CTRCrLATlON OF THE SAP. 135 



off. Hales many years ago, made some interesting experi- 

 ments, not only proving this general circulati<in, hut determi- 

 ning the force, with wliicli the fluid moved forward. 



179. By the aid of a glass tul)e, containing morcurv, at- 

 tached to the stalk of a vine cat off* two feet and nine inches 

 from the ground, the force of the sap at its maximum raised the 

 mercurv IVJj inches, which was on the twelfth day at'ter the 

 exj>eriment commenced, April 18, at 7 A. M., which force 

 was sufficient to raise water 36 feet. 



" In another like mercurial guatje. fixed near the hottom of 

 a vine which ran *20 feet liigh, the mercury was raised by the 

 force of the sap, 33 inches, equal to 43 feet 3 inches hight of 

 water, which force is more than five times greater than the 

 force of the blood, in the great crural artery of the horse, 

 seven times greater than the force of the blood in the like ar- 

 tery of the dog ; and eiglit times greater than the blood's force 

 in the same artery of a fallow doe." 



190. These ex|)eriments show not only circulation, but 

 that it is carried on with great force. The force with which 

 the sap moves in vegetables varies with the seasons and the 

 hours of the day. It is most powerful in the spring, and in 

 the morning of the day, and under the direct action of the 

 sun after a rain. The course whicli the sap takes in its gen- 

 eral circulation, is from the roots through the alburnum to the 

 leaves, and downwards through the bark, and laterally by the 

 medullary processes. These tacts may be shown by cutting 

 in early spring into the sugar maple, and we shall find the sap 

 running from tlie alburnum only, and mostly from the lower 

 surtace of the wound, showing the upward course of the sap 

 is through this part of the stem. If the same tree be cut in 

 mid summer, there will be little or no issue from the alburnum, 

 but the bark will now give out a fluid from the uj>per edge of 

 the wound, proving that the downward current is through the 

 bark. The reason that has been assigned for little or no sap 

 issuing from the cut alburnum in summer, is that the draft made 

 upon it by the evaporation, prevents the vessels from holding 

 enough sup to issue from the cut ends. 



191. That the sap, before elaboration, ascends within the 

 wood, and that most of it after this process, descends within 

 the bark, is proved by tying a ligature very tight round a 

 branch in spring, and the branch will greatly increase above 

 the ligature, and l)ut very little below ii ; thus showing that 

 the sap was not obstructed in its ascent, but was obstructed 

 in its descent. This operation will very much increase the 

 aize of fruit on any branch, for a single year, but it injures 



