154 



THE LEAVES. 



renchyma consists of only a single series of perpendicular cells. 

 Also in Fig. 178 (after Brongniart), which represents a similar view 

 of a thin slice of a leaf of the Garden Balsam. Fig. 179 represents 

 a similar section through the thickness of a leaf of the White Lily ; 

 where the upper stratum is composed of only one compact lay- 

 er of vertical cells. The parenchyma is alone represented ; the 

 woody portion, or veins, being left out. This structure shows why 

 the upper surface of leaves is of a deeper green than the lower. 



263. 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 holes in the epidermis 

 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 



FIG. 179. Magnified section through the thickness of the leaf of the White Lily, showing 

 the parenchyma, and the epidermis of both surfaces ; the lower pierced with stomata. (After 

 Brongniart.) 180. Two of the cells of the upper stratum of parenchyma, detached and more 

 magnified, showing the contained grains of chlorophyll. 



FIG. 181. Magnified view of the 10,000th part of a square inch of the epidermis of the lower 

 surface of the White Lily, with the stomata, or breathing pores, it bears. These are unusually 

 large in the Lily. One is shown more magnified in Fig. 182 : and widely open in Fig. 183. 



FIG. 184. Magnified perpendicular section through the thickness of the epidermis and upper 

 stratum of parenchyma in the leaf of the Oleander (after Brongniart); showing the epidermis 

 of three layers of thick-sided cells, and the upper parenchyma of very compact vertical cells. 



