8 LIGHT IN RELATION TO TREE GROWTH. 
In the case of trees growing near a wall or steep slope the tree 
may receive ight which is reflected back upon the tree and is called 
reflected side light. (Fig. 1, ¢.) 
In some cases, especially where trees are growing near bodies of 
water, their crowns are illumined by light which is reflected from the 
ground or from the water’s surface. (Fig. 1, d.) This is called 
reflected ground light, and is not so insignificant as it may appear 
at first thought. Thus, actual measurements have shown that, at a 
height of 1 meter (84 feet) the intensity of light reflected from a 
road illumined by the sun may be 1/12 of the overhead light inten- 
sity; the intensity of hght reflected from the water’s surface may 
amount to 1/6 of the overhead light intensity, measured at a height 
of about 5 feet from the surface. 
The intensity of light varies with the direction from which it 
comes. Thus, the following results were obtained from measure- 
ments made at the end of April, at noon, in Vienna. If the hght 
intensity coming from the north is taken as 1, then the intensity of 
light from the west is 1.19; from the east, 1.25; from the south, 3.12; 
while the intensity of the overhead light is 4.50. 
LIGHT INTENSITIES AND TREE GROWTH. 
Only in exceptional cases do forest trees make use of the total 
daylight. Isolated individual trees may do so, but, as a rule, the total 
daylight is considerably weakened by the configuration of the land 
and by the shade cast by the foliage of the individual tree itself or of 
neighboring trees. The bulk of forest trees and the interior parts 
or crowns of even isolated trees depend, therefore, only on a part of 
the total daylight. Actual measurements of lght intensity have 
shown that on a clear, sunny day the light intensity on the edge of 
a forest is only about half that of the total daylight, while in the 
shade of the trees, even when they were still without any foliage, 
the light intensity was one-fourth that of the total daylight. 
MINIMUM INTENSITIES. 
The minimum intensity of light in which photosynthesis can take 
place is not sufficiently determined for all species; it differs in differ- 
ent species with the sensitiveness of the chloroplasts. Trees not only 
accumulate energy by building up new organic substance, but they 
also expend energy from the organic substance which they produce. 
This expenditure of energy is accompanied by oxidation of carbon 
and exhaling carbon dioxid, or respiration. As long as the light in- 
tensity is above the necessary minimum for the given species, the 
process of assimilating carbon from the air, and thus building up 
new organic substance, goes on with greater energy than the opposite 
