[10] Make up three stock solutions as follows: (1) Copper sulphate 9 grams Water 250 cc. (2) Caustic potash 30 grams Water 250 cc. (3) Rochelle salts 49 grams Water 250 cc. For Fehling’s solution take one volume of each of (1), (2), and (3), and to the mixture add two volumes of water. [11] This solution of taka diastase should be made up cold. If it is heated to 60° C. or over it is destroyed. CHAPTER IX. HOW PLANTS OBTAIN THEIR FOOD. I. 1. Sources of Plant Food. =170. The necessary constituents of plant food.=—As indicated in Chapter 3, investigation has taught us the principal constituents of plant food. Some suggestion as to the food substances is derived by a chemical analysis of various plants. In Chapter 8 it was noted that there are two principal kinds of compounds in plant substances, the organic compounds and the inorganic compounds or mineral substances. The principal elements in the organic compounds are _hydrogen_, _carbon_, _oxygen_ and _nitrogen_. The elements in the inorganic compounds which have been found indispensable to plant growth are _calcium_,[12] _potassium_, _magnesium_, _phosphorus_, _sulphur_ and _iron_. (See paragraphs 54-58, and complete observations on water cultures.) Other elements are found in the ash of plants; and while they are not absolutely necessary for growth, some[13] of them are beneficial in one way or another. =171.= The carbohydrates are derived, as we have learned, from the CO₂ of the air, and water in the plant tissue drawn from the soil; though in the case of aquatic plants entirely submerged, all the constituents are absorbed from the surrounding water. =172. Food substances in the soil.=—Land plants derive their mineral food from the soil, the soil received the mineral substances from dissolving and disintegrating rocks. Nitrogenous food is chiefly derived from the same source, but under a variety of conditions which will be discussed in later paragraphs, but the nitrogen comes primarily from the air. Some of the mineral substances, those which are soluble as well as some of the nitrogenous substances, are found in solution in the soil. These are absorbed by the plant, as needed, along with water, through the root hairs. =173. Absorption of soluble substances.=—Since these substances are dissolved in the water of the soil, it is not necessary for us to dwell on the process of absorption. This in general is dwelt upon in Chapter 3. It should be noted, however, that food substances in solution, during absorption, diffuse through the protoplasmic membrane independently of each other and also independently of the rate of movement of the water from the soil into the root hairs and cells of the root.