142 



THE PLANT 



shade plants, moreover, the decrease in the number is compensated in some 

 measure by the ability of the epidermal cells to absorb g-ases directly 

 from the air. 



180. The differentiation of the chlorenchym. The division of the 

 chlorenchym into two tissues, sponge and palisade, is the normal conse- 

 quence of the unequal illumination of the leaf surfaces. Exceptions to this 

 rule occur only in certain monocotyledons, in which the leaf tissue consists 

 of sponge-like cells throughout, and in those stable species that 'retain more 

 or less palisade in spite of their change to dififuse light. The difference in 

 the illumination of tlie two surfaces is determined by the position of the 

 leaf. Leaves that are, erect or nearly so usually have both sides about equally 

 illuminated, and they may be termed isophotic. Leaves that stand more 



or less at right angles to the 

 stem receive much more light 

 upon the upper surface than 

 upon the lower, and may ac- 

 cordingly be termed diphotic. 

 Certain dorsiventral leaves, 

 however, absorb practically as 

 much light on the lower side 

 as upon the upper. This is 

 true of sun leaves with a dense 

 hairy covering, which screens 

 out the greater part of the 

 light incident upon the upper 

 surface. It occurs also in 

 xerophytes which grow in 

 light-colored sands and gravels 

 that serve to reflect the sun's 

 rays upon the lower surface. In deep shade, moreover, there is no essential 

 difference in the intensity of the light received by the two surfaces, and 

 shade leaves are often isophotic in consequence. From these examples it 

 is evident that isophotic and diphotic leaves occur in both sun and shade, 

 and that the intensity of the light is secondary to direction, in so far as the 

 modification of the leaf is concerned. 



The essential connection of sponge tissue with diffuse light is conclu- 

 sively shown by the behavior of shade ecads, but further evidence of great 

 value is furnished by diphotic leaves, and those with hairy coverings. The 

 sponge tissue, which in the shade leaf is due to the dift'use light of the 

 habitat, is produced m the hairy leaf as a consequence of the absorption and 

 diffraction of the light by the covering. In ordinary diphotic leaves, the 



Fi^. 42. Diphotophylls of Quercus novi- 

 niextcana: 1, sun leaf; 2, shade leaf of the same 

 tree (light .06). X 130. 



