In the jelly which fills the leaf-cells there are floating specks of green, so vivid in color, and so numerous, as to give their hue to the whole leaf. These specks are chlorophyll bodies, and they are the cause of the rich and tender green in summer fields and woodlands.
The closely-packed tissue of the upper side of the leaf contains many cells, and hence many chlorophyll bodies.
But Nature has not economized space in the arrangement of the lower leaf-cells, - and where cells are comparatively scarce chlorophyll is scarce also. Hence, the under surfaces of leaves are often pale in hue.
Chlorophyll is formed only under the direct light of the sun. Tender young leaves, which have been shut up under bud-scales in the dark, contain as yet but few of the useful little green grains. The sun has not yet given them their working outfit, so expanding foliage is seldom really green. The budding oaks are of a warm reddish-brown or gosling-gray, according to their species. The new leaves of the poplars are silvery, and those of the willows are almost golden.
Even the vegetable garden, when things are beginning to wake there, is a symphony of delicate color. The very smallest carrot-leaves are yellow or golden brown.
The asparagus, when it makes its debut, is of a bluish or purplish color, and the sprouting beets are of a rich Tyrian red, too sumptuous for such plebeians.
But as soon as the leaves come out into the sunlight, chlorophyll begins to form in them, and they grow greener and greener.
In some of the smaller fresh-water algae the chlorophyll bodies are flattened plates of very distinctive and beautiful forms (Fig. 17). But those which color the leaves of the wood are generally disk-shaped or oval, and are often called "grains" of chlorophyll.
Whatever its shape, the chlorophyll body consists of two substances, the green coloring-matter itself, and a small, dense, jelly-like mass which holds it.
If a leaf is put into alcohol the spirit draws the coloring-matter out of the chlorophyll bodies, and the leaf gradually becomes pallid while the liquor in which it flows shows a deepening tinge of green.
Now if we examine a piece of the leaf tissue with a powerful microscope we shall see that the chlorophyll bodies are still there, and are unchanged in form and size, but the green pigment which tinted them is gone.
In autumn, Nature withdraws the coloring-matter from the chlorophyll bodies, and at last the bodies themselves disintegrate, so that by time the leaf falls nothing is left of them but a few yellow granules.
Fig. 17. - Some common fresh-water algae. (Much magnified).
a, Zygnema, with two star-shaped chlorophyll bodies in each cell; 6, Mougeotia, with a single oblong chlorophyll body in each cell; c, Spirogyra, with chlorophyll bodies in the form of spiral bands.
When a plant which has been growing in the light is subjected to continued darkness, the green pigment fades out of the chlorophyll bodies so that they become pallid, and finally white. Eventually the little disks or ovals themselves disorganize and disappear, and then the hapless plant begins to starve to death.
For the office of chlorophyll in the vegetable economy is digestion. By its action lifeless gases and lifeless mineral-matter are changed into living vegetable tissue.
Green is essentially the color of life. Wherever we see it in the living world we know that inorganic substances are being changed into organic substances, and thus life is preparing the material which it will mould into many forms.
The tissues built up by green plants feed herbivorous animals, which in their turn feed carnivorous animals, and so vegetable life mediates between the mineral and the animal worlds.
Organisms which have no chlorophyll are entirely dependent for their food upon organisms which have chlorophyll. The living chain binding the live creature to Mother Earth may have as many links as there are in the chain of phases connecting the "priest all shaven and shorn" with "the house that Jack built." But always at the end of the chain we find a green plant.
In the teeming life of the ocean the same rule holds, for marine flesh-eaters feed upon marine vegetarians, and these in their turn are fed by minute chlorophyll-bearing seaweeds, which live near the surface of all but the very coldest waters, and are the floating pastures of the sea.
In plants which habitually bear richly-colored leaves - in the copper-leaved beech, for instance, or the copper-hazel - chlorophyll bodies are present and busy, just as they are in those plants which bear green foliage; but the leaf-sap contains some strong pigment which overpowers and masks the green. Some of those minute plants which have a great and evil reputation under the name of bacteria contain a purple coloring-matter which seems able to fulfil the office of chlorophyll.
By aid of this pigment they can form organic matter when they are exposed to the light.
A few other bacteria can form organic matter in the dark, and unaided by any pigment, green or purple.
But such "exceptions being excepted," the vividness of the green in stem or leaf is in direct proportion to the plant's self-helpful activity, for in the vegetable world the very young and the very shiftless are not green.
But when a plant begins to form habits of parasitism the leaves grow dim, and the more confirmed these bad habits become, the less chlorophyll is to be found in stems and foliage. The mistletoe is still of a dingy or yellowish-green, because it has not yet sunk to the lowest depths of shiftlessness. It steals its food from the tree upon which it grows, but steals it in an undigested or half-digested state, and does its own digesting. The yellow-rattle and the pretty painted-cup practice a like sort of thieving. Their roots draw moisture from the roots of their next neighbors, instead of taking it direct from the soil. But the sap thus appropriated cannot be used in the building of vegetable tissue till it has been worked over in the leaves, and as yellow-rattle and painted-cup make use of their foliage, they have retained it.