49, LIGHT IN RELATION TO TREE GROWTH. 
out the day. In densely foliaged trees in which the foliage has a. 
more or less horizontal position, as a rule the ight intensity in the 
interior of the crown at noon, or when the sun is at its highest alti- 
tude, is greatly reduced, since the position of the leaves prevents to 
a large extent the entrance of the vertical rays. In such trees the 
greatest light intensity within the interior of the crown occurs in the 
forenoon and in the afternoon when the sun is in the east and west— 
that is, when the light penetrates into the interior of the crown, not 
from above, but from the side. The light intensity at noon, there- 
fore, lies between two maxima, which occur in the forenoon and the 
afternoon. In thinly foliaged trees of species, such as locust, whose 
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Fic. 8.—Maximum and minimum light intensities within crowns of trees with different 
arrangement of foliage. a, Tree with horizontal arrangement of foliage, in which noon 
light intensity lies between two maxima; 6b, tree with vertical foliage in which the 
maximum illumination of the crown occurs at noon. 
leaves avoid the midday light by assuming a profile position, a maxi- 
mum intensity may be attained at noon. In trees of the same spe- 
cies which have a dense foliage, however, only the uppermost leaves 
assume a profile position at noon, and the relative light intensity 
within the interior of the crown, therefore, remains more or less 
constant during the day. (See fig. 8.) This difference in the occur- 
rence of maximum and minimum light intensities within the tree 
crowns during the day is clearly brought out in the diagrams (figs. 9 
and 10), which represent actual hourly measurements of light inten- 
sities in the interior of the crown of ailanthus (Adlanthus glandulosa) 
and black locust (Robinia pseudacacia). 
