LIGHT IN RELATION TO TREE GROWTH. 187 



the rays of the red portion of the spectrum. For this reason 

 light which goes through the green fohage may still be eflfective 

 in producing the green colour of the leaves inside the crown, 

 but is no longer eflfective in bringing about the process of 

 assimilation of carbon. 



In general the intensity of light varies directly with the height 

 of the sun. The maximum light intensity, however, does not 

 coincide with the intensity at noon, which is always less than 

 the maximum. Thus, from measurements made at Vienna, 

 the ratio of the average noon light intensity to the average 

 maximum light intensity for the year was found to be as i : i'o8. 

 This is analogous to the well-known fact that the highest 

 temperature during the day occurs not at noon, but later, and 

 is probably due to the same causes. 



The maximum light available for tree growth is, of course, 

 the total daylight. This varies, as has already been pointed 

 out, with latitude, altitude and the configuration of the earth. 



The highest light intensities found by Wiesner (1905) in the 

 United States were in Yellowstone Park. Thus, at Norris, on 

 ist September 1904, at i p.m., with the sun at an altitude of 

 52° 56', the chemical intensity of the light was found to be 17 

 in Bunsen-Roscoe units. ^ At Old Faithful, on 4th September 

 1904, at noon, angle of sun 52° 22', the light intensity was i"9, 

 and an hour later, with the sun forming an angle of 51° 47', 

 2'o83. These measurements were taken on cloudless days. 

 In Europe the highest light intensities measured were 1-5 near 

 Vienna, at an elevation of 550 feet above sea-level (Wiesner, 

 1896), between 1-5 and i-6 at Kremsmunster at an elevation of 

 1268 feet (F. Schwab, 1904), and i-8 in Steiermark, at an 

 elevation of 4550 feet (Thomas V. Weinzierl, 1902). 



The optimum light intensity at which different species thrive 

 best has not been fully determined, especially since this optimum 

 varies during the life of the tree, and is subject to variations 

 even in different parts of the same vegetative season. In a 

 general way it may be stated that with the majority of forest 

 trees the optimum light intensity at which the leaves function 

 best, and at which the production of flowers and fruits is most 

 abundant, lies nearer to the maximum amount of light available 



' Bunsen-Roscoe unit of chemical light intensity is the amount of light 

 required to produce a standard colour in one second on standardised sensitive 

 paper. 



