"Between the budding and the falling leaf, Stretch happy skies, With colors and sweet cries, Of mating birds in uplands and in glades. The world is rife." - T. B. Aldrich.
When spring, long waited for, has come indeed, and young leaves are unfolding in May sunshine, we find the ground beneath the branches strewed with half-transparent green or brownish scales. In city parks they litter the asphalt walks, and drift along their edges into little heaps.
They are bud-scales, whose day. of usefulness is over. They have braved all the rigors of storm and frost, while, folded safe within them, lay the foliage of the coming summer, destined to expand in tender colors under happy skies.
But the bud-scales seldom have any beauty, save the beauty of fitness.
They and the sleeping life which they enfold together constitute the winter bud. It contains very little water in its tissues, and so can withstand low temperatures without freezing.
The bud-scales live in a chill and sombre world, and when the sky is blue and full of light they fall and perish in the heart of spring.
Yet, they are themselves imperfectly-formed and partially-developed leaves. Under certain exceptional circumstances they have shown their possibilities, and developed into typical leaves. And under most circumstances there is in them the arrested power to become like the green foliage of summer.
Stunted, as they are, these scales have done work which perfect leaves could never do. Their horny substance has shed the cold rains of winter, resisted the frost, and protected the tips and shoots in which the life of the branches lav dor-mant.
We owe to the bud-scales most of the beauty of the summer world. Their highest usefulness has been attained through sacrifice of their complete development. Now their brief lives of service are ended, and as they fall the summer leaves unfold.
As soon as these new leaves have stretched and shaken themselves after their long winter sleep, they set to work, for, fair though they are, beauty is not their sole excuse for being, and there is plenty for them all to do in Nature's great garden.
Through every leaf there runs a network of delicate woody threads, curving, branching, and interlacing.
Its ramifications continue beyond the limits of unaided vision. We call it the "skeleton," and it does fulfil an office similar to that of the bones in the human frame, for it supports the leaf and gives it shape and strength.
But it also serves the leaf as veins and arteries serve the body, for the life-giving sap creeps through these woody threads in slow but continuous circulation.
With the aid of a powerful microscope we can see that the green pulp of the leaf looks somewhat like a honeycomb, as it consists of numberless cells laid row above row. Those on the upper side of the leaf are generally long and narrow, and stand upright, pressing together almost as closely as the bricks in the side of a house (Fig. 16).
But the lower leaf cells differ greatly one from another in shape and size, and they are laid together loosely, like the boulders in those gray walls which separate New England pastures.
In this lower leaf-tissue there are generally a number of irregular cavities or air-spaces.
Each separate leaf-cell is a little bag of delicate, transparent skin, filled with colorless jelly.
Fig. 16. - Magnified section of the green tissue of a leaf.
(From the Vegetable World).
This jelly is protoplasm, which Huxley has called "the physical basis of life." The living creature, animal or plant, is largely built of it. Science teaches that its chemical composition is closely akin to that of the white of an egg, and that its elements are oxygen, hydrogen, carbon, nitrogen, phosphorus, and sulphur.
But the proportions of these ingredients vary almost from moment to moment, and with them are mingled various accidental substances in varying proportions. For while the plant or animal lives new tissue is always being built up or old and waste tissue is being resolved into its elements and cast out of the body. This unceasing work is accompanied by unceasing changes in the protoplasm, which makes up the bulk of the living creature, and when death puts an end to these forms of activity decomposition sets in, and the protoplasm begins to change again. So the exact proportions in which six lifeless substances are blended in order to make the "basis of life" can never be accurately known, and the jelly which fills the cells of the summer leaves is one of the great mysteries of the physical world.
Because they are forever changing protoplasm and its chemical allies are called "proteids".
When protoplasm, existing alone, or mingled with other substances, is surrounded by a wall, we call the little bag and its contents a "cell." But the living jelly is the chief part of the combination. The wall which encloses it is of secondary importance, and is sometimes dispensed with altogether, for the Nature-student makes the acquaintance of cells, so called, which are merely little naked masses of protoplasm.
So "cell" is regarded as a sad misnomer for the minute particles of living substance which build up the animal or plant body, and some modern scientists are striving to get rid of it.
The name was chosen, in the first place, by a microscopist who looked through his lenses at a bit of cork, and found that it was made up of plates of thin tissue, meeting one another at right angles and enclosing empty chambers. He thought that the walls were the important part of the combination, as indeed they were in this particular case, so he called the tissue "cellular" and its component parts he named "cells".
Modern science teaches that in most cases the cell-wall is as subordinate to the cell-contents as a picture-frame is to the picture it encloses, and also that the living units which go to build up a plant or animal have a special form for each kind of tissue, so that "cells," far from being uniformly square, or uniformly six-sided, as their name might lead us to expect, assume shapes of almost infinite variety. But the old, misleading name is still in use, mainly because no one has yet been able to think of a satisfactory substitute for it.