FOLIAGE LEAVES : FUNCTION, STRUCTURE, ETC. 41 



C. Leiif 2»'ok'ctivn. 



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 

 as. water plants are not in danger 



from drought ; and plants of 

 the tropical lowlands are in no 



U.S. li.^. 



Fig. 31. Sections tlirougli leaves of the same plant, showing the efEect of exposure to 

 light upon the structure of the mesophyll. In both caj^et^ os indicates upper surface, 

 and «s 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 body, 

 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, 



