Page:Popular Science Monthly Volume 43.djvu/526

510 of plants known as proteids, starch, cellulose, etc., are formed in the downward steps of its progress with a liberation of a portion of the stored energy in the form of heat. The heat liberated in these first steps of destructive metabolism is not, however, sufficient to maintain an independent temperature in plants, as it is used in vaporizing the water exhaled in the processes of growth, or lost by radiation from the extended surface of foliage. The energy expended in vaporizing water must be considerable, as experiments show that for each pound of dry organic substance formed by the plant about three hundred pounds of water are exhaled in the form of vapor.

The products of the downward steps of metabolism are numerous, some of which, as waste matters, are either excreted, as in the case of carbonic acid and water, or deposited in the more stable tissues; while others called plastic products, including proteids, starch, fats, etc., are stored as reserve materials to be used in constructive metabolism when needed by the plant. These reserve materials are not as a general rule stored in the place where they are formed, and they can only be transported when changed to a soluble form, which is brought about by certain "soluble ferments," which are also products of the destructive metabolism of protoplasm. Starch formed in the leaves is changed to glucose and transported to other parts of the plants where it is reconverted into starch and stored for use, sooner or later, in constructive metabolism. Some of these reserve materials are apparently built up again into protoplasm before they are resolved into their ultimate products. This is seen in the starch deposited in oily seeds which is used in forming protoplasm, and the oil is then formed as a product of its metabolism.

The many forms of organic acids—as the malic, tartaric, oxalic, citric, etc.—and a variety of alkaloids and other bodies are also products of destructive metabolism, which may be deposited in the various tissues as waste materials, or some of them may be changed by soluble ferments into forms which may be again utilized in the economy of the plant. The organic acids and tannin of green fruits, for example, are converted into sugar in the process of ripening by ferments formed from the protoplasm of the fruit cells.

In general terms the processes of nutrition in plants may, then, be said to consist in the construction of protoplasm from the elements of their food, with a storing of energy, and the conversion of this protoplasm into the various organic substances entering into the composition of their tissues (starch, cellulose, etc.), with a liberation of energy, and all vital activities are included in