DEPARTMENT OF BOTANICAL RESEARCH. 87 



of gi-owth or actual shrinkage of growing joints after niidday and con- 

 tinuing until the following morning. These are destruction of enzymes 

 concerned in renewing building material, excessive transpiration by 

 which a plant might lose bulk by water-loss, and accumulation of acids 

 or other respiration products which would clog metabolism and reduce 

 the water-holding capacity of the protoplasm. There is evidence 

 available showing that the second and third conditions may operate to 

 check, cancel, or retract growth-elongations. 



Measurement of Light in Physiological Aspects, by D. T. MacDougal 



and H. A. Spoehr. 



Experimentation with the direct and indirect effects of light on organ- 

 isms requires sources of light under good control, screens for transmitting 

 special regions of the spectrums, and methods of measurement of the 

 relative intensity of the illumination falling on the organism. 



Sunlight may serve in some work w^hen the requisite screens are 

 available, but incandescent filaments, mercury, and amalgam vapor 

 arcs inclosed in glass or in quartz may be used as sources of hght down 

 to wave-lengths of 0.28 /x. A series of formula? for a number of glasses 

 which would transmit various parts of the spectrum has been devel- 

 oped in the laboratory of a prominent firm of glass-makers, of which the 

 following promise the greatest usefulness : 



Red: High tiansmission in red remove^ all light below 0.61 ix. 



Blue: Transmits only blue below 0.52m and may be made deeper to transmit only below 



0.50 m. 

 Yellow : High transmi.ssiou in red and infra-red and through screen to 0.48 m givine about 



75 per cent of incident white light. All ultra-violet absorbed. 

 Uviol: Transparent to visible spectrum, transmitting ultra-violet to 0.31 M in sheets 0.25 



inch thick, to 30 M through 0.125 inch thick. 

 Heat-absorbing: Absorbs most of infra-red ami 97 per cent of heat of Nernst lamp; gives 



a pyrhelioraeter reading about half that of good window-glass. Transmits Go per 



cent of incident white light. 



The blue- violet region of the spectrum is of the greatest interest to 

 the biologist, and as photometers, thermopiles, and pyrheliometers 

 do not register the dissociation effects, it is proposed to use the photo- 

 electric cell as developed by Elster and Geitel and by Dr. Jacob Kunz. 

 This instrument has the great advantages of extreme sensitiveness in 

 the blue- violet region and ease of manipulation; it records immediate 

 and directly proportional values, and can be used for extensive ranges 

 of intensities. 



A comparison of the results to be obtained by the use of two methods 

 is afforded by the data given below. Direct sunlight at the Desert Lab- 

 oratory is taken as 100, and the figures in both cases are percentages of 

 this total. The values from the pyrheliometer were calculated in calories 

 per square centimeter per minute, and those of the sodium photo-electric 

 cell are from readings of the high-sensitivity galvanometer. 



