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the organization of the roots and branches may be 
traced into the leaves, which present, however, a 
more perfect, 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 
transparent. 
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 
passes, 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 exposed 
to the sun, the epidermis is thick but transparent, 
and is composed of matter possessed of little or- 
ganization, 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 princi- 
pally constituted 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 
probably altogether by this surface that moisture 
and the principles of the atmosphere necessary to 
vegetation are absorbed. 
If a leaf be turned, so as to present its lower 
surface to the sun, its fibres will twist so as to 
bring it as much as possible into its original 
