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 gases directly 
from the air. 
180. The differentiation of the chlorenchym. The division of the 
chlorenchym into two tissues, sponge and palisadé, is the normal conse- 
quence of the unequai illurnination 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 diffuse light. The difference in 
the illumination of the 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 
Fig. 42. Diphotophylls of Quercus nowi- ; ; 
mexicana: 1, sun leaf; 2, shade leaf of the same xerophytes which grow in 
tree (light .06). >< 130. 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 tisstie, which in the shade leaf is due to the diffuse light of the 
habitat, is produced in the hairy leaf as a consequence of the absorption and 
diffraction of the light by the covering. In ordinary diphotic leaves, the 
