
and snow, while at the ends of the roots just back of 
the tip or growing point are found delicate root hairs, 
which draw in the moisture and plant food dissolved 
in the water which surrounds the soil particles. 
The stem and branches are the framework of the 
plant which support the leaves; the latter serve as 
the stomach and lungs at the same time. The soluble 
salts coming in through the delicate cell walls of the 
root hairs pass up through the sapwood to the leaves. 
There the water is combined with oxygen, and carbon 
dioxide found in the air is taken in through openings— 
“stomata”—found on the under side of the leaves. These 
two ingredients are combined in the presence of a 
green substance called chlorophyll, found in the leaf 
cells, to make starch and sugar. 
Any water that is not used by the leaves to manu- 
facture plant food passes out through the stomata as 
