32 SYLVA AMERICANA. 



delicate, their texture throughout, porous and transparent, and 

 the orifices of the epidermis (so essential to the process) numer- 

 ous beyond calculation, and so extremely minute, as to require 

 very powerfully magnifying glasses to detect them ; their diam- 

 eter being only adapted to the absorption and extrication of 

 vapory fluids in the highest degree of tenuity. 



The processes of transpiration and absorption, as peculiar to 

 the functions of the leaves, are indeed of a most highly interesting 

 character, and require a far more able pen than mine to do 

 justice to their illustration. Upon the new or ascending sap 

 reaching the leaves from the roots, the operation of transpiring a 

 portion of its watery particles commences from the smooth or 

 upper surface of the leaf, as soon as the sun rises, and continues 

 until the approach of night ; by which the sap acquires more 

 consistency, and is thus rendered fit to receive those materials, 

 which are to be imparted to it through the agency of absorption. 

 This evaporation is so considerable, that Dr. Hales, whose 

 experimental accuracy has never been questioned, has ascertained, 

 that a cabbage transmitted daily more than half its weight, and 

 that a sunflower, three feet high, transmitted in twenty-four 

 hours, a watery fluid equal to twenty ounces. 



While this watery evaporation is going on, an absorption by 

 the same surface of the leaf of the carbonic acid gas of the 

 atmosphere, and a decomposition of some of the water left in the 

 sap, are taking place ; by which, in the former instance, the 

 carbon is separated and fixed in the sap, and the oxygen gas is 

 set at liberty ; while in the latter, the hydrogen is communicated 

 to the sap, and its oxygen gas also becomes free ; by which 

 operations, the sap has acquired two of the leading principles 

 necessary to vegetables, the carbon and the hydrogen ; while a 

 double supply of oxygen, or the vivifying principle, is restored 

 to the atmosphere, by which its purity is preserved against the 

 deterioration to which it is uniformly exposed by animal respi- 

 ration, combustion and mineral absorption. 



During the night the under surface of the leaf absorbs moisture 

 from the air, or from the evening dew, to make up in some 

 degree the deficiency of the previous day's evaporation, and takes 



