Naturalists have formed various conjectures, to account for the mysterious phenomena occurring in vegetable nature ; and though unable to discover the primary source from which plants are enlivened, yet it is now agreed, and proved, that all vegetables originate from seeds, each of which comprehends three parts, namely : 1. The cotyledons, or two porous lateral bodies or lobes, that imbibe moisture: 2. The radicle, or eye, which appears between the lobes : and, 3. The plumula, a small round body attached to the radicle, though wholly concealed within the cotyledons.
If a seed be deposited in the earth, in a favourable situation, it imbibes moisture, and evolves carbonic acid gas; but, if any oxygen gas be present, it is gradually absorbed by the seed, and the farinaceous matter, contained in the cotyledons, acquires a saccharine taste. Numerous vessels then appear in the lobes which convey the nutriment to the radicle, that progressively increases in size, and at length assumes the form of a root; strikes downwards into the earth ; and thence derives the nourishment necessary for the support of the future plant. - Now the cotyledons shoot above the ground, become leaves, and form what botanists have termed the seminal leaves. Thus, the plumula is gradually enlarged, and rises out of the earth, spreading itself into branches, etc.; after which the seminal leaves wither and decay, while the different processes of vegetation are carried on in the plant, without their assistance.
Such is the. manner in which the growth of plants naturally takes place: it may, however, be artificially promoted, by applying certain chemical preparations; but, as we have already concisely treated on this subject, in the articles Food of Plants, and Manure, the reader will revert to those heads. - Many valuable hints respecting the phenomena of vegetation, are contained in Dr. Ingenhousz's "Experiments on Vegetables, " etc. (8vo. 6s.), and also in Mr. Gough's " Experiments and Observations on the Vegetation of Seeds, " inserted in the 4th vol. of the "Memoirs of the Literary and Philosophical Society of Manchester."