Roots differ not only in their form and habit, but also in color of tissue, character of bark or rind, and other features. It is excellent practice to _try to identify different plants by means of their roots_. Let each pupil bring to school two plants with the roots very carefully dug up, as cotton, corn, potato, bean, wheat, rye, timothy, pumpkin, clover, sweet pea, raspberry, strawberry, or other common plants. =Root Systems of Weeds.=--Some weeds are pestiferous because they seed abundantly, and others because their underground parts run deep or far and are persistent. Make out the root systems in the six worst weeds in your locality. CHAPTER VIII THE ROOT.--FUNCTION AND STRUCTURE =The function of roots is twofold=,--to provide _support or anchorage_ for the plant, and to _collect and convey food_ materials. The first function is considered in Chapter VII; we may now give attention in more detail to the second. The feeding surface of the roots is _near their ends_. As the roots become old and hard, they serve only as _channels through which food passes_ and as _hold-fasts or supports_ for the plant. The root-hold of a plant is very strong. Slowly pull upwards on some plant, and note how firmly it is anchored in the soil. [Illustration: FIG. 40.--WHEAT GROWING UNDER DIFFERENT SOIL TREATMENTS. Soil deficient in nitrogen; commercial nitrogen applied to pot 3 (on right).] =Roots have power to choose their food=; that is, they do not absorb all substances with which they come in contact. They do not take up great quantities of useless or harmful materials, even though these materials may be abundant in the soil; but they may take up a greater quantity of some of the plant-foods than the plant can use to advantage. _Plants respond very quickly to liberal feeding_,--that is, to the application of plant-food to the soil (Fig. 40). The poorer the soil, the more marked are the results, as a rule, of the application of fertilizers. Certain substances, as common salt, will kill the roots. =Roots absorb Substances only in Solution.=--Substances cannot be taken in solid particles. These materials are in solution in the soil water, and the roots themselves also have the power to dissolve the soil materials to some extent by means of substances that they excrete. The materials that come into the plant through the roots are _water and mostly the mineral substances_, as compounds of potassium, iron, phosphorus, calcium, magnesium, sulfur, and chlorine. These mineral substances compose the ash when the plant is burned. The carbon is derived from the air through the green parts. Oxygen is derived from the air and the soil water. [Illustration: FIG. 41.--NODULES ON ROOTS OF RED CLOVER.]