LEAVES 



25 



active mesophyll cells would soon lose all their water. In 

 some plants of very dry regions, what may be regarded as 

 several epidermal layers appear. 



(2) The palisade layer of the mesophyll ( 13) also is 

 very commonly present and tends to diminish transpira- 



FIG. 19. Section through a small portion of yew leaf, showing the epidermal 

 layer (e) with its cuticle (c), and the upper portion of the palisade layer (p). 



tion, exposing only the ends of elongated cells, which stand 

 so close together that there is no drying air between 

 them (Fig. 12). It is very characteristic of alpine and 

 desert plants to form two or three layers of palisade cells, 

 apparently as a protection 

 against unusual danger from 

 drought and intensity of 

 light. The accompanying 

 figure (Fig. 18) shows in a 

 striking way the effect of 

 light intensity upon the 

 structure of mesophyll, by 

 contrasting leaves of the 

 same plant exposed to ex- 

 treme conditions of light 

 and shade. The intense 

 light is dangerous to the 

 chloroplasts, and- it has 

 been observed that they are 



able to assume various positions, in very intense light mov- 

 ing to the more shaded depths of the palisade cells, and in less 

 intense light moving to the more external regions of the cell. 



FIG. 20. Section through a small portion 

 of carnation leaf, showing the epider- 

 mal cells (e) with their heavy cuticle 

 (c); a single stoma in the epidermal 

 layer, opening without into a broad 

 passageway through the cuticle, and 

 within into an air-chamber ; and the 

 upper portion of palisade cells (p) con- 

 taining chloroplasts. 



