The _secretions_ may be produced either in ordinary cells and found in their vacuoles, or in special secretory cells and stored in cavities in the secreting glands (as in the leaves of mints, skin of oranges, etc.), or in special ducts (as in pines, milkweeds, etc.) or on the epidermis (as the "bloom" of plums, cabbages, etc., the resinous coating of many leaves, etc.). As a general rule, the glucosides, pigments, and enzymes are the products of unspecialized cells and have some definite connection with the metabolic processes of the plant; while the volatile oils and the alkaloids are usually secreted by special cells and have no known rôle in metabolism. CHAPTER III PHOTOSYNTHESIS Photosynthesis is the process whereby chlorophyll-containing plants, in the presence of sunlight, synthetize organic compounds from water and carbon dioxide. The end-product of photosynthesis is always a carbohydrate. Chemical compounds belonging to other groups, mentioned in the preceding chapter, are synthetized by plants from the carbohydrates and simple raw materials; but in such cases the energy used is not solar energy and the process is not photosynthesis. Under the ordinary conditions of temperature, moisture supply, etc., necessary to plant growth, photosynthesis will take place if the three essential factors, chlorophyll, light, and carbon dioxide are available. PHYSIOLOGICAL STEPS IN PHOTOSYNTHESIS There are five successive and mutually dependent steps in the process of photosynthesis, as follows: (1) There must be a gas exchange between the plant tissue and the surrounding air, by means of which the carbon dioxide of the air may reach the protoplasm of the chlorophyll-containing cells. (2) Radiant energy must be absorbed, normally that of sunlight, although photosynthesis can be brought about by the energy from certain forms of artificial light. (3) Carbon dioxide and water must be decomposed by the energy thus absorbed, and the nascent gases thus produced combined into some synthetic organic compound, with a resultant storage of potential energy. (4) This first organic synthate must be condensed into some carbohydrate suitable for translocation and storage as reserve food. (5) The oxygen, which is a by-product from the decomposition of the water and carbon dioxide and the resultant synthetic process, must be returned to the air by a gas exchange. Of the five steps in this process, the first two and the last are essentially purely physical phenomena, the chemical changes involved being those of the third and fourth steps. Hence, it is only these two parts of the process which need be taken into account in a consideration of the chemistry of photosynthesis. FORMALDEHYDE, THE SIMPLEST CARBOHYDRATE STRUCTURE