PECAN ROSETTE. 3 



arrangement of the tissues, a poorly developed palisade tissue of 

 one-cell layer, three layers of sponge cells, and an epidermis of 

 large thin- walled cells with delicate cuticle. On the other hand, 

 when grown in dry sand or in undrained sphagnum bogs where the 

 soil temperature was several degrees below that of the air the leaves 

 were thickened, reduced in size, and revolute margined. The meso- 

 phyll tissue was more compact, with two or three layers of palisade, 

 and two layers of sponge cells. The epidermal cells were smaller, 

 with outer walls and cuticle thickened. In addition to the develop- 

 ment of these other xerophilous characters, drops of oil or resin, 

 characteristic of bog plants, were formed on the epidermis and cells 

 adjacent to the bundles; these are absent under moisture conditions . 

 more favorable for this species. 



Warming (82) states that in acid soils intimately associated with 

 high water content, in a cold or temperate climate, the tendency of 

 plants is toward the development of leaf coatings of hairs, papillae, 

 or wax; thickened cuticle; mucilage; erect and cricoid, terete, or 

 filiform leaves; with bilateral internal structure. Since these char- 

 acters develop on wet, moor soils the world over, he considers that 

 there must be a connection between these soils and the xeromorphic 

 structure, and that consequently these soils must be " physiologically 

 dry." These facts also account for the xeromorphic structure of 

 plants in the extreme north or at high altitudes. -I 



The experimental results obtained by Mrs. Clements (23) show 

 that the xerophyte tendency is toward the development of a diplo- 

 phyll palisade (bilateral) tissue with restricted air spaces and with 

 or without water-storage cells. This prolate closely packed type of 

 cell tends to reduce transpiration. The mesophyte type, on the other 

 hand, approximates an equal development of palisade and sponge 

 cells with moderate looseness of structure. The hydrophyte type 

 consists in the development of simple globose cells and large air 

 spaces. She found that decreased light and increased water absorp- 

 tion caused an increase of leaf surface but a decrease in thickness, 

 while increased light and decreased water absorption brought about 

 a reduction in leaf surface but an increased thickness. Extremes 

 of any factors not at the optimum tended toward dwarfing. 



Hanson (39) found differences in total thickness between leaves 

 from the south periphery and the center of the same tree usually 

 greater than the differences hitherto reported between leaves of meso- 

 phytic and xerophytic forms of a species. Leaves from the south 

 periphery, as a whole, developed more palisade, greater compactness 

 of structure, and thicker epidermis and cuticle than leaves from 

 within the crown. 



Halophytes, or " salt-loving plants." usually develop thick, fleshy 

 leaves which are more or less translucent, owing partly to the abun- 



