HOW SOME FAMILIAR THINGS LOOK 101 
which fit closely together. There are peculiar 
little windows in the walls between the cells. 
The regular wall is replaced by a fine skin, and, 
in order to protect this skin, a sort of saucer 
covers it on both sides. Each saucer has a hole 
in its center. These windows are called pits . 
They allow the water to pass from one tube, or 
cell, to another and so up through the trunk 
into the leaves. The cell walls are made of 
woody fibers. Sometimes we find them rein¬ 
forced by a spiral band, like a spring, which is 
coiled about the outside of the cell wall. The 
cell walls are the framework of the tree or plant. 
They support the tree and through them the 
water is transported to the leaves. These cells 
and cell walls are built up by the process which 
takes place in the leaves. The trunk or woody 
part of the plant is actually manufactured in 
the leaves from air and water, with the help 
of the sun. 
If we shave of? a piece of wood crossways, 
instead of up and down, and look at it that way, 
we see that the cells are often perfectly round. 
What does wood 
loo\ li\e? 
