VEGETATION

VEGETATION, in physiology, the act whereby plants receive nourishment and grow. See PLANT.

The process of nature in the vegetation of plants is very accurately delivered by the excellent Malpighi, to the effect following: The egg or seed of the plant being excluded out of the ovary, called pod or husk, and requiring further fostering and brooding, is committed to the earth; which having received it into her fertile bosom, not only does the office of incubation by her own warm vapours and exhalation, joined with the heat of the sun, but by degrees supplies what the seed requires for its further growth; as abounding everywhere with canals and sinuses, wherein the dew and rain-water, impregnated with fertile salts, glide, like the chyle and blood in the arteries, &c. of animals. This moisture meeting with a new-deposited seed, is percolated, or strained through the pores or pipes of the outer rind or husk, corresponding to the secundines of the foetuses, on the inside whereof lies one or more, commonly two, thick seminal leaves, answering to the placentas in women, and the cotyledons in brutes.

These seed-leaves consist of a great number of little vesiculæ, or bladders, with a tube corresponding to the navel-string in animals. In these vesiculæ is received the moisture of the earth, strained through the rind of the seed; which makes a slight fermentation with the proper juice before contained therein. This fermented liquor is conveyed by the umbilical vessel to the trunk of the little plant; and to the germ or bud, which is contiguous thereto: upon which a vegetation and increase of the parts succeed.

Such is the procedure in the vegetation of plants; which the illustrious author exemplifies in a grain of wheat, as follows: The first day the grain is sown it grows a little turgid; and the secundine, or husk, gapes a little in several places: and the body of the plant, being continued by the umbilical vessel to a conglobated leaf (which is called the pulp or flesh of the seed, and is what constitutes the flower) swells; by which means, not only the germ or sprout (which is to be the future stem) opens, and waxes green, but the roots begin to bunch out; whence the placentas, or seed-leaf, becoming loose, gapes. The second day, the secundine, or husk, being broke through, the stem, or top of the future straw, appears on the outside thereof, and grows upwards by degrees: in the mean time the seed-leaf guarding the roots, becomes turgid with its vesiculæ, and puts forth a white down. And the leaf being pulled away, you see the roots of the plant bare; the future buds, leaves, and rest of the stalk, lying hid. Between the roots and the ascending stem the trunk of the plant is knit by the navel knot to the flower-leaf, which is very moist, though it still retains its white colour and its natural taste. The third day, the pulp of the conglobated, or round leaf, becomes turgid with the juice which it received from the earth fermenting with its own.

Thus the plant increasing in bigness, and its bud or stem becoming taller, from whitish turns greenish; the

lateral

Vegetation. lateral roots also break forth greenish and pyramidal from the gaping sheath, which adheres chiefly to the plant; and the lower root grows longer and hairy, with many fibres shooting out of the same.

Indeed there are hairy fibres hanging all along on all the roots, except on their tips; and these fibres are seen to wind about the saline particles of the soil, little lumps of earth, &c. like ivy; whence they grow curled. Above the lateral roots there now break out two other little ones.

The fourth day, the stem mounting upwards, makes a right angle with the seminal leaf: the last roots put forth more; and the other three growing larger, are clothed with more hairs, which straitly embrace the lumps of earth; and where they meet with any vacuity, unite into a kind of net-work.

From this time forward the root pushes with more regularity downward, and the stalk upward, than before. There is, however, this great difference in their growth, that the stalk and branches find no resistance to their shooting up, while the roots find a great deal to their shooting downward by means of the solidity of the earth, whence the branches advance much faster and farther in their growth than the roots; and these last often finding the resistance of a tough earth unfurmountable, turn their course, and shoot almost horizontally.

As to the manner in which vegetation is effected, very little can be said. That heat, air, and light are necessary for this purpose, is known to every one; but in what manner they act, seems to be totally undiscoverable by us. The lowest degree of vegetation seems to be that of the crystallization of salts. This indeed has scarce been allowed really to be a vegetation, tho' it certainly is owing to the same causes. Whether crystallization will succeed in vacuo, indeed, has not been determined; but it certainly may go on without light, and in much lower degrees of heat than those in which any vegetable will grow. Light indeed seems not absolutely necessary to vegetation; for the roots of all plants grow below ground, where they are very much deprived of light. This element seems only necessary for giving a green colour to the upper part of vegetables, and perhaps strengthening them, so that they can better bear the inclemency and changes of the weather. But whatever may be the differences between the powers which produce vegetation and those of crystallization, water is the medium by which only both can act; for if a salt is perfectly dry, it will not crystallize, more than a seed will grow in dry sand or dust. Some kinds of crystallization indeed resemble vegetation so much, that we can scarce avoid ascribing them to the same cause. Of these the most remarkable is the arbor Dianæ, or compound of nitrous acid and silver †. Several curious crystallizations, or vegetations as they are called, may also be formed with other metals; but that which most of all resembles a true vegetation, is the caput mortuum of Glauber's spirit of nitre. This is a combination of very pure vegetable fixed alkali with vitriolic acid. If a large quantity of the latter has been employed in the distillation, and the mass is exposed to a moist air, a great number of crystallizations will take place on the surface, exactly resembling the vegetations of some kinds of shrubs. See CRYSTALLIZATION, PLANTS, GERMINATION, &c.