STUDIES IN TOLERANCE OF NEW ENGLAND FOREST TREES 5 



far enough from the forest margin to be beyond the influence of side 

 light. He found that all trees absorb between Frauenhof er lines B and 

 C in the red end of the spectrum, then in the region about F in the 

 blue, and beyond H in the violet. Some tree species absorb practically 

 all rays from F to H, while others allow many rays in the indigo re- 

 gion to pass through unabsorbed. Intolerant pine and larch absorb 

 about the same in the red, blue, and violet sections, using practically 

 all the red, but only small amounts of blue and violet. On the other 

 hand the tolerant spruce and beech, besides red, absorb some orange 

 and a large amount of blue, indigo, and violet rays. Ash and oak fall 

 between these two extremes and allow many indigo rays to pass through 

 unabsorbed. 



Zederbauer's results, in part at least, are admitted to be inaccurate, 

 because of defects in the apparatus. It will appear later that many 

 other errors must be eliminated before reliable results can be ob- 

 tained. The fact that leaves of various species of trees absorb dif- 

 ferent rays of light is interesting, but of itself proves nothing. 



Knuchel (10) undertook an exhaustive study of this subject and 

 published a great mass of data which was reviewed by Sponsler (13). 

 His methods were much more refined than those of Zederbauer, and 

 his results are accordingly more trustworthy. His study was 

 devoted especially to the variations in the composition of light 

 which had passed through leaves, through forest crowns and 

 through individual crowns. In the first case he used both sun 

 and shade leaves of basswood, beech, ash, maple, etc. On account 

 of their size no attempt was made to use softwood leaves. Only one 

 illustration of his work with leaves need be given. On July 8, 1911, 

 he took measurements of the composition of diffuse light after it had 

 passed through leaves of the beech and obtained the following results : 



TABLE I COMPOSITION OF LIGHT AFTER PASSING THROUGH BEECH LEAVES 



(KNUCHEL) 



Rays passing Sun leaves Shade leaves 



One leaf Two leaves One leaf Two leaves 



percent percent percent percent 



average average 



Since the sun leaves receive the light first and use the greater 

 part of it, it would seem reasonable to assume that the filtered light 

 is not of much use to the leaves within the crown. 



