FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 41 
C. Leaf protection 
34. Need of protection. Such an important organ as 
the leaf, with its delicate active cells well displayed, is ex- 
posed to numerous dangers. Chief among these dangers 
are intense light, drought, and cold. All leaves are not 
exposed to these dangers. For example, plants which grow 
in the shade are not in danger from intense light ; many 
O .s. water plants are not in danger 
from drought ; and plants of 
the tropical lowlands are in no 
FIG. 31. Sections through leaves of the same plant, showing the effect of exposure to 
light upon the structure of the mesophyll. In both cases os indicates upper surface, 
and us under surface. In the section at the left the growing leaf was exposed to 
direct and intense sunlight, and, as a consequence, all of the mesophyll cells have 
assumed the protected or palisade position. In the section at the right the leaf was 
grown in the shade, and none of the mesophyll cells have organized in palisade 
fashion. After STAHL. 
danger from cold. The danger from all these sources is be- 
cause of the large surface with no great thickness of hody, 
and the protection against all of them is practically the 
same. Most of the forms of protection can be reduced 
to two general plans : (1) the development of protective 
structures between the endangered mesophyll and the air ; 
(2) the diminution of the exposed surface. 
35. Protective structures. The palisade arrangement of 
mesophyll may be regarded as an adaptation for protection, 
