128 



A DISCOURSE 



BOOK II. abundant nourislnnent ; and therefore sometimes there may be as much 

 ^"^^y^-^ occasion to use the lancet, as veniesection to animals ; especially if the hy- 

 pothesis hold, of superfluous moisture's descent into the roots, to be re- 

 concocted ; but where, in case it be more copious than can be there 

 elaborated, it turns to corruption, and sends up a tainted juice, which 

 perverts the whole habit of the tree : in this exigence, therefore, it were, 



nent naturalists to suppose a regular circulation of the vegetable juices. M, Perrault, 

 M. Major, M. Mariotte, Malpighi, and our countryman Grew, contended, much about the 

 same time, for the circulation of the sap. According to their microscopical observations, 

 the wood of trees, and the flesh of plants, consist of fine capillary tubes, which run pa- 

 rallel from the root, through the trunk and branches. These they looked upon as arteries. 

 Other minute vessels were observed running between the wood and inner bark, which they 

 distinguished by the name of veins. They also described, very correctly, the tracheae, 

 or air-vessels, which take their course through the fibres of the wood. These anatomical 

 preliminaries being settled, they proceeded to reason in this manner. The root having 

 absorbed a quantity of juice from the earth, it is made to ascend through the vessels of the 

 wood, by the alternate expansion and contraction of the tracheae, assisted by the natural 

 absorption of the sap-vessels themselves. They supposed the sap to be rarefied to the. de- 

 gree of a fine vapour, in which state it mounted upwards to the extreme parts of the plant, 

 where meeting with the external air, it became condensed into a liquor, and in that form 

 returned to the root by the venal system, between the wood and bark. Dr. Hales, in the 

 most satisfactory manner, set aside this doctrine, and substituted another in its place, more 

 consonant to reason and experiment. It is something remarkable that Dr. Hervey should 

 have been the first who established the circulation of the blood, in opposition to most of the 

 anatomists in Europe ; and that Dr. Hales should have clearly disproved the circulalion of 

 the sap, contrary to the opinion of almost every naturalist of his time. In order that we 

 may have a distinct view of the motion of the sap, it will be necessary to reflect, that the 

 root, stem, branches, and leaves are constructed in the same manner. Sallows, Willows, 

 Vines, and most shrubs, will grow in an inverted state, with their tops downward in the 

 earth. Dr. Bradley describes the manner of inverting a young Cherry-tree, the roots of 

 which will put forth leaves, and the branches become roots. Hence it is obvious that the 

 nutritive matter may be conveyed as well by the leaves as the roots, their vascular struc- 

 ture being the very same. We have now settled the anatomical structure of a plant. Upon 

 it depends much of what we know of the vegetable oeconom}^ The motion of the sap comes 

 next to be examined. 



During the heat of a summer's day, all plants perspire freely from the pores of their leaves 

 ' and bark. At that time the juices are highly rarefied. The diameters of the trachese, or 



air-vessels, are enlarged, so as to press upon and straiten the vessels that carry the sap. In 

 consequence of which, their juices, not being able to escape by the roots, are pressed up- 

 ward, where there is the least resistance, and perspire off the excrementitious parts by the 

 leaves and top-branches, in the form of vapour. When the solar heat declines, the tracheae 

 are contracted ; the sap-vessels are enlarged, and the sap sinks down in the manner of the 

 spirits of a thermometer. In consequence of this change, the capillary vessels of the leaves 



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