EFFECTS OF REGIONS OF VISIBLE SPECTRUM 779 



lamp, by supplementary radiation from a carbon arc, promoted the 

 absorption of nitrate by young wheat plants grown in water culture. 

 Potassium proved a more efficient carrier of the nitrate than sodium, 

 which was thought to corroborate the findings of Nemec and Gracanin 

 concerning increased absorption of potassium under violet light. 



REFLECTION, ABSORPTION, AND TRANSMISSION OF DIFFERENT PARTS 



OF THE SPECTRUM BY LEAVES 



The importance of a knowledge of reflection, absorption, and trans- 

 mission of radiation by plants has long been recognized, but accurate 

 information has been forthcoming only during the last few years. Prac- 

 tically all of the reported investigations have been restricted to a study 

 of leaves, since these organs are probably the most important absorbers of 

 radiant energy in the higher plants, particularly in connection with 

 photosynthesis and transpiration. 



A discussion of the relative merits of different methods of measuring 

 reflection, transmission, and absorption of radiation will not be under- 

 taken in this report. The reader desiring information on this subject is 

 referred to the papers by McNicholas (25, 26), Waldram (53), and 

 Nuernbergk (28) as well as to those of the investigators whose work will 

 be mentioned in this report. 



Ijrsprung (50) reviews the work up to 1903 on the physical properties 

 of leaves with respect to light. In many of the earlier investigations no 

 attempt was made to determine reflection, absorption, and transmission 

 of different regions of the spectrum. These factors were considered only 

 in relation to the total energy of the source or the total visible radiation 

 of the source. The most widely quoted of the earlier investigations is 

 that of Brown and Escombe (4) in which an attempt was made to account 

 for all of the solar energy incident to the leaf surface, that is, to draw up a 

 balance sheet of energy income and outgo. Shull (44) criticizes the results 

 of Brown and Escombe on the basis of the fact that they failed to take 

 into account the important factor of reflection from the leaf surface, 

 which his own as well as the measurements of others have shown to be 

 often as great as the amount of radiation transmitted. Seybold (39) and 

 Schanderl and Kaempfert (36) have also criticized the method of 

 Brown and Escombe. Seybold (39) from his own investigation gives the 

 following tentative light-energy balance (Table 3), in which only white 

 light exclusive of infra-red is considered. The figures in parentheses are 

 estimated values; the others, measured. 



Seybold himself states that it is not possible to set up a general 

 energy balance because of the great variability of leaves, as well as the 

 variation in the quality of radiation incident upon them. Further 

 discussion of this question will not be considered here, since it falls more 

 logically under the section in this monograph on photosynthesis. 



