You know that in autumn nearly all the leaves fall from the trees. To be sure, a few trees (such as the pines and hemlocks) and some plants (such as the laurel and wintergreen and partridge vine) do hold fast their leaves all winter; but these are so few as compared with the many plants which lose their leaves, that they hardly count.
Perhaps you never stopped to wonder why most plants get rid of their leaves before winter comes on; but you feel pretty sure now that there is some good reason for a habit that is adopted by nearly all the plants that live in this part of the country.
When we were talking about the way in which leaves defend themselves from different dangers, we found that evergreen leaves, the leaves which hold fast to the tree and keep fresh all winter, manage to keep their water safe inside their cells by wearing a very thick skin, and by not having too many little leaf mouths. For when a leaf has a thin skin and a great many mouths, its water leaks away very quickly. And if many such leaves should remain upon a plant into the winter, might it not happen that they would let off all its water at a time when its roots could not find any more in the frozen ground? And thus might not the leaves kill the plant by draining it quite dry?
So you can see why it is well for most plants to shed their leaves before winter comes on and the root’s drinking water is turned into ice.
But when a plant is about to shed its leaves, it takes care not to waste the precious food which they hold. This food it draws back into its stem and roots, laying it away in safe places beneath the buds which are to burst another year.
It is this action on the part of the plant which changes the color of the leaves every fall. That material which makes them green is broken up, and part of it is taken away. That which is left is usually yellow or brown or reddish, and gives the leaves the beautiful colors we see in our October woods.
So whenever you see the woods changing color, losing their fresh green and turning red and yellow, you can be sure that the trees have begun to prepare for winter. You know that they are stowing away their food in warmer, safer places than can be supplied by the delicate leaves. And when all the food has been drawn out of the leaves, and packed away in the right spots, then the plant finishes a piece of work it began some time before. This piece of work is the building-up of a row of little cells just where the leafstalk joins the stem or branch. When this row is complete, it acts almost like a knife, loosening the stalk from the stem.
Then the leaf’s life work is over; and with the first breeze, the empty shell, which is all that is left, breaks away from the parent plant, and drifts earthward.
