104 PLANT HOCIOWGY 



(1923), Lundegardh (1930), Morton (1927), and Klugh (1927). I hys- 

 ically unimpeachable methods of measuring Hght are so comphcated 

 that they are not available to the plant sociologist. Approximate and 

 comparative measurements can be had by the photographic-paper 

 method of Wiesner, which is still generally used. It depends on the 

 darkening of a sensitive silver chloride paper. Vouk and Eder- 

 Hecht have elaborated the method of Wiesner. The Graukeil photom- 

 eter of Eder-Hecht, as now used, serves for determining both the 

 momentary light intensity and the summation of light for definite 

 periods of time. It can be had at a reasonable price, with instructions 

 for use and accessory equipment from the Photographische Industrie- 

 gesellschaft Herlango, Vienna. Lundegardh has constructed a self- 

 registering apparatus for measuring light. 



Intensity of light is measured by exposing a sensitive paper to the 

 light for a definite time and comparing the resulting coloration with a 

 standard. Bunsen and Roscoe give as unity the intensity that gives 

 the standard color in one second. 



The absolute illumination of a habitat is expressed as a fraction 

 whose denominator is the number of seconds required to produce the 

 standard color (3^^ = 5 sec. exposure). 



The relative light requirement {Lichtgenuss of Wiesner) is the rela- 

 tion of the light intensity of the habitat to full sunshine measured 

 at the same time. This is also expressed as a fraction. Relative light 

 requirement of }i signifies that the light of the habitat is one-third that 

 of full sunlight. 



On account of the great and often sudden changes in intensity of 

 light, absolute measurements in a definite habitat or in a definite plant 

 community (average summation of light) are useful only when con- 

 tinuously registered. This may be accomplished, to a certain extent, 

 by the use of a pair of Livingston's atmometers: one with a white bulb, 

 the other with a black, mounted side by side. All factors influencing 

 the rate of evaporation from the two instruments are the same except 

 that radiant energy is absorbed by the black bulb only.^ The resulting 

 rise in temperature increases the evaporation of water from the black 

 atmometer. The difference in the water loss of the two atmometers 

 thus gives a measure of the amount of radiant energy during the period 

 of exposure. 



The measurement of relative hght intensity in two adjacent habi- 

 tats gives useful results when accomplished by means of a simple 

 photographic apparatus like "Wynne's infallible exposuremeter. " 

 The relative light requirement may be approximated by single simul- 



* A very small amount of radiant energy is absorbed by the white bulb. 



