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- 
Fic. 19.—Section througa 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 Fic. 20.—Section through a small portion 
same plant exposed to ex- of carnation leaf, showing the epider- 
treme conditions of light 
and slade. The intense 
light is dangerous to the 
chloroplasts, and it has 
been observed that they are 
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. 
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. 
