6 LIGHT IN RELATION TO TREE GROWTH. 
terial assistance to students of the subject, and it is hoped that it may 
also stimulate an interest to further research in determining more 
accurately the light requirements of our forest trees, especially by 
actual measurements of light in the forest. 
KINDS OF LIGHT. 
DIRECT AND DIFFUSED LIGHT. 
In discussing light with reference to tree growth a distinction must 
be made between direct sunlight and diffused light. If the earth 
were not enveloped by an atmosphere, all hght would be direct sun- 
light, and its intensity at every point on the earth could be calcu- 
lated mathematically from the position of the sun. The presence of 
the atmosphere, however, modifies essentially the distribution of 
light and heat on the earth’s surface. Only a part of the light which 
emanates from the sun reaches the earth as direct sunlight. An- 
other part is reflected from the small particles contained in the air, 
such as dust and minute drops of water, and forms diffused light. 
Still another part is absorbed and entirely disappears. The diffused 
hght illumines the atmosphere and forms the skylight. When the 
sky is cloudless the total daylight consists of both direct and diffused 
hght. On cloudy days all the light is diffused light. 
Trees in the forest make use chiefly of the diffused skylight, and 
for this reason it plays the most important part in their life. Indeed, 
many plants have developed special contrivances for protecting them- 
selves from the direct rays of the sun. There are, however, trees and 
other plants which, in addition to diffused light, need direct light 
either during their entire life or during a definite period of their life, 
as, for instance, during the period of fiowering and leafing. Thus, 
the opening of the buds in many trees proceeds much faster when 
the tree is exposed not only to diffused light, but also to the direct 
rays of the sun. Therefore, in determining the effect of light on 
vegetative processes it may be essential to know what portion of the 
entire light affecting the tree is diffused and what is direct. 
Both direct sunlight and diffused ight decrease with increase in 
latitude, but not in the same proportion. The intensity of direct 
sunlight decreases much more rapidly with increase in latitude than 
does the intensity of diffused or sky light. This is well brought out 
in Table 1, computed on the basis of measurements taken by Bunsen 
and Roscoe, which gives the relative chemical intensities of the radia- 
tion received directly from the sun and from the sky upon a horizon- 
tal surface during a whole day at the spring equinox. It is of 
considerable interest as showing what kind of lght.is mostly avail- 
able for tree growth at various latitudes. 
