EXTERNAL CONDITIONS ON PLANT LIFE. 177 



are even more marked than the corresponding differences in 

 position. Plate No. 13 shows two sections of leaves from 

 Rhododendron maximum, the one from the sunlight, the other 

 from the shade. Section a, from the sun, shows four super- 

 imposed rows of palisade cells, extremely long and narrow, 

 arranged with their long diameters pointing to the surface of 

 the leaf in a way to afford the best protection from the heat 

 rays. Section b, from the shade, has no very well marked 

 palisade system, for no such protection is needed. The section 



PLATE No. 12. 



from the sun is much thicker than that from the shade, but 

 while the loosely arranged mesophyll in the under part of the 

 leaf, where assimilation is most active, occupies in the sun 

 section only one-half of the space of the whole leaf, yet in the 

 shade section it includes more than two-thirds. The explana- 

 tion is simple. The exposed leaf requires less space for the 

 work of assimilation on account of the strong light. The shaded 

 leaf, on the contrary, as much less light reaches it, must have a 

 greater number of cells in which to do a given amount of work. 

 (4) The food supply of the plant may exert a very strong 

 influence upon its development and life. It is a well-known 



