THEIR ANATOMICAT. STRUCTURE. 147 



upper stratum shows why the upper surface of leaves is of a deeper 

 green than the lower. 



2G3. The object which this arrangement subserves will appear 

 evident, when we consider that the spaces between the cells, filled 

 with air, communicate freely with each other throughout the leaf, 

 and also with the external air by means of openings in the epider- 

 mis (presently to be described) ; and when we consider the powerful 

 action of the sun to promote evaporation, especially in dry air; and 

 that the thin walls of the cells, like all vegetable membrane, allow 

 of the free escape of the contained moisture by transudation. The 

 compactness of the cells of that stratum which is presented immedi- 

 ately to the sun, and their vertical elongation, so that each shall 

 expose 'the least possible surface, obviously serve to protect the 

 loose parenchyma beneath from the too powerful action of direct 

 sunshine. This provision is the more complete in the case of plants 

 which retain their foliage through a season of drought in arid re- 

 gions, where the soil is usually so parched during the dry season, 

 that, for a long period, it affords only a scanty supply of moisture to 

 the roots. Compare, in this respect, a leaf of the Lily (Fig. 221), 

 where the upper stratum contains but a single layer of barely oblong 

 cells, with the firm and more enduring leaf of the Oleander, the 



upper stratum of which consists of two layers of long and narrow 

 vertical cells as closely compacted as possible (Fig. 222). So dif- 



FIG. 221. A magnified section through the thickness of a minute piece of the leaf of the 

 White Lily of the gardens, showing also a portion of the under side with some breathing-pores. 



