PLANT STUDIES 
but it usually occurs, and does not necessarily imply ex- 
treme conditions of any kind. However, if the cells of the 
palisade tissue are unusually narrow and elongated, or 
p 
FIG. 32. Section through a portion of the leaf of the yew (Taxus), showing cuticle 
(c), epidermis (e), and the upper portion of the palisade cells (p). 
form two or three layers, we might infer the probability of 
exposure to intense light or drought. The accompanying 
illustration (Fig. 31) shows in a striking way the effect of 
light intensity upon the structure of the mesophyll, by 
contrasting leaves of the same plant exposed to the extreme 
conditions of light and shade. 
The most usual structural adaptations, however, are 
connected with the epidermis. The outer walls of the epi- 
dermal cells may become thickened, sometimes excessively 
so; the other epidermal 
walls may also become 
more or less thickened; 
or even what seems to 
be more than one epi- 
dermal layer is found 
protecting the meso- 
phyll. If the outer 
walls of the epidermal 
cells continue to 
thicken, the outer re- 
gion of the thick wall 
loses its structure 
and forms the cuticle, 
which is one of the 
FIG. 33. Section through a portion of the leaf of 
carnation, showing the heavy cuticle (cu) 
formed by the outer walls of the epidermal 
cells (ep). Through the cuticle a passageway 
leads to the stoma, whose two guard-cells are 
seen lying between the two epidermal cells 
shown in the figure. Below the epidermal 
cells some of the palisade cells (pal) are shown 
containing chloroplasts, and below the stoma 
is seen the air chamber into which it opens. 
