The green membranous substance may be Consi- 

 dered as an extension of the parenchyma, and the 

 fine and thin covering as the epidermis. Thus the 

 organization of the roots and branches may be traced 

 into the leaves, which present, however, a more per- 

 fect, refined, and minute structure. 



One great use of the leaves is, for the exposure 

 of the sap to the influence of the air, heat, and light. 

 Their surface is extensive, the tubes and cells 

 very delicate, and their texture porous and trans- 

 parent. 



In the leaves much of the water of the sap is 

 evaporated ; it is combined with new principles, and 

 fitted for its organizing functions, and probably pas- 

 ses, in its prepared state, from the extreme tubes of 

 the alburnum into the ramifications of the cortical 

 tubes and then descends through the bark. 



On the upper surface of leaves, which is expos- 

 ed to the sun, the epidermis is thick but transparent, 

 and is composed of matter possessed of little organi- 

 zation, which is either principally earthy, or consists 

 of some homogeneous chemical substance. In the 

 grasses it is partly siliceous, in the laurel resinous, 

 and in the maple and thorn, it is principally constitut- 

 ed by a substance analogous to wax. 



By these arrangements any evaporation, except 

 from the appropriated tubes, is prevented. 



On the lower surface the epidermis is a thin 

 transparent membrane full of cavities, and it is proba- 

 bly altogether by this surface that moisture and the 

 principles of the atmosphere necessary to vegetation 

 are absorbed. 



