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 Nymphasa, 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, pei-vade 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, then* 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. EXHALATION 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 



