86 ON VEGETABLE PHYSIOLOGY. 



vegetables, which, to the best of my recollection 

 I have omitted. 



Upon this interesting subject which, at various 

 periods, has engaged the attention of philosophers 

 without any satisfactory results, my time will not 

 allow me to dilate. All I can briefly say, is, 

 that the ascent of the sap varies according to the 

 season of the year, and the state and temperature 

 of the weather; being suspended during the win- 

 ter, and most active in the spring, when vegeta- 

 tion recommences, and previously to the full ex- 

 pansion of the leaf; that at the vernal season, 

 Dr. Hales has ascertained by experiments on the 

 vine, in the heat of the day it will rise in glass 

 tubes adjusted for the purpose, at the rate of an 

 inch in three minutes, and attain in these tubes the 

 height of more than twenty feet; and that, by its 

 force upwards, it will sustain a column of quick- 

 silver, of thirty-eight inches, equivalent to the 

 pressure of a column of water of more than forty- 

 three feet; which force, he says is, " five times 

 greater than that of the blood in the crural artery 

 of a horse, seven times greater than that of a dog, 

 and eight times greater than the blood's force of 

 the same artery in a fallow deer." 



The cause of this ascent of the sap has been va- 

 riously explained by different naturalists; some 

 attributing it to the action of temperature upon 

 the fluid, by which it ascends from the diminution 

 of its own gravity ; some to capillary attraction, 



