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. =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. [Illustration: FIG. 41.—NODULES ON ROOTS OF RED CLOVER.] =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, sulphur, 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. 42.—NODULES ON VETCH.] =Nitrogen enters through the Roots.=—All plants must have nitrogen; yet, although about four-fifths of the air is nitrogen, plants are not able, so far as we know, to take it in through their leaves. It enters through the roots in combination with other elements, chiefly in the form of nitrates (certain combinations with oxygen and a mineral base). The great family of leguminous plants, however (as peas, beans, cowpea, clover, alfalfa, vetch), _use the nitrogen contained in the air in the soil_. They are able to utilize it through the _agency of nodules_ on their roots (Figs. 41, 42). These nodules contain bacteria, which appropriate the free or uncombined nitrogen and pass it on to the plant. The nitrogen becomes incorporated in the plant tissue, so that these crops are high in their nitrogen content. Inasmuch as nitrogen in any form is expensive to purchase in fertilizers, the use of leguminous crops to plough under is a very important agricultural practice in preparing the land for other crops. In order that leguminous crops may acquire atmospheric nitrogen more freely and thereby thrive better, _the land is sometimes sown or inoculated with the nodule-forming bacteria_.