=180. The differentiation of the chlorenchym.= The division of the chlorenchym into two tissues, sponge and palisade, is the normal consequence 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 accordingly 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 conclusively 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 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 absorption of light in the palisade reduces the intensity to such a degree that the cells of the lower half of the leaf are in diffuse light, and are in consequence modified to form sponge tissue. The sponge tissue of the diphotic leaf is just as clearly an adaptation to diffuse light as it is in those plants where the whole chlorenchym is in the shade of other plants or of a covering of hairs. As is indicated later, all these relations permit of ready confirmation by experiment, either by changing the position of the leaf or by modifying the intensity or direction of the light. [Illustration: Fig. 43. A plastic species, _Mertensia polyphylla_, showing the effect of water upon the sponge: 1, chresard 25%; 2, chresard 12%. × 130.]