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 ericoid, 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. 
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- 
