ADJUSTMENT TO WATER 



57 



cross-section of a leaf shows the three primary regions, epidermal, 

 cortical, and vascular. Each of these is modified in a manner 

 characteristic of the leaf. In the epidermis this modification is 

 the stoma, in the cortical region the differentiation of the green 

 tissue or chlorenchym, and in the vascular region the great reduc- 

 tion and division of the bundles. The upper and lower epidermis 

 of a leaf are very similar. In the vast majority of leaves, they 

 consist of a single layer of cells. Because of a difference of expo- 

 sure, the tendency of the upper epidermis is to develop cutinized 



Fig. 14. Cross-section of the leaf of the monks-hood, Aconitum colum- 

 hianum, showing the palisade tissue above and the sponge tissue 

 below. 



hairs, and that of the lower to develop stomata. Thus while hairs 

 and stomata often occur on both surfaces of a leaf, hairs are 

 often more numerous upon the upper surface than upon the lower, 

 while stomata are regularly more numerous upon the lower. In 

 many leaves, hairs are found only upon the upper epidermis, 

 while in others, stomata occur only upon the lower. The epi- 

 dermal cells proper have their outer walls cutinized and usually 

 thickened also. In these likewise the outer wall or cuticle is 

 generally thicker on the upper surface than upon the lower. 



73. The chlorenchym. As a rule, the cortical region of the 

 leaf consists wholly of parenchyma cells filled with chloroplasts. 

 From its nature, this tissue is called chlorenchym. The latter 

 comprises two distinct parts, viz., the palisade parenchyma and 

 the sponge parenchyma. In the normal leaf, the palisade tissue 

 occurs in the upper half, and the sponge tissue in the lower half. 

 The probable causes of this differentiation are discussed under 

 Light. At present it is sufficient to point out that the position 

 and development of these two tissues are directly connected with 

 differences in the degree of exposure to light and liumidily 



