Page:Popular Science Monthly Volume 20.djvu/466

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Respiration of Plants.—The leaf-respiration, so called, of plants is not an excretory process, but rather a nutritive one. In effect it is precisely the reverse of true respiration. It is a deoxidizing process—separating the components of carbonic acid under the influence of the sun's rays, and depositing the carbon; whereas respiration is an oxidizing process the production of carbonic acid. However, during germination and flowering, and in darkness, decomposition takes place within the plant, resulting in the production and elimination of carbonic acid a—true respiration.

Many biologists now hold that there is a constant decomposition of crude nutriment in the interior of plants, and therefore a slight respiration, but that it is masked by the more prominent nutritive process. The leaves are commonly, but wrongly, called the lungs of the plant, for their chief function, as we observe, is not respiration, but nutrition. It were more correct to regard them as the stomach of the plant.

PSM V20 D466 Gills of annelida and of a bivalve mollusk.jpg
Fig. 3.—Gills, a, b, c, or Annelida: d, of a Bivalve Mollusk. a, Nauphanta celox (Greeff), enlarged to three diameters, with broad gill-fins, b, foot of Vanadis ornata (Greeff), with two broad gill-fins, c, section of a segment of Eunice: br, the ramified gill-appendages of the rudimentary foot. d, Mytilus edulis, with br, the gill-folds, and l, the lips separated from them.

Organs of Respiration.—As the function of respiration is so simple in principle, being a single physical action, it can be conducted almost anywhere in the body, or wherever the blood can be conveniently exposed to the surrounding medium. The nature of an animal, as regards other less easily modified functions, and its peculiar circum-