176 



PLANT PHYSIOLOGY AND ECOLOGY 



spiration. The fact that light is the most potent factor in pro- 

 ducing sponge tissue is proved by the uniform occurrence of the 

 latter in the lower half of the leaf, when the stomata are more 

 numerous upon the upper surface. 



193. Palisade tissue. The formation of palisade tissue is 

 brought about in response to sunlight or to low water content. In 

 very many cases the two act together and produce a striking 



Fig. 60. Cross-section of leaves of the sun (1) and shade (2) form of a 

 sunflower, Helianthus pumilus, in which the chlorenchym consists 

 entirely of palisade tissue. 



modification. In amphibious and floating plants, and in many 

 mesophytes where the water supply is adequate, the palisade is 

 due to the action of light. On the other hand, it is possible to 

 increase the number of rows of palisade cells in the leaves of most 

 sun plants by growing them in drier conditions. As a consequence, 

 it is a difficult task to decide which factor is the more important 

 in producing this tissue. When it is recalled that the light energy 

 is almost entirely used in causing evaporation from the plastids, 



