SOME PROBLEMS IN LIGHT AS A FACTOR OF FOREST GROWTH 227 



importance for the different life processes as it was thought before. 

 Thus, for instance, the formation of chlorophyl takes place, although 

 with a different degree of rapidity, in all visible rays between A and H. 

 The light which penetrates through the dense crown canopy and lacks 

 the red rays can still cause the formation of chlorophyl grains. Assi- 

 milation, although it takes place most vigorously in the presence of red 

 rays, still may go on in the presence of blue, green, and even ultra- 

 violet rays. If this is the case, then the dift'erence in the quality of 

 the light in the forest cannot have an important effect since the same 

 life processes can go on in the presence of different rays and the 

 quality of the light in the shade of the forest may for all practical pur- 

 poses be considered the same as the ordinary daylight but of weaker 

 intensity. 



If this could be definitely settled once for all it would facilitate the 

 solution of many forest problems. Thus, for determining whether a 

 certain tree species could come up and grow in the shade of other tree 

 species, all that would be needed is a determination of the minimum 

 light intensities under which this species can grow without going to the 

 trouble of determining the quality of the light. The measurement of 

 tight intensities can be made very quickly and very simply either with 

 Wiesner's or with Clements' photometer, while the determination of the 

 amount of the different rays of the spectrum would require numerous 

 observations by means of complicated and cumbersome spectro-pho- 

 tometers, ill adapted for work in the forest. 



There are two schools : the one led by Professor Wiesner claims that 

 the light in the forest is practically of the same quaHty as that of the 

 open and can be measured by its action on sensitive paper; the other, 

 at the head of which is Zederbauer, claims that the quality of the light 

 in the forest is different from that in the open and that each portion 

 of the spectrum must be measured separately if we are to understand 

 the effect of light upon forest growth. Zederbauer's investigations 

 show in particular that the foliage of so-called shade-enduring species, 

 such as spruce, beech and fir, absorb most of the red, blue to violet 

 rays while the foliage of the light-needing species, such as pine, larch 

 and birch, absorb less of the red, blue to violet rays than the shade- 

 enduring species. In the shade of a forest stand composed of some 

 shade-enduring species, the quality of the light must therefore be 

 different from the kind of light that is to be found in the shade of a 

 forest stand made up of light-needing species. Under the shade of a 



