62 A TEXTBOOK OF GENERAL BOTANY 

Fig.45. Cross section of a leaf of oleander 
(Nerium) with upper and lower palisade, three- 
layered upper epidermis, and stomata in a pit 
protected by trichomes. ( x 285) 
entirely (Fig. 450). In 
dry regions the reverse 
is the case. Here the 
spongy chlorenchyma 
is usually less conspic- 
uous than the palisade. 
There may be a thin 
layer of spongy chlo- 
renchyma near the 
lower epidermis; or 
the layer of spongy 
chlorenchyma may be 
between two layers of 
palisade (Fig. 45), one 
below the upper epi- 
dermis and the other 
just above the lower 
epidermis; or, again, 
the spongy chloren- 
chyma may be entirely 
replaced by palisade 
chlorenchyma. Note 
the difference in the de- 
velopment of the pali- 
sade and the spongy 
chlorenchymaas shown 
in Fig. 35 and also in 
Figs. 24 and 26. The 
relative development 
of the palisade and the 
spongy chlorenchyma 
may be suited not only 
to various rates of evaporation but also to various intensities of 
light. In damp ravines the light is usually of low intensity, and 
in such situations not much light would be likely to pass through 
thick layers of tissue. On the other hand, in dry regions the 
