THE LIGHT FACTOR IN PHOTOSYNTHESIS 



1039 



Visible. — Recent determinations have confirmed the older results 

 that the red and yellow-red regions of the spectrum are the most effective 

 in photosynthesis. Results obtained by Briggs (8) are given in Table 3. 

 He has given his results in cubic centimeters of oxygen evolved for each 

 500 calories of light incident on the leaf, because, on the average, the heat 

 of evolution of 1 cc. of oxygen from organic material is 5.0 cal. This 

 convention was adopted in order to avoid all assumptions as to the 

 mechanism of the process and the troublesome estimations of the energy 

 absorbed by the leaves. For all practical purposes, however, these values 

 are comparable to those reported in per cent by other investigators. 

 The values of Briggs were apparently not obtained with the same incident 

 light intensity and may, therefore, be subject to some error due to the 

 fact that photosynthetic activity is not always directly proportional to 

 the intensity. 



To what extent and in what manner the presence of pigments other 

 than chlorophyll influences photosynthesis has not yet been definitely 

 determined. Schmucker (101) found that Cryptocoryne ciliata, which 

 was red on the under surface, gave only half the photosynthesis when the 

 red side was turned to the light as compared to the activity with normal 

 exposure. According to Ehrke (21) the green alga. Enter omor-pha com- 

 pressa, exhibited a higher rate of photosynthesis in red than in green or 

 blue light; the red alga, Delesseria sanguinea, showed very little differ- 

 ence in the three spectral regions. Harder (37) has criticized these 

 results on the basis of lack of uniformity of treatment previous to the 

 photosynthesis determinations and because of the fact that photosynthe- 

 sis never exceeded respiration, though he considers that in the main the 

 results present a true picture. Similar results were obtained by Klugh 

 (51), which are summarized in Table 4. 



Table 4. — Relative Rates of Photosynthesis op Gkeen, Brown, and Red 

 Algae in Different Spectral Regions According to Klugh 



Ultra-violet.— Little work has been done on the effect of ultra-violet 

 light on photosynthesis. Arnold (2) found that irradiation with light 

 of X2537 A reduced the rate of photosynthesis of Chlorella pyrenoidosa 

 in proportion to the amount of radiation. He concluded that the 

 effect of the ultra-violet light was directly on the photosynthetic appara- 

 tus of the plant. 



