ADAPTATION TO LIGHT 175 



is furnished by diphotic leaves and those with hairy coverings. 

 In ordinary diphotic leaves the absorption of sunlight by the 

 chloroplasts of the palisade cells reduces the intensity to such a 

 degree that the plastids of the lower half of the leaf are in diffuse 

 light. In consequence, the cells that contain them become 

 elongated or oblate, and form sponge tissue. In this case the 

 latter is just as truly an adaptation to diffuse light as in the pre- 

 ceding, where the whole chlorenchym is in the shade of other 

 leaves or plants. A cover of hairs reflects and absorbs the greater 

 part of the light which falls upon the leaf, and thereby changes 



Fig. 59. Leaf of a bog orchid, Gyrostachys stricta, in which the chlorenchym 



consists wholly of sponge cells. 



the interior into sponge tissue. Consequently, lanate sun leaves 

 usually possess chlorenchym essentially like that of shade leaves. 



From the preceding it seems clear that sponge tissue serves 

 primarily to increase the light-absorbing or chlorophyll surface 

 under all conditions that make such an increase beneficial. In 

 addition, it is intimately connected with aeration, largely owing 

 to the fact that it is in contact with the lower epidermis, which 

 usually contains the larger number of stomata. Indeed, the 

 spongy nature of this tissue is due to the presence of air spaces, 

 which are the means of carrying on effective aeration. This point 

 is nicely brought out by submerged leaves, in which the form of 

 the cells and the arrangement of the plastids are those of sponge 

 tissue. The usual air spaces are altogether lacking, however, 

 since the air is now obtained in solution in the water. Finally, 

 the abundance and size of the air spaces in the sponge makes the 

 latter subject in some degree to the modifying influence of tran- 



