174 PLANT PHYSIOLOGY AND ECOLOGY 



shade, but represents an ancestral feature that has persisted in 

 spite of the change. The difference in the amount of light received 

 by 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 may in consequence be termed isophotic. 

 Leaves that stand more or less at right angles to the stem usually 

 receive much more light upon the upper than upon the lower 

 surface. They really receive two intensities of light, and may 

 accordingly be termed dvphotic. Certain horizontal or dorsiventral 

 leaves, however, absorb nearly or quite as much light on the lower 

 side as on the upper. This is true of sun leaves with a dense 

 hairy covering, which screens out the greater part of the light 

 falling upon the upper surface. It occurs in some degree also in 

 xerophytes which grow in light-colored sands and gravels that 

 serve to reflect the sun's rays upon the lower side of the leaves. 

 In deep shade, moreover, there is little or no difference in the 

 strength of the light received by the two surfaces, and shade 

 leaves are often isophotic in consequence. Isophotic leaves are 

 typical of shaded habitats, but they occur also in the sun. Dipho- 

 tic leaves are found chiefly in sunshine, but the diphotic structure 

 often persists in shade that is not too dense. It should also be 

 noted that the isophotic sun leaf consists chiefly or entirely of 

 palisade tissue, while the isophotic shade leaf is similarly composed 

 of sponge tissue. Thus, while the direction of the light determines 

 whether a leaf shall be isophotic or diphotic, its intensity determines 

 the kind of tissue formed. 



192. Sponge tissue. All of the cases just cited make it fairly 

 clear that sponge tissue is developed primarily to increase the 

 light-absorbing surface. It is found practically without exception 

 in all leaves where the light is diffuse, regardless of the cause 

 of the latter. The leaves of shrubs and herbs which grow regu- 

 larly in forests consist largely or entirely of sponge tissue. The 

 interior leaves of the foliage of trees and shrubs contain much 

 more sponge than the leaves of the same plant which are exposed 

 to the sun. Sun species transferred to the shade usually change 

 much or all of their palisade tissue into sponge. This is also 

 true of the leaves of amphibious or floating species that become 

 submerged, while the leaves of submerged plants consist entirely 

 of sponge or sponge-like cells. Further evidence of the develop- 

 ment of sponge tissue in consequence of reduced light intensity 



