This placentation of vegetable buds is clearly evinced from the sweetness of the rising sap, and from its ceasing to rise as soon as the leaves are expanded, and thus compleats the analogy between buds and bulbs. Nor need we wonder at the length of the umbilical cords of buds since that must correspond with their situation on the tree, in the same manner as their lymphatics and arteries are proportionally elongated.
It does not appear probable that any umbilical artery attends these placental absorbents, since, as there seems to be no system of veins in vegetables to bring back the blood from the extremities of their arteries, (except their pulmonary veins,) there could not be any vegetable fluids to be returned to their placenta, which in vegetables seems to be simply an organ for nutrition, whereas the placenta of the animal foetus seems likewise to serve as a respiratory organ like the gills of fishes.
NOTE XXXVI—VEGETABLE CIRCULATION.
And refluent blood in milky eddies bends.
CANTO IV. l. 420.
The individuality of vegetable buds was spoken of before, and is confirmed by the method of raising all kinds of trees by Mr. Barnes. (Method of propagating Fruit Trees. 1759. Lond. Baldwin.) He cut a branch into as many pieces as there were buds or leaves upon it, and wiping the two wounded ends dry he quickly applied to each a cement, previously warmed a little, which consisted principally of pitch, and planted them in the earth. The use of this cement I suppose to consist in its preventing the bud from bleeding to death, though the author ascribes it to its antisceptic quality.
These buds of plants, which are thus each an individual vegetable, in many circumstances resemble individual animals, but as animal bodies are detached from the earth, and move from place to place in search of food, and take that food at considerable intervals of time, and prepare it for their nourishiment within their own bodies after it is taken, it is evident they must require many organs and powers which are not necessary to a stationary bud. As vegetables are immoveably fixed to the soil from whence they draw their nourishment ready prepared, and this uniformly not at returning intervals, it follows that in examining their anatome we are not to look for muscles of locomotion, as arms and legs; nor for organs to receive and prepare their nourishment, as a stomach and bowels; nor for a reservoir for it after it is prepared, as a general system of veins, which in locomotive animals contains and returns the superfluous blood which is left after the various organs of secretion have been supplied, by which contrivance they are enabled to live a long time without new supplies of food.
The parts which we may expert to find in the anatome of vegetables correspondent to those in the animal economy are, 1. A system of absorbent vessels to imbibe the moisture of the earth similar to the lacteal vessels, as in the roots of plants; and another system of absorbents similar to the lymphatics of animal bodies, opening its mouths on the internal cells and external surfaces of vegetables; and a third system of absorbent vessels correspondent with those of the placentation of the animal foetus. 2. A pulmonary system correspondent to the lungs or gills of quadrupeds and fish, by which the fluid absorbed by the lacteals and lymphatics may be exposed to the influence of the air, this is done by the green leaves of plants, those in the air resembling lungs, and those in the water resembling gills; and by the petals of flowers. 3. Arterial systems to convey the fluid thus elaborated to the various glands of the vegetable for the purposes of its growth, nutrition, and various secretions. 4. The various glands which separate from the vegetable blood the honey, wax, gum, resin, starch, sugar, essential oil, &c. 5. The organs adapted for their propagation or reproduction. 6. Muscles to perform several motions of their parts.
I. The existence of that branch of the absorbent vessels of vegetables which resembles the lacteals of animal bodies, and imbibes their nutriment from the moist earth, is evinced by their growth so long as moisture is applied to their roots, and their quickly withering when it is withdrawn.
Besides these absorbents in the roots of plants there are others which open their mouths on the external surfaces of the bark and leaves, and on the internal surfaces of all the cells, and between the bark and the alburnum or sap-wood; the existence of these is shewn, because a leaf plucked off and laid with its under side on water will not wither so soon as if left in the dry air,—the same if the bark alone of a branch which is separated from a tree be kept moist with water,—and lastly, by moistening the alburnum or sap-wood alone of a branch detached from a tree it will not so soon wither as if left in the dry air. By the following experiment these vessels were agreeably visible by a common magnifying glass, I placed in the summer of 1781 the footstalks of some large fig-leaves about an inch deep in a decoction of madder, (rubia tinctorum,) and others in a decoction of logwood, (haematoxylum campechense,) along with some sprigs cut off from a plant of picris, these plants were chosen because their blood is white, after some hours, and on the next day, on taking out either of these and cutting off from its bottom about a quarter of an inch of the stalk an internal circle of red points appeared, which were the ends of absorbent vessels coloured red with the decoction, while an external ring of arteries was seen to bleed out hastily a milky juice, and at once evinced both the absorbent and arterial system. These absorbent vessels have been called by Grew, and Malphigi, and some other philosophers, bronchi, and erroneously supposed to be air-vessels. It is probable that these vessels, when cut through, may effuse their fluids, and receive air, their sides being too stiff to collapse; since dry wood emits air-bubles in the exhausted receiver in the same manner as moist wood.