. Research methods in ecology. Plant ecology. 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 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 palis
. Research methods in ecology. Plant ecology. 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 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 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 â as much light on the lower side as upon the upper. This is true of sun leaves with a dense hairy covering, which screens â¢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
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