raw materials for the synthesis of organic compounds, with the aid of solar energy, or that of previously produced synergic foods. In all later chapters, the term "food" will be used to mean the organic compounds which serve as the synergic food for the green parts of green plants and as the sole supply of nutrient material for the colorless parts of green plants and for parasitic or saprophytic forms (see page 16).

PLANT FOOD ELEMENTS

The raw materials from which the food and tissue-building compounds of plants are synthetized include carbon dioxide, oxygen, water, nitrogen, phosphorus, sulfur, potassium, calcium, magnesium, and iron. The two gases first mentioned are derived directly from the air, through the respiratory organs of the plant. Water is taken into the plant chiefly from the soil, through its fibrous roots. All the other elements in the list are taken from the soil, nitrogen being derived from decaying organic matter (the original source of the nitrogen is, however, the atmosphere, from which the initial supply of nitrogen is obtained by direct assimilation by certain bacteria and perhaps other low forms of plant life), and the remaining ones from the mineral compounds of the soil.

Carbon dioxide and oxygen, being derived from the air, are always available to the leaves and stems of growing plants in unlimited supply; but the supply available to a seed when germinating in the soil, or to the roots of a growing farm crop, may sometimes become inadequate, especially in soils of a very compact texture, or "water-logged" soils. In such cases, the deficiency of these gaseous food elements may become a limiting factor in plant growth.

Water is often a limiting factor in plant growth. Experiments which have been repeated many times and under widely varying conditions show that when water is supplied to a plant in varying amounts, by increasing the percentage of water in the soil in which the plant is growing by regular increments up to the saturation point, the growth of the plant, or yield of the crop, increases up to a certain point and then falls off because the excess of water reduces the supply of air which is available to the plant roots. Hence, abundance of water is, in general, a most essential factor in plant growth.

Under normal conditions of air and moisture supply, however, the plant food elements which may be considered to be the limiting factors in the nutrition and growth of plants are the chemical elements mentioned in the list above.

AVAILABLE AND UNAVAILABLE FORMS

The plant food materials which are taken from the soil by a growing plant must enter it by osmosis through the semi-permeable membranes which constitute the epidermis of the root-hairs, and circulate through the plant either carried in solution in the sap or by osmosis from cell to cell. Hence, they must be in water-soluble form before they can be utilized by plants. Obviously, therefore, only those compounds of these elements in the soil which are soluble in the soil water are available as plant food. The greater proportion of the soil elements are present there in the form of compounds which are so slightly soluble in water as to be unavailable to plants. The processes by which these practically insoluble compounds become gradually changed into soluble forms are chiefly the "weathering" action of air and water (particularly if the latter contains carbonic acid) and the action of the organic acids resulting from decaying animal or vegetable matter or secreted by living plants.

THE VALUE OF THE SOIL ELEMENTS AS PLANT FOOD

Analyses of the tissues of plants show that they contain all of the elements that are to be found in the soil on which they grew. Any of these elements which are present in the soil in soluble form are carried into the plants with the soil water in which they are dissolved, whether they are needed by the plant for its nutrition or not. But in the case of those elements which are not taken out of the sap to be used by the plant cells in their activities, the total amount taken from the soil is much less than is that of the elements which are used in the synthetic processes of the plant. Hence, much larger proportions of some elements than of others are taken from the soil by plants. The proportions of the different elements which are used by plants as raw materials for the manufacture of the products needed for their growth varies with the different species; but a certain amount of each of the so-called "essential elements" (see below) is necessary to every plant, because each such element has a definite rôle which it performs in the plant's growth. A plant cannot grow to maturity unless a sufficient supply of each essential element comes to it from the soil.

From the standpoint of their relative value as raw materials for plant food, the elements which are present in the soil may be divided into three classes; namely, the non-essential, the essential and abundant, and the critical elements.

The first class includes silicon, aluminium, sodium, manganese, and certain other rarer elements which sometimes are found in soils of some special type, or unusual origin. These elements seem to have no rôle to play in the nutrition of plants; although silicon is always present in plant ash and sodium salts are found in small quantities in all parts of practically all plants. Nearly all species of plants can be grown to full maturity in the entire absence of these elements from their culture medium. Occasional exceptions to this statement in the case of special types of plants are known, and are of interest in special studies of plant adaptations, but need not be considered here.

The second group includes iron, calcium, magnesium, and, generally, sulfur. All of these elements are essential for plant growth, but are usually present in the soil in ample quantities to insure a sufficient supply in available form for all plant needs. Recent investigations have shown, however, that there are many soils in which sulfur is present in such limited quantities that many agricultural crops, when grown on these soils, respond favorably to the application of sulfur-containing fertilizers. In such cases, sulfur is a "critical" element.

The "critical" elements are those which are essential to the growth of all plants and which are present in most soils in relatively small proportions and any one may, therefore, be the limiting factor in plant growth so far as plant food is concerned. These are nitrogen, phosphorus, potassium, and (possibly) sulfur.

RÔLE OF PLANT FOOD ELEMENTS IN PLANT GROWTH

The use which a plant makes of the elements which come to it from the soil has been studied with great persistency and care by many plant physiologists and chemists. Many of the reactions which take place in a plant cell are extremely complicated, and the relation of the different chemical elements to these is not easily ascertained. It is probable that the same element may play a somewhat different rôle in different species of plants, in different organs of the same plant, or at different stages of the plant's development. But the usual and most important offices of each element are now fairly well understood, and are briefly summarized in the following paragraphs. It should be understood that a thorough and detailed discussion of these matters, such as would be included in an advanced study of plant nutrition, would reveal other functions than those which are presented here and would require a more careful and more exact method of statement than