30 SYLVA AMERICANA. 



or at least furnishes the materials. All this, however, is a mere 

 probability, as we know very little with certainty connected 

 with it. 



The ascent of the sap varies according to the season of the 

 year, and the state and temperature of the atmosphere ; being 

 suspended during the winter, and most active in the spring, when 

 vegetation recommences, and previously to the full expansion 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 mercury, 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." 



It is difficult to determine by what means the sap is propelled 

 through the vessels : the agitation of the winds, the form of the 

 vessels, the action of the heat, the pressure of certain plates, 

 called silver grain, in the oak, are all supposed to contribute to 

 this end ; and very possibly they do this to a certain extent 

 We confess, however, that they do not appear to our minds 

 adequate causes. It is a matter of some moment to ascertain 

 how the function is performed ; but our knowledge of facts is so 

 very imperfect, that it is impossible to frame any reasonable 

 hypothesis on the subject. In this, as in every other department 

 of physics, men are too prone to step beyond the limits within 

 which their actual knowledge should confine them. 



Of the Leaves. 



This part of the plant which contributes so much to its beauty, 

 (though infinitely diversified in its forms), is in all cases similar 

 in its organization as well as its functions. It consists of a thin 

 and flat substance usually of a green color, issuing generally 



