METHODS OF DETERMINING TOLERANCE, 49 
photographic paper—that is, the ultraviolet, violet, and blue rays— 
are absorbed by the leaves first of all. 
It is evident, therefore, that the hght intensities in the forest meas- 
ured by the photochemical method are necessarily too low. Further- 
more, the composition and intensity of light in the forest are influ- 
enced by the character of the bark of the tree and of the under sur- 
faces of the leaves. 
SPECTROSCOPIC MEASUREMENT OF LIGHT INTENSITY. 
All these facts tend to show that the photochemical method of 
measuring light in the forest determines only that the denser the for- 
est canopy and the fewer the openings in it, the less will be the chem- 
ical intensity of light in the forest. The more light demanding is 
the species, the more open is its crown, the more will be the chemi- 
cal intensity of light under its shade. The photochemical method, 
however, apparently can not establish the absolute amount of light 
which the green leaf utilizes. There were, therefore, many criticisms 
of measuring light in the forest merely by this method. It was sug- 
gested that light measurement in the forest should be qualitative as 
well as quantitative; in other words, that it 1s necessary to know not 
only how much light in general, but how much of each kind of light 
a plant receives. 
ZEDERBAUER’S INVESTIGATIONS. 
The most recent attempt in measuring separately the intensity of 
the different rays of the solar spectrum was made by Dr. Zederbauer 
(1907; 825-830) of the Austrian forest experiment station at Maria- 
brunn. Since individual green leaves, as is now well established, 
exercise a selective power of absorption, he concludes that the tree 
crown, merely an aggregation of leaves, should show a similar if not 
identical absorption. It would seem, therefore, that the light in the 
forest must be of a different composition from that in the open. 
This conclusion is apparently verified by his observations with the 
spectroscope. He finds that the leaf canopy exerts a selective power 
of absorption similar to that of individual leaves, and that the degree 
and kind of absorption varies with the species. Thus the absorption 
in a beech stand is found to be different from that in an oak or a 
pine stand. Common to all species is the absorption between the 
Frauenhofer lines B and C in the red portion of the spectrum, also 
in the blue portion approximately at the line F, and in the violet 
beyond H. Some species absorb practically all of the rays between 
F and H (blue-violet), while others allow most of the indigo rays 
to pass through unabsorbed. He finds that the species generally 
