the soil differs in composition from the air above the surface. As a rule, the soil air contains less oxygen and more carbonic acid (CO2), ammonia, and vapor of water. The increased amount of carbonic acid and ammonia have their origin in the organic matter or humus. A soil is not in the best condition for the production of crops unless there is within its depths a free circulation of air. This is true because oxygen in the soil is as essential for the life of the plant as it is for the animal. . . .

"When the soil is full of water to within a few inches of the surface, there can be no circulation of air among its particles. Adequate ventilation can be provided for such a soil only by drainage. Drainage ventilates the soil by lowering the ground water three or four feet, and thus makes it possible for the roots of plants to penetrate soil more deeply. In time these roots die and decay and afford passageways throughout the soil for the ready movement of the air."1

124. Relation of soil to heat.- The influence of the temperature of the soil on crop production is a factor of considerable importance. The life processes of a plant are practically suspended below a certain minimum temperature, which is about 40 degrees Fahrenheit for most, cultivated crops. Above this temperature all the vital activities, as germination and growth, increase until the optimum is reached. Above this point these life processes decrease in activity until the point is reached when they cease. The soil is a great factory that has its production vastly increased as the temperature rises. . . . The minimum temperature at which corn germinates and also the minimum for its growth is 48° or 49° F. Its optimum is about 93° F. . . .

"The sources of the heat of the soil are the internal heat of the earth, the sun, and decaying vegetable matter. It is difficult to estimate to just what extent the internal heat of the earth, which itself is very great, affects the temperature near the surface of the earth. However, the amount of heat from this source is insignificant, is a constant factor, and is entirely beyond the control of man. 1 Bailey's "Cyclopedia of Agriculture," Vol. IX, "Farms," p. 357.

Decaying organic matter furnishes some heat to the soil. For example, manure heats the soil to a limited extent when it is spread on the surface and plowed in. . . . The sun is by far the most important source of heat for the soil. When its rays are nearly vertical there is tropical heat; when its rays are withheld, the land

is locked in snow and ice. The heat received at the surface passes downward by conduction." 1

125. Cultivation of the soil.

A moment's thought will convince us that since all the food of man is ultimately derived from plants, any measures that tend to improve crops and reduce the cost of crop production are of vital interest to all of us. In the past, before much was known in regard to scientific principles, farmers put their seeds in the ground, cultivated them relatively little, and trusted Nature to do the rest. In recent times, however, man has learned a great deal in regard to soils, crops, and methods of cultivation, so that the modern farmer is often able to double the yield of a given area. The investigations of the National and State Departments of Agriculture have done much to make farming a science, and the future will doubtless see far greater improvements.

FIG. 50.- A plow.

For the cultivation of plants the first requisite is a suitable preparation of the soil. This involves, in the first place, plowing, which turns under any weeds or other plants that may have grown there before and which prepares for the work of the harrow, an implement which pulverizes the soil so that

1 Bailey's" Cyclopedia of American Agriculture," Vol. I, "Farms."

pp. 355, 356.

ready penetration of the roots of the growing plant is possible. In small garden plots this work is done by the use of spades, hoes, and rakes. It is often found necessary to add wellrotted manures to increase the humus of the soil and chemically prepared fertilizers, which furnish available mineral

FIG. 51. A harrow.

food for the crops. We have already called attention to the necessity of proper drainage of the soil before crops are planted (122). Scientific investigation has demonstrated, too, that frequent and thorough

stirring of the soil is most important not only to prevent the growth of weeds, but also, and this is even more essential, to conserve the soil moisture, and insure proper aëration of the roots. It has been found that it is possible to produce large crops on semiarid land if the top-surface of the ground is kept in a thoroughly pulverized condition. This is the so-called method of "dry farming."

IV. THE STRUGGLE FOR EXISTENCE AND ITS EFFECTS

126. Variation among plants. We have all heard the common expression "as nearly alike as two peas." In reality, however, if our powers of observation were sharp enough, we should probably find that no two peas are exactly alike in shape, color, size, and weight. The plants grown side by side from any two peas would also vary in height, in number and position of leaves, and in the number and vigor of flowers and seeds. In other words, as every human being has certain distinguishing characteristics, so, too, we should bear in mind that every individual plant, however small, shows certain differences or variations from every other individual of its class.

- A second fact

127. The numbers of seeds produced by plants. which is evident to all is that plants produce an enormous number of seeds. Suppose we consider the case of a vigorous pea-vine. In the course of a season it should produce at least 20 pods, each containing at least 5 seeds. Hence, at the end of a single season, one pea seed would, if conditions were favorable, have multiplied itself 100 times. If each one of these seeds were to be planted where it

had plenty of moisture, light, food, air, and favorable temperature, it likewise should give rise to 100 seeds, and so at the end of the second season we ought to have 100 x 100, or 10,000 pea seeds, all propagated from a single pea seed. Simple multiplication shows us that at the end of five years a moderately prolific plant like the garden pea would have given rise, had all conditions been favorable to 10,000,000,000 new seeds. Bergen has made a patient count of the number of seeds produced by an average morning glory plant, and finds it to be rather more than 3000; hence, at the end of the fifth year, if such a rate of reproduction were

to be continued, there would be 243,000,000,000,000,000 morning glory seeds.1

It is evident, however, that no pea vine or morning glory plant, if left to itself, would be able to produce anything like the number of seeds we have named, for otherwise at the end of a short term of years there would not be room on the whole surface of the globe for any other kinds of plants than these. As a matter of fact, the number of individuals of a given kind of organism does not vary much from year to year. In the first place, many seeds are eaten by birds and other animals. Again, many other seeds are not carried to a place where they find all the conditions that are essential for germination (118). Still other seeds, even if planted in good soil and in favorable surroundings, fail to germinate. Because of the great losses of seeds in one or the other of these three ways, we can get some idea of the reason why plants must produce a great abundance of seeds if their kind is to be perpetuated.

But even if

128. The struggle for existence among plants. seeds finally germinate and get a foothold on the soil, a great many

of the plants thus started will never reach maturity and ripen their seeds. In the first place, each plant is struggling to lift up its leaves

1 See Bergen's "Essentials of Botany" (1910), p. 202.