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sulphur dioxide, a liquid which boils at low temperature, took the place of water.

These various solar engine plants all seem to have had moderate success, but either from lack of financial support or from just failing to compete on equal terms with coal, they have dropped out of operation. But we are living on the resources of former ages. Our coal, gasoline and crude oil supplies, though immense, are not inexhaustible. The water power of rivers is not enough for present needs. Unless other power sources are invented, we must sometime fall back on the solar rays for the principal part of the world's power.

CHAPTER XIX

HOW THE SUN MAKES PLANTS GROW

NEARLY everybody has seen potatoes sprout as spring comes on. These white vinelike sprouts grow towards the dim light of some distant window or crack till sometimes they are a yard long. But all this growth merely shrivels the potato. It is no real gain of weight, and only robs the store of starchy food material collected the year before to nourish a new plant. All is changed if the potato is planted out of doors. The sprouts put out green leaves the moment they come to the light above ground, and instead of stretching out into long tender vines, they become sturdy potato plants.

It is light that makes the difference. Artificial light like the electric arc would do, but our sun furnishes a much greater supply of light without cost or trouble to anybody. Many people believe that plants of all kinds get their main nourishment from the ground, and especially from the fertilizers that are placed there. This is a mistake, although small quantities of indispensable chemicals are furnished by the soil. Water, to be sure, comes up from the ground in the sap, and water makes up from 70 to 90 per cent of

a green plant's weight. But it is a truer measure of the value of a plant to remove all of its water and note the dry weight that remains. Of this, strange as it may seem at first sight, nearly all comes from the air and not the ground.

This paradox is all the stranger when we consider what the air is, and what the plant mainly takes from it. About one-fifth of the air is oxygen, the indispensable gas without which a man cannot breathe. The remaining four-fifths of the air are almost entirely of nitrogen, a very inactive gas, difficult to unite with other chemicals. As far as man's breathing is concerned, air is oxygen diluted with enough nitrogen to make it safe to breathe. But for the plants, air is very different. They actually give up oxygen to the air as they grow, and most of them make no use of atmospheric nitrogen. Some few kinds of plants are, to be sure, in partnership with bacteria which invade their roots, and which work over the nitrogen of the air into forms which these plants can use.

The most valuable part of the air to all plants is carbonic acid gas, of which the atmosphere contains an average proportion of only 3 parts in 10,000. It is a compound made up of one atom of carbon with two of oxygen. But this gas is of no use to the plants in darkness and without moisture. With both light and water present and at a suitable temperature the plant breathes in the carbonic acid gas through millions of little microscopic mouths, generally most plentiful on the under side of its leaves. Inside the leaves there

is a chemical change which results in the setting free of part of the oxygen, contained in carbonic acid gas combined with carbon. The oxygen set free escapes from the leaves into the air, but the carbon combines with water and other chemicals found in the sap, and the plant builds these up into complex compounds by mysterious methods not at all like those of chemists. Thus are formed in nature many very complicated "organic" chemicals quite beyond the present skill of chemists to imitate. But in this natural laboratory the power which acts to do the chemical work is that of light. Chemists, on the other hand, usually employ heat and various forms of chemical energy in their compound making.

We have spoken of the large percentage of water in green plants. But by no means all of the water which ascends in the sap stays with the plant. Much of it escapes as a sort of plant perspiration. An ordinary tree may lose nearly a hundred pounds of water through its leaves in a single day, and in an acre of forest the evaporation from the trees in a single season could reach 1,500 tons of water. of this water the sun supplies, by maintaining that gigantic still whose reservoir is the ocean, and whose condenser is the atmosphere from which the water comes to earth in snow and rain. So this is the second great service of the sun to plants.

All

A third action of sun rays is no less needful for them. It is to warm the earth enough, but not too much, to make plant growth possible. Practically

none of the higher forms of plant life will grow at temperatures below freezing or above 120° Fahrenheit. Measured from the absolute zero, where heat ceases, these figures are about 491° and 610°, so that a change of the earth's mean temperature (which is about 26° Fahrenheit, or 517° of the absolute Fahrenheit scale) by 10 per cent either way would pretty nearly kill the plant world. In other words, if the sun's rays grew much hotter or much less hot, plant life and all animal life which feeds on it would be destroyed.

Let us turn back now to the action of light to enable plants to extract carbon from the carbonic acid of the air. Sun rays contain a mixture of all of the rainbow colors. Not only are there these colored rays, but others beyond the red and violet ends of the visible spectrum. It is only red rays and blue-violet rays that help the plant to take in carbon. Green rays are no better than darkness for this purpose.

We have seen how the potato sprouts grow long in dim light. Trees show the same tendency. If pine trees grow well separated, where the light falls all around them, they have great spreading tops on short trunks. But where they are in a forest, so closely crowded that they shade one another, the trunks shoot up very tall and slender as if trying to outstrip each other to reach the light above.

Also in the tropical primeval forests whose dense tops make a perpetual darkness on the ground beneath, the vines trail their stems without a leaf from the high

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