Spring or early summer is the best time to study the leaves of trees.
They are clean, and fresh, and new. Every tree is a great mound of green.
The broad-leaved trees seem to be thatched or shingled with overlapping
blades so that no sunlight can get into the darkened room, which is empty
except for the bare branches that support this outer dome of leaves. A
sugar maple, or a linden tree, shows best this outer thatch, which is so
thick that the sun is unable to look through. The bird flying overhead
sees only a solid mass of leaves. The one on its nest in a forked limb
looks up and sees the inside of this leafy tree cover. She is glad for
the twilight that surrounds her, and for the coolness of this shady
place; but more glad that her nest is hidden from sight of hawks that
sail overhead, while she keeps a close watch for sly, thieving red
squirrels that may come to steal her eggs, by climbing up the branches.
What are the leaves for? Why does the tree put out in spring young shoots
with rows of leaves along their sides? Why does the tree hold these
branches out as far as possible from the trunk, and bend the leaf stems
and the twigs so as to face the leaf blades towards the sun?
The reason is this: the life of the trees is in the green layer which we
see on the surface of all green shoots, and which we can discover under
the older bark of twigs, which has turned brown. Following the twig back
from its tip, all of the leafy part is green. Behind it the smooth twig
is no longer green, but a thumb nail easily strips off the layer of
brown, and reveals the green under bark. Go a little further back, and
gradually the outer bark thickens, and it is more difficult to get at the
soft under layer. After a while, we shall need a knife to reach it, for
old bark is hard and tough.
When the bark gets so thick that the sun cannot reach the green layer,
the colour fades out. The living part of the trunk of the tree is the
soft, juicy layer between the bark and wood. Through this portion of the
tree the sap rises from the roots, and finally reaches the leaves. This
sap needs to be changed before it can be useful to the tree as food.
The leaves are the places where these changes take place. Through little
doorways in the under sides of the leaf air passes in. With it goes
carbonic acid gas, an important food element. The soft green leaf pulp,
which is the green juice of a bruised leaf, has a wonderful work to do.
It cannot do this work unless the sun is shining upon it. On a bright day
every leaf is making starch, and sending it down through the twigs and
branches as food. This starch is contained in the sugary sap that flows
back constantly from the leaves to the farthest root tips. It is made in
the leaves out of the sap brought up from the roots and the carbonic acid
gas which the leaves absorb from the air.
As long as the leaves do their work, the tree is able to grow, and to
blossom, and to ripen its seeds. When the leaves have done their work the
summer has passed; the tree lets go the leaves, and rests without growing
all winter.
It is not easy to explain the work of the leaves, nor even to understand
the wonderful work accomplished there all through the summer. When we
eat, our food must go into the stomach to be changed by the processes
called digestion. It is hours before the digested food is poured into the
blood and carried to all parts of the body. The tree takes its food from
the air, and from the soil. Neither the dirty water that rises as sap to
the leaves, nor the gas which enters the leaf doorways from the air, is
useful as food to the growing tree until they have been combined and
changed. The leaves are, then, in a sense, the stomachs of the trees, for
in them the raw foods must be “digested” before they are ready to be
poured into the life blood that flows down through all the live parts of
the tree. Now they are fit to feed the growing cells, which are always
hungry.