pO EDITH SCHWARTZ CLEMENTS 



mits the fewest intercellular spaces and hence reduces harmful 

 transpiration where the water-supply is limited. Moreover, as 

 noted by Heinricher and brought out in this paper, the isophotic 

 structure of leaves is incompletely present in those which grow in 

 wet sunny habitats. In such cases the ancestral type is being 

 modified by wetter conditions. Heinricher has also observed that the 

 vertical leaves of sunny swamp plants and water plants are not com- 

 pletely isolateral. This indicates that dryness is a necessary factor. 

 The layer of palisade on the under side of these leaves has been 

 explained as due to reflected light in the case of horizontal leaves 

 and to either incident or reflected light for those vertically placed. 

 It is worthy of note that the species covered in this paper, which 

 have isophotic leaves, whether of the staurophyll or diplophyll type, 

 are not generally vertical. As a rule, however, the diplophyll with 

 internal sponge, such as is characteristic of the Agoseris group, is 

 typical of vertical leaves in moist situations, while the diplophyll 

 with interior water-storage tissue (Solidago, Mertensia, etc.), and 

 the staurophyll, whether with (Helianthus} or without (Pentste- 

 mon} water-storage cells, include both vertical and horizontal leaves 

 and are characteristic of habitats with high light values and dry- 

 ness of soil, connected as a rule with reflected light and heat. 



Between the two extremes of physical factors and of leaf struc- 

 ture lie the sun and shade mesophytes which furnish examples of 

 all intergrades. A typical mesophyll unites equal percentages of 

 palisade and of sponge tissues with moderate looseness in the cell- 

 arrangement. The amount and combinations of the physical factors 

 for any mesophyte can be very nearly approximated by a study of the 

 amount and character of the chlorenchym cells, r 



Experiments with respect to sun-leaves and shade-leaves have 

 been numerous. Dufour's have led to the conclusions that sun- 

 leaves are larger and better developed in every way than shade- 

 leaves, and that the size of shade-leaves as generally observed is 

 due to water-content. His results are not convincing, since his 

 light and shade experiments were made with typical sun-plants. 

 These under normal conditions would doubtless thrive and show 

 better development in every way than when grown under abnormal 

 light conditions. Also, in proving that it is water and not shade 

 which causes development in leaf surface, he has used extremes of 

 water-content, ignoring the fact that extreme conditions dwarf 



