no PHOTOSYNTHESIS 



spectrum may vary independently of one another. Kniep and Minder, 

 working in Naples, consider that the intensity of sunlight about the noon 

 hours is relatively constant, and checked each determination with direct 

 measurement of the energy falling on the leaf. For measuring the in- 

 tensity of radiation they used a Rubens thermopile with 20 elements which 

 permitted the determination of less than 0.0000001° and was calibrated 

 to a scale of gram calories per unit time. As a source of light sunlight 

 between 11 A.M. and 2.30 P.M. was used; the light was filtered through 

 water to absorb the heat rays. 



Filters of colored glass were used to produce different spectral regions : 

 (1) a red glass fiker passing wave-lengths in \i \i, with the coefficient of 

 transmission D from infra-red to about 620 \i \i, and (2) a blue filter. 



1. 



2. 



^^ 644 578 546 509 



D 0.846 0.00056 0.000057 0.000 



Uti 546 509 480 436 405 384 361 340 332 



D 0.00 0.0109 0.177 0.455 0.395 0.267 0.078 0.010 0.000 



Green light was produced by the use of a filter made by mixing a solution 

 of potassium chromate and ammoniacal copper hydroxide. This solution 

 allowed light of wave-lengths 512 to 524 \i \i to pass through, though no 

 quantitative data are given. From the data on the coefficients of trans- 

 mission and from the curve of distribution of energy in the sun's spectrum 

 it was possible to construct curves of the distribution of energy in the 

 various regions passing through the light filters. 



The plant employed was Elodea canadensis and for determining the 

 rate of photosynthesis the bubble counting method was used (see chapter 

 on Methods of Measuring Photosynthetic activity). Unfortunately this 

 method is subject to many errors, as was later recognized by the authors 

 who subsequently made a careful analysis of the sources of error involved. 

 As a consequence they had to reduce the intensity of radiation by a series 

 of screens which to a measure diminished the value of their careful deter- 

 minations of radiation distribution. The conclusion is drawn that red 

 and blue light of the same intensity produce about the same rate of photo- 

 synthesis. The light intensity in these experiments was very low and 

 probably was the factor of relatively minimum intensity, though little 

 regard was apparently paid to the other factors. The real value of Kniep 

 and Minder's investigations is the demonstration of the fact that there 

 is no sense in considering the photosynthetic activity in different colored 

 light without at the same time determining the energy relations of the 

 light employed. The maximum photosynthesis in the red-yellow portion 

 of the spectrum, observed by earlier investigators could thus be ascribed 

 to the higher intensity of these wave-lengths in the spectra used. 



