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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 suitablepreparation 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

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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

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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.

127. The numbers of seeds produced by plants. - A second fact 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

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FIG. 52. Variations in the corn ears produced in a single field. (Courtesy of Dr. E. M. East, Bussey Institution, Harvard University.)

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 × 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.

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

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FIG. 53. The struggle for existence and the survival of the fittest among turnips.

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.

to the light and air, and those that are most vigorous usually get above and shade the others. Again, the supply of water and mineral food in the soil of a given area is limited; hence, plants that cannot get what they need are dwarfed and finally starved to death. In

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the third place, injurious insects destroy an enormous amount of vegetation, the loss of cultivated crops alone from this cause being estimated at $700,000,000 annually. Frosts, dry seasons, heavy rains, and fungous diseases are other important factors in the life of many plants. And so if we were able to see what is actually going on in each square foot of the earth's surface, whether of forest,

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