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taste of which was sweetish, aud slightly astringent, 

 and which was colourless. 



The increase of trees and plants must depend 

 upon the quantity of sap which passes into the organs 

 upon the quality of this sap; and on its modification 

 by the principles of the atmosphere. Water, as it is 

 the vehicle of the nourishment of the plant, is the sub- 

 stance principally given off by the leaves. Dr. Hales 

 found, that a sunflower, in one day of twelve hours, 

 transpired by its leaves one pound fourteen ounces of 

 water, all of which must have been imbibed by its 

 roots. 



The powers which cause the ascent of the sap 

 have been slightly touched upon in the second and 

 third Lectures. The roots imbibe fluids from the soil 

 by capillary attraction; but this power alone is insuffi- 

 cient to account for the rapid elevation of the sap into 

 the leaves. This is fully proved by the following fact 

 detailed by Dr. Hales, Vol. I. of the Vegetable Statics, 

 page 1 14. A vine branch of four or five years old was 

 cut through, and a glass tube carefully attached to 

 it; this tube was bent as a siphon, and filled with quick- 

 silver; so that the force of the ascending sap could be 

 measured by its effect in elevating the quicksilver. In 

 a few days it was found, that the sap had been propel- 

 led forwards with so much force as to raise the quick- 

 silver to 38 inches, which is a force considerably su- 

 perior to that of the usual pressure of the atmosphere. 

 Capillary attraction can only be exerted by the sur- 

 faces of small vessels, and can never raise a fluid into 

 tubes above the vessels themselves. 



