There is a lower depth of parasitism than this, in which the plant steals digested food from its victim. When this stage of degradation is reached the foliage of the parasite dwindles, and its green color disappears. We have seven or eight native plants which suck their food, already prepared, from the roots of herbs and trees. They are representatives of three widely-differing botanical families, but similarity of practice has brought about among them a certain similarity of aspect, so that we may almost say that there is a rogue type, even in the vegetable world. All these plants are destitute of green coloring-matter, and are of creamy hues, tinged with purple, straw-color, or golden-brown, and the leaves of all are mere reminiscent scales. It is believed that the Indian-pipe and its next of kin, popularly known as pine-sap (Fig. 18), begin life as parasites on living roots; but, as they mature, their habits undergo still further deterioration, till the full-grown plants suck their nourishment from the decaying leaves which carpet the forest. The Indian-pipe is entirely white all over, and though it is own cousin to the bonny heather, its substance looks like that of the fungi, which stand far below it in the scale of nature, and yet share its tastes and bear it company.
Fig. 18. - Pine-sap (Mono-tropa Hypopitys).
(From Natural History of New York).
But green plants make their own food. The chlorophyll which they contain is a lure to catch the sunbeams, which, when caught, are set to work to help the protoplasm in the work of food-and tissue-building.
This work can prosper only under certain conditions. Sunshine must fall upon the plant, carbon dioxide gas must be mingled with the air which surrounds it, the temperature must not be too low, and water must come up from the roots into the leaves and green stems. Under these circumstances food-making goes merrily on.
The first evident product of the plant's industry is starch. This is a much less complex substance than the proteids, for it contains but three elements, carbon, hydrogen, and oxygen, and they are mingled in accurately known and unvarying proportions.
The carbon comes out of the carbon dioxide which the leaves breath in; the hydrogen is a chemical constituent of the water which the roots suck up, and the oxygen comes in as the other element of the water, or is inhaled from the atmosphere by the green stems and the foliage.
Some surplus oxygen is left after the starch-making, and this is exhaled by the leaves. When the sun shines brightly upon a pool, in which pond-weeds are growing lustily, we can see oxygen rising in bubbles from the submerged leaf-laboratories to the surface of the water.
Oxygen-making is carried on even more actively by the smallest of the fresh-water algae. These little plants are fine green filaments, which have no roots and float in tangled bunches near the surface of still and sunny water. When the weather is warm and bright, the oxygen given out by these little algae forms great bubbles, which become entangled in the cobwebby meshes and float the plants to the surface.
Here algae and bubbles together form a green scum, which froths as we look at it. One might readily suppose that all manner of impurities were festering in the water, and that evil gases and malaria were being distributed throughout the neighborhood. But, as we learned in our copybook days, "appearances are deceitful".
The frothing is caused by one of the most active processes of constructive life. The bubbles which are being cast up are life-giving oxygen, which enables the grass to grow and the animal's heart to beat.
And the little algae, so busy and beneficent, are seen under the microscope to be beautiful also (Fig. 18).
The newly-made starch in leaves appears in tiny grains inside the chlorophyll bodies, or close beside them. It does not remain there and grow into larger starch-grains, but with the withdrawal of sunlight it seems to melt away and disappear. The starch has been dissolved, or rather changed, into fluid glucose, and this is gradually drawn through cell-wall after cell wall till it reaches some actively-growing part of the plant, where it is used at once, or some permanent tissue, where it is turned into starch again, and stored away to meet the needs of the future.
In spring all the starch which the leaves can make is changed to glucose and used immediately for growth. But in latter summer the plant puts it away. In some cases the starch is saved in wood, pith, bark, or tubers to feed next spring's shoots; in others it is packed into seeds, where it supports the plant's children in their infancy.
If a tree is hewn down in winter the cells of its wood are found to contain innumerable starch-grains. When nature takes her course these are converted into glucose during the first warm days of spring, and the pushing buds are fed with it.
But even when man has interfered with this programme the starch-grains are not without their use. They close the pores of the wood, making it almost impenetrable, and hence peculiarly adapted to certain economic uses. "Winter-hewn timber is almost exclusively employed for staves," says the Scientific American. "With staves made from summer-wood the contents of the barrel are subject to evaporation through the pores".
The stored-up starch-grains in tubers and seeds have very characteristic forms.
Those which we find in the tubers of the Indian-shot look like clam-shells, and those of the potato are uneven ovals. Those which we find in grains of corn are very small and angular, like particles of sand, and those of barley, wheat, and rye are lens-shaped (Fig. 19). When these starchy roots and seeds begin to grow the starch will be changed into fluid glucose and then drawn from cell to cell till it reaches the pushing tips of stems and roots.
Fig. 19. - Starch-grains of the potato (a) and of wheat (b).
(Much magnified.) (From the Vegetable World).