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circulation of the fap, moving downwards in the root, and feeding the trunk upwards.
31. Some difference indeed there is between learned men, concerning the proper use of certain parts of vegetables. But whether the discoverers have rightly guessed at all their uses or no, thus much is certain, that there are innumerable fine and curious parts in a vegetable body, and a wonderful fimilicude or analogy between the mechanism of plants and animals. And perhaps some will think it not unreasonable to suppose the mechanism of plants more curious than even that of animals, if we consider not only the several juices secreted by different parts of the same plant, but also, the endless variety of juices drawn and formed out of the same soil, by various species of vegetables ; which must therefore differ in an endless variety, as to the texture of their absorbent vessels and fe
32. A body, therefore, either animal or vegetable, may be considered as an organised system of tubes and vessels, containing several sorts of Auids. And as Auids are moved through the vessels of animal bodies, by the systole and diaftole of the heart, the alternate' expansion and condensation of the air, and the oscillations in the membranes and tunicks of the vessels; even so by means of air expanded and contracted in the tracheæ or vessels made up of elastic fibres, the fap is propelled through the arterial tubes of a plant, and the vegetable juices, as they are rarefied by heat or condensed by cold, will either ascend and evaporate into air, or descend in the form of a grofs liquor.
33. Juices therefore, first purified by straining through the fine pores of the root, are afterwards exalted by the action of the air and the vessels of the plant, but, above all, by the action of the sun's
light; which at the same time that it heats, doth wonderfully rarefy and raise the fap; till it perspires and forms an atmosphere, like the effluvia of animal bodies. And though the leaves are supposed to perform principally the office of lungs, breathing out excrementitious vapours, and drawing in alimentary; yet it seems probable, that the reciprocal actions of repulsion and attraction are performed all over the surface of vegetables, as well as animals. In which reciprocation, Hippocrates supposech the manner of nature's acting, for the nourishment and health of animal bodies, chief ly to consist. And, indeed, what share of a plant's nourishment is drawn through the leaves and bark, from that ambient heterogeneous Auid called air, is not easy to say. It seems very considerable and al. together necessary, as well to vegetable as animal life.
34. It is an opinion received by many, that the sap circulates in plants as the blood in animals: that it ascends through capillary arteries in the trunk, into which are inofculated other vessels of the bark answering to veins, which bring back to the root the remainder of the fap, over and above what had been deposited, during it's ascent by the arterial vessels, and secreted for the several uses of the vegetable throughout all it's parts, Item, branches, leaves, flowers, and fruit
. Ochers deny this circulation, and affirm that the sap doth not return through the bark vessels. It is nevertheless agreed by all, that there are ascending and descendo ing juices; while some will have the ascent and descent to be a circulation of the same juices through different vessels: others will have the ascending juice to be one fort attracted by the root, and the descending another imbibed by the leaves, or extremities of the branches : Saftly, others think that
the same juice, as it is rarefied or condensed by heat or cold, rises and subsides in the same tube. I shall not take upon me to decide this controversy. Only I cannot help observing, that the vulgar argument from analogy between plants and animals Joseth much of it's force, if it be considered, that the supposed circulating of the sap, from the root or lacteals through the arteries, and thence returning, by inosculations, through the veins or bark vessels to the root or lacteals again, is in no sort conformable or analogous to the circulation of the blood.
35. It is sufficient to observe, what all must acknowledge, that a plant or tree is a very nice and complicated machine (a); by the several parts and motions whereof, the crude juices admitted through the absorbent vessels, whether of the root, trunk, or branches, are variously mixed, separated, altered, digested, and exalted in a very wonderful manner. The juice as it passeth in and out, up and down, through tubes of different textures, shapes, and sizes, and is affected by the alternate compression and expansion of elastic vessels, by the vicisitudes of seasons, the changes of weather, and the various action of the solar light, grows still more and more elaborate.
36. There is therefore no chemistry like that of nature, which addeth to the force of fire, the most delicate, various, and artificial percolation (b). The incessant action of the fun upon the elements of air, earth, and water, and on all sorts of mixed bodies, animal, vegetable and foffil, is supposed to perform all sorts of chemical operations. Whence it should follow, that the air contains all sorts of chemic productions, the vapours, fumes, oils, falts,
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and spirits of all the bodies we know : from which general aggregate or mass, those that are proper being drawn in, through the fine vessels of the leaves, branches, and stem of the tree, undergo in it's various organs, new alterations, secretions, and digestions, till such time as they assume the most elaborate form.
37. Nor is it to be wondered, that the peculiar texture of each plant or tree, co-operating with the solar fire and pre-existing juices, should lo alter the fine nourishment drawn from earch and air (a), as to produce various specific qualities of great efficacy in medicine : especially if it be considered that in the opinion of learned men, there is an influence on plants derived from the sun, besides it's mere heat. Certainly doctor Grew, that curious anatomist of plants, holds the solar influence to differ from that of a mere culinary fire, otherwise than by being only a more temperate and equal heat.
38. The alimentary juice taken into the lacteals, if I may so fay, of animals or vegetables, consists of oily, aqueous, and saline particles, which being difsolved, volatilised, and diversly agitated, part thereof is spent and exhaled into the air ; and that part which remains is by the economy of the plant, and action of the sun, strained, purified, concocted, and ripened into an infpiffated oil or balfam, and deposited in certain cells placed chiefly in the bark, which is thought to answer the panniculus adiposus in animals, defending trees from the weather, and, when in sufficient quantity, rendering them evergreen. This balfam, weeping or sweating through the bark, hardens into resin ; and this moft copiously in the several species of pines and
firs, whose oil being in greater quantity, and more tenacious of the acid spirit or vegetable soul (as perhaps it may not improperly be called) abides the action of the sun, and attracting the sun beams, is thereby exalted and enriched, so as to become a most noble medicine ; such is the last product of a tree, perfectly maturated by time and fun.
39. It is remarked by Theophrastus, that all plants and trees while they put forth have most humour, but when they have ceased to germinate and bear, then the humour is strongest and most sheweth the nature of the plant, and that, therefore, trtes yielding resin should be cut after germination. It seems also very reasonable to fup. pose the juice of old trees, whose organs bring no new fap, should be better ripened than that of others.
40. The aromatic Aavours of vegetables seem to depend upon the sun's light, as much as colours. As in the production of the later, the reflecting powers of the object, so in that of the former, the attractive and organical powers of the plant co-operate with the sun (a). And as from Sir Isaac Newton's experiments it appears, that all colours are virtually in the white light of the sun, and shew themselves when the rays are separated by the attracting and repelling powers of objects, even so the specific qualities of the elaborate juices of plants, seem to be virtually or eminently contained in the solar light, and are actually exhibited upon the separation of the rays, by the peculiar powers of the capillary organs in vegetables, attracting and imbibing certain rays, which produce certain flavours and qualities, in like manner as certain rays, being reflected, produce certain coIcurs.
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