

66 ON VEGETABLE PHYSIOLOGY. 



is rendered broad and extensive, the tubes and 

 cells exquisitely fine and delicate, their texture 

 throughout, porous and transparent, and the ori- 

 fices of the epidermis (so essential to the process,) 

 numerous beyond calculation, and so extremely 

 minute, as to require very powerfully magnifying 

 glasses to detect them ; their diameter 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 jus- 

 tice to their illustration. Upon the new or as- 

 cending 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 ab- 

 sorption. 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 



