Before July is gone the amount of water taken up by the roots has perceptibly diminished. The food supply is proportionately lessened.
The whole leaf system must be re-adjusted to the cutting off o1 supplies. The leaves cautiously begin to close the (lours through which water was wont freely to escape. As the sap-flow from the roots grows gradually weaker, the making of starch dwindles. Cooler weather warns the tree that the tender shoots need thickened bark. and that the buds must be sealed up warm and tight. To save the leaves is out of the question, for their walls are thin. So the tree makes preparations to abandon them. It is quite worth our while to pick up a leaf now and then as it flutters to our feet during the autumn. Each one tells a most interesting bit of personal history, to any one who will carefully examine and question it. NO two are alike. but all tell the same story of the withdrawal of the leaf pulp" into the twig—a story of the thriftiness of the tree. The monotony of green gives place to patches of vivid, contrasting colors, or to dead russets. The last traces of leaf green are likely to be seen along the veins, Ivhich are the channels that drained the leaf dry of its soft living matter.
We can well understand the browns of dead leaves. They are dead colors. But why should other leaves die like the painting our autumn landscapes with the changeful splendor of sunset skies? Once we said, is the frost." But now we know better. The dying leaf still holds sonic patches of leaf green. The waxy granules gradually change to a yellow liquid which shows through the transparent leaf walls as plainly as when its elements were still green. During the summer the leaves accumulated a considerable burden of mineral sub stances that came np to them in the crude sap, and, being in the way. it was lodged in leaf cells, to their great disadvantage. As the leaf suffers the withdrawal of its living substance, these useless mineral deposits chemically decompose. The gradual breaking• down of all the residual substances in the leaf is the true cause of the brilliant and wonderful variations of color we see in the foliage of our woods in autumn. Because these changes occur at the season when warm days and frosty nights are common, we have erroneously put the two phenom ena together as cause and effect.
As the leaf -ripens." a layer of healing tissue forms between leaf and twig. and when they part, we have no reason to think that the separation is cause fur regret on either side. Now the tree is to sleep. As the cold increases:, much of the waiter which is within the cells of the living layer. filters through the cell walls and forms into ice crystals in the spaces outside. There is room here for the expansion due to freezing. and no danger of rupturing the delicate cell walls. The cold may for a :season e severe enough to stiffen the mucilaginous substance still left in the cells. Then the tree is in a death-like trance. But with the milder weather, the protoplasm thaws, and life stirs once more. With this explanation, one call understand how it is that trees freeze solid in winter without injury. There is an important difference between freezing and freezing to death.
Look out at the trees in these warm, showery days of early April! The frost is out of the ground, and every little root is happy. The buds are shining and swelling and bursting with secrets they are soon to reveal. The twigs are green with the rising tide of sap. The very bark, rough and dead, seems feel in its barrenness some touch of Out-of-the-way cells give up stores of starchy, sugary substance that they have been saving all winter against this day. There is food enough and to spare fur every hungry cell.
Yesterday the great buds of the poplars were sound asleep. They roused themselves and threw off their shiny scales. To-dav the little gray-green leaves are trembling on every twig. To-morrow the tree will he in full leaf, hold and self-stnlicient, as if it had never been hare and shivering.
The botanist dissects and analyzes and experiments. So does the chemist and the physicist. Nature has told them how some of her wonders are performed. But outside the laboratory, in the April sun shine, the sum of human knowledge seems very small. The miracle of the creation is repeating itself on every hand.
The unfathomable mystery of the coining of spring!