that three elements (i. e. hydrogen, oxygen and carbon) make up 981⁄2 per cent of the entire composition of the plant, the remaining elements constituting only 11⁄2 per cent. (NOTE. All of the elements mentioned above as occurring in the ash, with the exception of chlorine, are combined with oxygen. In the table the names under "ash" represent these combinations, i. e. potash is composed of potassium and oxygen; phosphoric acid is phosphorus and oxygen; lime is calcium and oxygen, etc.) CHAPTER II ATMOSPHERE AS A SOURCE OF PLANT FOOD Importance of Water to the Plant.-One of the most striking points brought out by the chemical analysis is the large proportion of water that enters into the composition of the plant. A reference to the table shows that nearly 800 of the 1,000 pounds of the matured corn plant consist of water in a form that can be driven off at a heat not above the boiling point. In the organic matter is found 12.7 pounds of hydrogen and 88.9 pounds of oxygen which practically all came originally from water, making a total of nearly 900 pounds derived from this source. These figures represent but a small part of the water actually required by the crop. Water is being continually given off into the air by the plant-leaves. This exhaled, or "transpired," water is in the form of a vapor and is invisible, but that it actually exists can be proved by a simple experiment. Invert a wide mouthed bottle or fruit jar over a small plant, and after a short time the inner surface of the bottle will be found to be covered with moisture. The earth around the plant should first be covered with a piece of oil cloth or oiled paper to make sure that the water does not come from the soil. If the underside of a leaf is examined with a magnifying glass or microscope it will be found that the surface is not entire, 12 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. Amount of Water Required by Crops. -European experi ments have shown that approximately 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 uses 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. 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 PRO- 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 |