10 LIGHT IN RELATION TO TREE GROWTH. 
in the shade where the light is only 1/70 of the total daylight. In the 
Tropics the last vestige of green vegetation disappears from under 
the -tree cover when the minimum light intensity falls to 1/120 of 
the total daylight. 
The minimum intensity of hght which is needed for the formation 
of chlorophyll is much smaller than the minimum required for photo- 
synthesis. Thus, while the green leaves of Acer platanoides cease to 
break up the carbon dioxide of the air and form organic substance 
in a light whose intensity is 1/55 of the total daylight, the forma- 
tion of chlorophyll still takes place in a hght which is 1/400 of the 
total daylight; in some herbaceous plants, such as peppergrass 
(Lepidium sativum), the formation of chlorophyll still goes on at a 
light intensity of 1/2,000, while in light whose intensity is 1/50 of 
the total daylhght the leaves are just as green as in higher light 
intensities. 
The decomposition of carbon dioxid is produced chiefly by the rays 
of the red portion of the spectrum, and the leaves in the interior of 
the crown are able to assimilate, because the largest part of the light 
which they receive penetrates not through the green leaves, which 
absorb the red rays, but through the places between the foliage. 
Chlorophyll may form in light which lacks the rays of the red 
portion of the spectrum. For this reason light which goes through 
the green foliage may still be effective in producing the green color 
of the leaves inside the crown, but is no longer effective in bringing 
about the process of assimilation of carbon. 
MAXIMUM INTENSITIES. 
In general the intensity of light varies directly with the height of 
the sun. The maximum light intensity, however, does not coincide 
with the intensity at noon, which is always less than the maximum. 
Thus, from measurements made at Vienna, the ratio of the average 
noon light intensity to the average maximum light intensity for the 
year was found to be as 1:1.08. This is analogous to the well-known 
fact that the highest temperature during the day occurs not at noon, 
but later, and is probably due to the same causes. 
The maximum light available for tree growth is, of course, the total 
daylight. This varies, as has already been pointed out, with latitude, 
altitude, and the configuration of the earth. 
The highest light intensities found by Wiesner (1905) in the 
United States were in Yellowstone Park. Thus, at Norris, on Sep- 
tember 1, 1904, at 1 p. m., with the sun at an altitude of 52° 56’, the 
chemical intensity of the light was found to be 1.7 in Bunsen-Roscoe 
units.t At Old Faithful, on September 4, 1904, at noon, angle of sun 
1 Bunsen-Roscoe unit of chemical light intensity is the amount of light required to pro- 
duce a standard color in one second on standardized sensitive paper. 
