538 
HERBERT C HANSON 
measured in the same positions in the crown or at the south periphery 
of each tree. Cytological material was collected from leaves growing 
in these positions respectively. Pieces from about twenty leaves from 
each position were killed in chromo-acetic acid and in Juel's killing solu- 
tions. The material from Juel's solution gave the best specimens for 
study. The ordinary paraffine method was used in preparing the 
material. 
The response of the leaves to the environmental factors was shown 
in transpiration; in the green and dry weights and water content of 
given leaf areas; in the thickness of the leaf and its parts, palisade, 
sponge, upper and lower epidermis and the cuticle; in the compactness 
of the tissues; in the structure of individual cells; and in the macro- 
scopic characters, as area and lobing. 
The Physical Factors 
I. Light 
The light was measured by means of the Clements' photometer 
between ii A.M. and 2 P.M. in August, 1915. Four readings were 
taken within the crown of isolated trees and among the lowest leaves 
of the forest trees for four or five individuals of ten species. From 
these 16 to 20 readings for each species the light values, as arranged in 
the following table, were averaged. 
Table i 
Shoiving the Light Values in the Crown of Isolated Trees and at the Base of 
Forest Trees 
Position of 
Light Value in Crown 
Species 
Trees* 
or at Base 
0.0175 — crown 
Forest 
0.0076 — base 
0.0688 — crown 
Isolated (L.) 
0,1000 — crown 
Forest 
0.0086 — base 
Quercus macrocarpa Michx 
Isolated 
0. 1 1 32— crown 
Forest 
0.0754 — base 
0.0425 — base 
0.0100 — base 
Acer saccharinum L 
Isolated 
0.0406 — crown 
Isolated (L.) 
0.0380 — crown 
Acer negundo L 
Isolated 
0.0979 — crown 
0.0770 — crown 
Fraxinus pennsylvanica Marsh, 
0.0497 — crown 
Celtis occidentalis L 
Isolated (L.) 
0.0591 — crown 
* Trees marked (L.) in Lincoln, all others in Minneapolis. 
