Plants obtain food from the soil.—Contrast this with the condition of a seedling which has been grown in soil. It still flourishes, even when the seed food is used up, for it is drawing up food from the soil—food which could not be obtained from the damp sawdust or clean sand. That the plant really has taken up some solid matter from the soil can be proved by a few simple experiments. A plant which has been growing in soil for some time after its seed food is used up is dried and burnt, and the ashes are weighed. The weight of ash or mineral matter thus obtained is found to be considerably greater than that of the ash obtained from an ungerminated seed, or from a seedling grown in damp sawdust or sand which has only been supplied with pure water. The mineral food of plants.—The composition of the ash obtained from various plants has been carefully determined by chemists, and in this manner they have been able to find out what substances must be present in soil in order that the plant may obtain all the mineral food it requires. A mixture of potassium nitrate (nitre), sodium chloride (common salt), calcium sulphate (plaster of Paris), magnesium sulphate (Epsom salts), calcium phosphate, and chloride (or sulphate) of iron—dissolved in water in the proportions specified on p. 27—has been found to supply the necessary elements of the mineral food in a form which the plant can readily use. That such a mixture is capable of [Pg 30] supporting the plant, while water alone is incapable of doing so, may be seen by growing a plant—in the manner shown in Fig. 19—in this solution. If, in addition, the plant is supplied with light and fresh air, it will grow in a perfectly healthy and normal manner. If any of the constituents (except the common salt) are omitted, the plant will suffer. On the other hand, a plant which is growing in pure water will presently die, from the lack of the necessary mineral food. Fig. 19.—Plant growing in a nutritive solution of salts. The bottle should be covered with a roll of paper to keep out the light. The work of the roots.—These experiments show that water—of which a large proportion of a plant consists—and the mineral constituents of its food (dissolved in the soil-water) are taken up from the soil by the roots. In ordinary soil the rootlets spread out on all sides, dividing and subdividing, seeking for this very weak solution of mineral salts. Even when soil appears practically dry, a very thin film of moisture covers each little particle of earth, and the root hairs become closely applied to these little particles (Fig. 20), so that the water passes through their walls and gradually makes its way to the main root, the stem, and the leaves. Fig. 20.—Tip of a root hair with adhering particles of soil. (× 240.)