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ascend, till it reaches the extremities of the branches. This 

 sap is absorbed from the soil, by the extremities of the capil- 

 lary rootlets, and conveyed upwards, through the vessels of 

 the root, to the trunk. In its ascent, it rises only through 

 the wood, and the albmnum, in tubes of various sizes, and 

 is prepared or elaborated by the leaves. That process, ac- 

 cording to some, is effected by means of an alternate con- 

 traction and dilatation of the sap-vessels, and still more, by 

 a respiration perceptible and imperceptible in the leaves, 

 which is peculiar to plants, whether woody or herbaceous, 

 and by the action of the atmosphere : But, according to 

 others, it is rather the exhalation from the leaves, than what 

 is properly their respiratory functions, that effects the ascent 

 of the sap. When this has taken place, the sap is then 

 converted into the proper juice, or what has been by some 

 called the cambium, that is, juice fitted for nutrition ; and it 

 descends by the returning vessels of the leaf-stalk, and the 

 longitudinal vessels of the rind, or inner bark. Thus, the 

 circulation is carried on by a double process, the ascending 

 and the descending ; whereby the vessels terminate down- 

 wards in absorbents, by which Uie fluids are received, and 

 they terminate upwards in exbalents, by which those fluids 

 are discharged. This doctrine of the two currents of sap 

 was originally struck out by Malpighi and Grew : But the 

 first who showed the organs of communication between the 

 two currents to be the leaves, was unquestionably Darwin ; 

 a discovery, which the ingenuity of Knight subsequently 

 extended and confirmed, and traced the existence of the 

 circulation of the sap. 



During the descent of the proper juice, it further appears, 

 that each branch is nourished by the juice prepared by itself, 

 and that the surplus, beyond what is recjuired for that pur- 

 pose, descends from the junction of the branch with the 

 stem, and contributes to the increase of the steu). and at 

 last of the roots, which originally supplied it. The descend- 



