THE STEM AND THE BRANCHES 
21 
and down again to nourish each living part. The 
branches are part of the trunk and do the same work. 
If you look at the drawing you will see how a tree 
trunk is built, and you will understand how the tree 
grows. 
On the outside, as you know, is the rough bark. This 
is the tree’s coat, to keep it warm, and the tree’s armor to 
keep it from being bruised or punctured. Right next in¬ 
side the bark grows a thin layer of delicate wood called 
the cambium. When a boy takes a block of wood to 
carve out a whistle, he will find that the easiest way to 
strip off the bark is to run his jack knife down this cam¬ 
bium layer. The cambium is the vital part of the trunk, 
the only part which never sleeps nor withers from the 
birth of the tree to its death. All up and down this thin 
cambium layer is stored the winter long food for the 
young buds and roots of next Spring, because, as you 
will remember, the buds and roots must be fed until 
they grow strong enough to gather nourishment for them¬ 
selves. A storehouse of food means to us shelves of 
apples and potatoes and nuts and sugar, so that if we cut 
down a tree in winter we might think that next Spring’s 
buds and roots would have a very thin diet, because the 
starch and the sugar in the cambium storehouse, like all 
other stored plant food, is invisible to us. 
