Whatever the source from which plants derive their nutriment, no organic substance is appropriated by them; but in order to their use, it must first undergo decomposition. Their food consists wholly of inorganic matter, and the value of organic substances as manures depends not only on the abundance winch they contain of the proper elements, but of the readiness with which they undergo decomposition so as to present these ele• mentS in the most suitable form; which is not, however, as elements uncombined. but in various combinations with each other. Thus carbon and oxygen enter plants together in the form of carbonic acid, oxygen and hydrogen together in the form of water, hydro gen and nitrogen in the form of ammonia. Carbonic acid absorbed by the leaves from the air is decomposed within the plant, under the influence of light, and particularly of the direct rays of the sun, and its carbon enters into new combinations to form vegetable substances, whilst its oxygen is exhaled again into the atmosphere, which is thus main tained in a state fit for the support both of vegetahle and animal life by the opposite and balanced action of animals and plants. Of the elements which enter into the composi tion of vegetable substances, carbon is the most abundant; and, along with it, oxygen, hydrogen, and nitrogen constitute the chief part of every plant. Other elements, both metallic and non-metallic, are found in comparatively small quantity, although some of them are very generally present in plants. as calcium, potassium, sodium, sulphur, phos phorus, silicon, iron, aluminium, magnesium, chlorine, and iodine. Among the ele ments found in plants are also to be enumerated bromine, manganese, and copper, which occur only in minute quantities, and copper very rarely.
There is no circulation in plants like that of the blood in animals, nor any organ at all analogous to a heart: although there is a constant motion or circulation of their juices, both throughout the whole organism and within individual cells. And although the term respiration has been often employed with reference to plants, and particularly to leaves, yet there is not only no action analogous to that of lungs, but no oxygenation of the juices by their being brought into contact with the air; carbonic acid and ammo nia—not oxygen—being imbibed from it for nutrltio–. And there is nothing in the vegeta ble kingdom having the slightest resemblance to a brain or a nervous system. In the possession of sexual organs, however, there is a wonderful agreement, where it might least have been expected, between plants—or at least all phanerogamous plants—and animals. As to this and other important points concerning the life of plants, see VEG
ETABLE PHYSIOLOGY. See also the article FLOWER, and those on the different organs of which the flower is made up; the articles FRUIT, SEED, SPORE; CELLS, CELLULAR TIS SUE, VASCULAR TISSUE; METAMORPHOSIS OF ORGANS; LEAVES, STEM, etc. The great divisions of the vegetable kingdom arc noticed in the article BOTANY, in connection with the subject of classification, and in separate articles. The GEOGRAPHICAL DISTRIBUTION OF PLANTS, and the DISEASES OF PLAwrs, are noticed under these heads.
Besides the relations of the animal and vegetable kingdoms already noticed in this article, in their joint and balanced action, keeping the constitution of the atmosphere such as is fit both for animal and vegetable life, reference may be here made, in conclu sion, to similar relations subsisting in plants and animals as to temperature and as to their mutually providing food for one another. " It would almost seem as if plants possessed a power of producing cold analogous to that exhibited by animals in producing heat, and of this beneficent arrangement man enjoys the benefit in the luxurious coolness of the fruit which nature lavishes on the tropics" (sir J. E. Tenuent). Flowers indeed produce heat; but the juices of plants are colder than the soil or surrounding atmosphere during the time of active vegetation: and the coolness of groves is owing not only to shade, but to the transpiration of moisture by the innumerable leaves.—Inorganie sub stances are appropriated by plants, as food, and converted by a "high and mysterious" chemistry into organic substances of many kinds, many of them suitable food for ani mals, which feed on organic substances alone. But the excrements of animals again furnish food for plants; and when animals die, their bodies undergo a series of changes by decomposition, which terminate in the production of the substances most suitable for the nourishment of plants. There is, moreover, not only this conversion of the same matter into animal and vegetable substances alternately; but there is also a continual transformation of matter which has remained inorganic throughout long geologic periods into organic substances, and in this some of the lowest kinds of plants are particularly employed, as lichens, which decompose and feed upon the very rocks on which they grow; whilst, on the other hand, the fossil remains of remote periods, and all the products of decomposition, exhibit matter which once formed part of living organisms returned to an inorganic state.