CLIMATIC FACTORS : RADIANT ENERGY 



139 



type in full sunlight, probably because of the favorable water bal- 

 ance maintained by the extensive root system of the tree. Certain 

 advantages of shade leaf development are more obvious than the 

 causes. 



In strong light, cells elongate parallel to the light source. The 



FlG. 70. The anatomical characteristics associated with so-called sun and 

 shade leaves of two chaparral species. (A) Arctostaphylos tomentosahom nor- 

 mal xeric habitat, (B) from mesic habitat. (C) Adenostoma jasciculatum from 

 normal xeric habitat, (D) from stump sprout. Note differences in thickness of 

 leaf and cuticle, and proportion of palisade to sponge tissue.— From Cooper™ 



more intense the light, the deeper its penetration into the leaf and 

 the more layers of palisade there will be. Desert and alpine plants 

 may have the mesophyll entirely made up of palisade cells. Leaves 

 subject to reflected light from below commonly have palisade on 

 the lower surface as well as the upper, and leaves growing ver- 

 tically regularly have palisade on both sides. 



When illumination is intense, chloroplasts arrange themselves 

 along the side walls, and thus in palisade cells they receive a mini- 

 mum of direct insolation. On the other hand, with weak light the 

 chloroplasts tend to appear along the walls at right angles to the 

 light source, and the form of sponge cells permits exposure of 

 more chloroplasts to the greatest effectiveness of available light. 



