but is perforated by numerous small openings. These openings are called "stomata" (little mouths), and it is through these that the water is exhaled. This power of transpiration continues during the life of the plant, the water being obtained from the ground through the roots. Very large quantities of water are used in this way.

300 pounds of water Experiment to show that water is given off passes through the plant for each pound of dry matter produced, so that 1,000 pounds of corn use at least 30 tons of water

from the leaves of plants. The bottle on the left has been over the plant for some time and is cloudy from the moisture which has collected on the inside. The one on the right has just been placed over the plant and is transparent.

(Drawn from photograph)

during its growing period. As this quantity of corn can be raised on one-thirtieth of an acre, it follows that to mature an acre of corn the crop must be supplied with 900 tons of water, or an amount that would make a layer over the acre about 8 inches deep.

This again takes no account of the quantity of water lost from the land by percolation or drainage. It has been estimated that this amount is at least equal to that used by vegetation, so that one acre of corn probably requires a precipitation of at least 1,800 tons of

water. These statements show clearly the necessity of carefully conserving the moisture of the soil, a point that can not be too strongly emphasized.

King found in investigations made at Wisconsin that the amount of water used by the crop was from 300 to 500 times the weight of the dry matter. His results are summarized in the following table.

AVERAGE AMOUNT OF WATER USED TO PRODUCE ONE TON OF DRY MATTER

From this and other data he calculated the minimum amount of available water necessary to produce the various yields of the more common grain crops. These interesting figures are given below.

LEAST AMOUNT OF WATER PER ACRE REQUIRED TO PRODUCE DIFFERENT YIELDS OF GRAIN

Functions of Water.-Water is important to the plant in several different ways. It is first of all the most essential plant food, in the sense that it composes about 80 per cent of the mature crop. It also supplies the hydrogen and oxygen found in the dry matter, which amounts to 10 per cent more, making a total of 90 per cent of the weight of the plant which is derived directly from the water.

Water is necessary to dissolve the plant food in the ground, and enable it to enter the plant, as will be noted later. It is needed to give stiffness or rigidity to the more succulent parts of the plant. This fact is shown by the drooping or wilting of plants during the hot hours of the day when the water is not furnished by the roots with sufficient rapidity to replace the loss by evaporation from the leaves. It is probable that water performs an important function in controlling the temperature of the plant. The chemical processes in the plant cells produce heat, and the excess of heat is removed by transpiration of water through the leaves, just as it is removed from the human body by the transpiration (perspiration so-called) through the skin. Water is also necessary for the movement of food within the plant. The food materials absorbed by the roots, and that manufactured by the leaves can be transported to the different parts of the plant where they are needed only when in solution in water.

Of such consequence to vegetation is the water supply that some investigators claim that the question of fertility is wholly one of having present in the ground the proper amount of moisture, and that it is independent of the chemical composition of the soil, except as

this composition affects its power to furnish the plant with water. This view is undoubtedly extreme, and is not generally accepted. There is no doubt, however, that the proper condition of moisture is the most important single factor in determining the fertility of the land, and that more soils fail to produce good crops for lack of it than for any other cause. It seldom happens that the water supply is sufficient to grow the maximum crop of which the soil is capable, as is demonstrated by the fact that a large increase in yield can be obtained by irrigation even in sections of very heavy precipitation. While the amount of water that falls on the land can not be controlled, much can be done to save the water so as to tide over the periods of scanty rainfall; a fact that will be emphasized throughout this book. Too much stress can not be laid upon the importance to the plant of an adequate supply of water in the soil, and the knowledge that certain methods increase the amount of moisture available to the crop should be sufficient reason for their adoption. The reader is asked to carry this thought with him as he reads the following pages.

Part of the Oxygen from the Air.—A small quantity of the oxygen in the plant probably comes from the air. One-fifth of the volume of the air is oxygen, and the plant uses this to some extent. Plants breathe in much the same manner that animals do, for all cells must have a supply of oxygen in order to live. The oxygen of the air combines with the materials in the cells one of the results being the production of heat, just as the oxidation taking place in the animal body produces heat, That heat is evolved by the living

vegetable cell can easily be proved experimentally by confining the plant in such a way as to prevent radiation. The rapid heating of silage in the silo is doubtless due to the breathing process of the cell, the heat in this case being unable to escape.

Carbon in Plants Derived from the Air.-Nearly one-half of the dry matter in the plant consists of the element carbon, all of which is derived from the carbonic acid gas which is present in the atmosphere. Carbonic acid is the colorless gas which is formed when a piece of coal or charcoal is burned, and is a compound containing the two elements carbon and oxygen. Charcoal is a good example of nearly pure carbon. Carbonic acid gas is found everywhere in the atmosphere, although present in very small quantities, constituting only four one-hundredths of one per cent of the volume of the air, or about four parts in 10,000. It seems strange to think that so large a proportion of the solid material of the plant should be formed from this gas, but it is well known that green plants have the power to decompose this gas, retaining the carbon and setting free the oxygen. This process is known as the "fixation (sometimes assimilation) of carbon," and takes place chiefly in the leaves. The power to fix carbon is dependent in some way on the presence of the green coloring matter (chlorophyll), so that it is only those plants having green leaves that can use the carbonic acid. Such plants as mushrooms and other fungi, for instance, can not obtain their carbon in this manner but must procure it through the decomposition of organic matter, or in other words, must have their food previously prepared for them. Green