MINUTE ANATOMY OF FLOWERING PLANTS 227 



rather close together, leaving scanty vacant spaces ; those 

 of the lower are commonly irregular in shape, most of them 

 with their long diameter parallel to the face of the leaf, 

 and are very loosely arranged, leaving many and wide air 

 chambers. The green color of the lower is therefore 



381. Magnified section of a leaf of White Lily, to exhibit the cellular struc- 

 ture, both of upper and lower stratum, the air passages of the lower, 

 and the epidermis in section; also a little of the lower face, with 

 some of its stomates. 



diluted, and paler than that of the upper face of the leaf. 

 The upper part of the leaf is so constructed as to bear the 

 direct action of the sunshine ; the lower so as to afford 

 freer circulation of air, and to facilitate the escape of mois- 

 ture. It communicates more freely than the upper with 

 the external air by means of pores in the epidermis. 



526. The upper cylindrical cells are known as the pali- 

 sade cells. The lower, irregular, or sometimes slightly 

 branching cells make up tlie sj^ongi/ parenchyma^ so called. 



527. The epidermis is usually composed of a single layer 

 of more or less flattened cells, devoid of chlorophyll, and 

 mostly of irregular outline (Figs. 382, 383). 



The walls of the epidermis are commonly thickened 

 externally by the addition of a layer of a waterproof sub- 

 stance. This layer is easily distinguished in the cross 

 section from the original exterior walls of the cells. It 

 is termed the cuticle. The several walls of each epider- 

 mal cell are impregnated with the same waxy or fatty 



