238 LEAVES 



as a function of the chlorenchyma than it is to refer to it 

 as a function of the leaves? 



Mesophyll is quite distinctly differentiated into palisade 

 tissue and spongy tissue. (Study Figure 79, page 220.) It is 

 believed that the palisade tissue has more to do with photo- 

 synthesis, and the spongy tissue more to do with relations to 

 the air. Evidently, on account of their positions, the palisade 

 is better related to light, and the spongy tissue better re- 

 lated to air. 



a. Palisade. In an ordinary leaf the palisade is on the 

 upper and the spongy tissue on the under side. Some ver- 

 tical leaves have palisade on both sides. The common wild 

 lettuce develops palisade on both sides in intense light, on 

 only one side in diffuse light, and in dense shade it de- 

 velops none at all. Should we be justified in concluding 

 from this that as a general rule the development of pali- 

 sade depends upon light ? Surely not. In the first place 

 the wild lettuce may be quite an exception among plants 

 in this matter, and in the second place the behavior noted 

 may have been due to other conditions than the differences 

 in light alone, as, for example, differences in the amount of 

 transpiration. Under ordinary conditions of plant growth, 

 transpiration increases as light increases, and decreases also 

 about as light decreases. 



It has also been noted that the palisade tissue is much 

 more extensively developed in dry than in wet soil, the 

 exposure to light being equal in both cases. Moisture 

 evidently has an effect upon the development of this tissue 

 as well as light. The greatest development of palisade 

 tissue is in plants which grow in deserts, where intense light 

 goes along with danger from too much evaporation. 



So it appears that excess of light and excess of transpi- 



