PLANT WORLD FOR JINE, 1019, WAS ISSUED DECEMBER 3, 1919 



SUNLIGHT AND ITS MEASUREMENT 



HOWARD E. PULLING 



Wellesley College, Wellesley, Mass. 



The term "sunlight" applies strictly only to that portion of the 

 sun's radiation that is capable of producing sensations of light in 

 man. Although it cannot be said that only this portion affects 

 plants it undoubtedly presents a greater share of today's botanical 

 problems than any other portion of the sun's spectrum. For 

 this reason the peculiar difficulties presented by the ultra-violet 

 and infra-red portions are largely omitted in the following dis- 

 cussion. The first part of this paper is intended as a brief, non- 

 technical statement of our present knowledge concerning the 

 distribution and the amount of energy in the incoming radiation 

 from the sun. The second part only outlines the complex subject 

 of the measurement of the sun's radiation but it is hoped that the 

 notes on some of the necessary precautions of manipulation and 

 interpretation attending the use of the various methods will be 

 helpful. From the standpoint of the expert in these fields the 

 treatment of some methods is unnecessarily detailed and that of 

 others too fragmentary to serve as a guide for beginners. Since 

 it is out of the question to prepare such a guide in the form of a 

 journal article the author has been guided in his selection by two 

 considerations : the state of knowledge of many graduate students 

 in biology regarding the interpretation of data secured by 

 methods that plant physiologists and ecologists have used; 

 the author's intention to direct attention to some of the ad- 

 vantages, disadvantages and peculiarities of other methods in 

 the hope that the beginner will be induced to become really 

 familiar with a method before publishing data secured by its 

 use and will not adhere to the belief that if an instrument is 

 well built the user need only consider the scale-readings. A 

 short literature list is added to indicate the locations of some 

 of the more thorough discussions of the important points. 



151 



PLANT WORLD, VOL. 22, NO. 6 

 JUNE, 1919 



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