THE LIGHT FACTOR IN PHOTOSYNTHESIS 1023 



it. In any consideration of the role of light in photosynthesis the optical 

 properties of the chlorophyll-bearing organ is of great importance, 

 especially if any information regarding quantitative relationships is to 

 be gained. Advance in this problem has been greatly aided by the 

 application of recent developments in physical and optical apparatus for 

 the measurement of light and of useful artificial sources of light. Helpful 

 compilations of the theory and use of the methods and apparatus for work 

 in this field, with special application to plant physiological problems, 

 have been prepared by Nuernbergk (86) and by Gaffron (30). Space 

 does not permit discussion of this important aspect of the experimental 

 treatment of the problems concerned. This subject is treated in another 

 section of this report to which reference must be made and to the works 

 just cited. The older literature has been reviewed by Ursprung (122), 

 and a compilation of transmission data of a large variety of leaves has 

 been made by Schanderl and Kaempfert (97). 



A not insignificant portion of the light falling on a leaf is reflected from 

 its surface. The quantity depends, of course, upon the angle of incidence 

 of the light, and also upon the texture of the leaf surface, its age, and the 

 spectral composition of the light. In the leaves of land plants reflection 

 occurs at the outer surface and also within the leaf, at the surfaces of the 

 intercellular spaces, in fact, wherever the light strikes an interface of 

 material of different refractive index. The leaves of land plants, there- 

 fore, offer a very complex system; this may be somewhat simpler in the 

 case of aquatics. 



In the spectrophotometric measurements of Pokrowski (90) and of 

 Shull (107) the percentage of 90 deg. reflection of light from the upper 

 surface of a variety of leaves of trees and herbaceous plants was found to 

 vary from about 3 to 15 per cent. From green leaves the amount of 

 reflection is maximum for light of X5400 to 5600 A. In this region the 

 reflection is 5 to 10 per cent in the darkest green leaves and 20 to 25 per 

 cent in the lightest green foliage. A reflection minimum has been 

 observed at the maximum absorption band of chlorophyll, X6600 to 

 6800 A, though this is not always the case. Albino leaves reflect very 

 much more light than green ones, amounting to 40 to 50 per cent and 

 these reflect mainly the radiations of longer wave-length. The under 

 surface of leaves reflects more light than the upper surface, though the 

 values for different wave-lengths run about parallel. It is interesting 

 that hairiness or smoothness of the cuticle does not regularly result in high 

 reflection. The extensive investigations of Seybold (104, a, h, c) on the 

 basis of energy measurements, in the main, confirm the results of Pok- 

 rowski and of Shull. Seybold has made a thorough study of the complex 

 system which the green leaf presents as an absorbing and reflecting 

 medium. The order of magnitude of the values for reflection of a parallel 

 beam of light falling on a leaf and for diffuse light is the same. In 



