98 THE LEAF. 
green above, where it is more compact; but paler beneath, where the cells are 
more loose and separate. 
264. The empty spaces between the cells, called intercellular, communicate 
with the external air by means of the stomata (37 —39), which are generally 
found only in the lower surface. In those leaves, however, whose position is nat- 
urally vertical instead of horizontal, stomata are found equally on both surfaces. 
In other leaves, as in the Nymphza, they are found upon the upper surface alone, 
the lower being in contact with the surface of the water. 
265. The vessels of the latex (34) are distributed through the under layer of the 
parenchyma. These are prolongations of the ramified veins, which, having 
reached the edge of the leaf, double back upon themselves, pervade the lower sur- 
face, and are again collected into the petiole, through which they are finally 
returned into the bark. 
266. A singular structure occurs in the Oleander of Barbary, and other plants 
of hot and arid regions. The epidermis on the upper surface is double and very 
compact, and there are few if any stomata on the lower surface, their places 
being supplied by cavities within its substance, opening outwards by a small aper- 
ture, and covered within by minute hairs. These peculiarities are adapted to the 
conditions of the air and soil in which the Oleander flourishes. ‘The hairs absorb 
moisture from the air, which the cavities readily retain, while the double epider- 
mis effectually restrains its evaporation. 
§12, OF THE FUNCTIONS OF LEAVES. 
267. These are exhalation, absorption, respiration, and diges- 
tion, and the result of their combined action is the conversion 
of the crude sap, absorbed from the soil by the roots, into the 
proper juice or latex, for the nourishment and increase of the 
plant, with its various products. 
268. The crude sap consists of water holding in solution 
minute quantities of various kinds of solid and gaseous matter 
derived from the soil. In its passage from the root to the 
leaves, its composition is somewhat modified by dissolving the 
previously formed secretions, which it meets with on its way. 
269. Exuaxartton is the process by which the superabundant 
water of the sap is given off to the atmosphere, so that the re- 
maining sap is reduced, as it were, by concentration, and con- 
tains a greater proportion of solid matter. It is analogous to 
perspiration in animals. 
270. It is to be distinguished from evaporation; the latter 
depending solely upon heat and the state of the air, and being, 
in plants, almost wholly restrained by the epidermis. 
271. Exhalation appears to take place through the stomata 
