184 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



different parts of the crown. The light which reaches the 

 crowns of trees from aboYe is called overhead light. This 

 determines the arrangement of leaves on the shoots, their 

 position in relation to the sun, and the arrangement of the 

 branches. It is the strongest light, whether it consists of either 

 direct or diffused light alone or both together. In the case of 

 dominant trees it equals the total daylight. 



The light which reaches the crowns of trees from the side 

 is called side light. It stimulates the development of buds 

 on the lateral branches, and is responsible for the development 

 of the branches facing an opening in the forest. 



In the case of trees growing near a wall or steep slope, the 

 tree may receive light which is reflected back upon the tree 

 and is called reflected side light. 



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. 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 i metre (3^ feet) the intensity of light 

 reflected from a road illumined by the sun may be 1/12 of the 

 overhead light intensity ; the intensity of light 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 

 measurements made at the end of April, at noon, in Vienna. 

 If the light intensity coming from the north is taken as i, then 

 the intensity of light from the west is i'i9 ; from the east, i"25 ; 

 from the south, yi2; while the intensity of the overhead 

 light is 4-50. 



Light Intensities and T?ee Gro-w fh. — On\y 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 neighbouring 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 measure- 

 ments of light intensity have shown that on a clear, sunny day 

 the light intensity on the edge of a forest is only about halt 



