456 Forestry Quarterly. 



S. Graves. Bulletin 92. U. S. Forest Service. June 191 1. 



Pp- 59- 



> 



As stated by the authors, the aim of the Bulletin whose title is 

 given above, is to bring together the principal facts with regard 

 to the part which light plays in the life of the forest, and the 

 different methods of measuring it. To this end the principal sub- 

 jects discussed are : Light Intensities and Tree Gr'owth, Tolerance 

 and Intolerance of Trees, Factors Influencing Tolerance and 

 Methods of Determining Tolerance. 



That tolerance and intolerance have a physiological basis is 

 shown by the experiments of Lubimenko who found that the 

 chloroplasts of different species are sensitive to light in different 

 degrees, the chloroplasts of shade enduring species being more 

 sensitive than those of light needing species ; that species with the 

 more sensitive chloroplasts begin to decompose carbon dioxide 

 and reach a maximum of assimilative energy in light of much 

 lower intensity than species of less sensitive chloroplasts. The 

 same fact was demonstrated by Grafe who found that when ex- 

 posed to a light intensity of 1-50 full sunlight, birch leaves showed 

 no presence of starch while beech leaves still continued vigorously 

 to form it. A physiological basis for tolerance and intolerance 

 of trees is also indicated by the experiments of Zederbauer who 

 found that the crowns of various species exercise a selective 

 power of absorption of light rays. The species generally recog- 

 nized as most light demanding absorb, in addition to the red, 

 only small portions of the blue and violet rays, while the shade 

 enduring species absorb, besides the red, some orange and a large 

 amount of the blue, indigo and violet rays. It may be that, as 

 it were, the shade endurers make use of the crlimbs which fall 

 from the tables of their more fortunate neighbors. 



The authors discuss the methods of determining the relative 

 tolerance of the various species under three heads, namely : Obser- 

 vational, anatomical and instrumental methods. Under obser- 

 vational methods, one notes the density of the crbwn, self-pruning, 

 number of branch orders, natural thinning of the stand, conditions 

 of reproduction, relative height and the results of artificial shad- 

 ing. The anatomical method consists in comparing leaf struc- 

 tures, it being assumed that a high development of palisade tissue 

 is due to exposure to strong light. The Bulletin closes with de- 



