14 LIGHT IN RELATION TO TREE GROWTH. 
exposed to hight of various intensities and tested for the presence of 
starch. At a hght intensity of 1/6 of total daylight the leaves of 
beech showed distinctly the presence of starch, while birch gave a less 
distinct reaction. At a light intensity of 1/10 beech still gave a pro- 
nounced reaction for starch, while in birch only traces of starch could 
be detected. At a lght intensity of 1/50 birch leaves showed no 
presence of starch, while the leaves of beech still continued vigorously 
to form it. These facts tend to show the same point brought out by 
Lubimenko, that there is a distinct difference in the sensitiveness of 
the chloroplasts of beech and birch. 
FACTORS INFLUENCING TOLERANCE. 
Tolerance varies not only with species, but even within the same 
species, according to conditions under which the tree is growing. 
These variations are due largely to changes in the structure of the 
leaves brought about by changes in transpiration. Among the impor- 
tant factors influencing transpiration, and therefore tolerance, are 
climate, altitude, moisture and nourishment in the soil, age of tree, 
and vigor and origin of the individual. 
CLIMATE. 
Plants need less light the higher the temperature and more light 
the lower the temperature. Consequently the higher the temperature 
of a given locality, the more shade a tree can stand. This explains, 
in part, the frequent differences of opinion regarding the tolerance of 
the same species when the observations are taken in different regions. 
For example, white pine in Maine is less tolerant than in the Southern 
Appalachians. In Vienna the minimum lght intensity in which the 
leaves of Norway maple (Acer platanoides) can exist is 1/55 of the 
total daylight. In Hamar, Norway, it is 1/37; in Drottningholm, 
1/28; and in Tromso, 1/5. 
This substitution of light for heat in plant growth, and vice versa, 
was well demonstrated by Wiesner (1907) with regard to annual 
meadow grass (Poa annua). The minimum light necessary for 
annual meadow grass at the beginning of March, at Cairo, is equiva- 
lent to about 53 calories, while at Vienna it is equivalent to 109. The 
mean temperatures are 59.9° and 35.6° F., respectively. 
ALTITUDE. 
The light requirement of a species increases also with increase in 
altitude, but only to a certain limit, beyond which it remains con- 
stant or even decreases. For example, observations by Wiesner 
showed that lodgepole pine, at an elevation of 6,400 feet, required a 
