1038 



BIOLOGICAL EFFECTS OF RADIATION 



would indicate that the rate of gaseous exchange, and consequently also 

 the compensation point, may be influenced by other means than the 

 direct effect of external factors. 



THE LIMITS OF LIGHT INTENSITY UTILIZABLE IN PHOTOSYNTHESIS 



Ehrke (20) quotes Lubimenko as stating that in a depth of 50 meters 

 in the Black Sea there is still perceptible photosynthesis with the green 

 as well as with the red algae. Gail (32) found that in Puget Sound the 

 lower limit at which photosynthesis takes place in both the red and brown 

 algae is 35 meters. The light penetrating the water at these depths is 

 probably of the order of 1 X 10^'^ and 1 X 10~^ that of sunlight and 

 composed largely of violet light (Oberdorfer, 87). For comparison, this 

 is probably of the same order of intensity as moonlight. Kostytschew 

 (57) found that in arctic regions some plants are photosynthetically active 

 throughout the whole 24 hours in July. Miiller (81) also states that 

 from the position of the compensation point and the light intensity at 

 69 deg. North it is possible to have photosynthesis during the whole 

 24-hour day. Little work is available on the upper limit of light intensity 

 for photosynthesis. Pantanelli (89) reported that the evolution of gas 

 still continued when Elodea canadensis was illuminated with light 64 times 

 as strong as sunlight. 



EFFECT OF WAVE-LENGTH ON PHOTOSYNTHETIC RATES 



Infra-red. — Very meagre information is available for the influence of 

 infra-red radiation on the photosynthetic process. What data there are 

 indicate that this region of the spectrum is not capable of inducing 

 photosynthesis (Warburg and Negelein, 137). Schmucker (101) removed 

 all light of wave-length shorter than 7700 A from a light source rich in 

 infra-red and found no photosynthesis. Burns (12) concludes that infra- 



o 



red to XI 1,000 A is detrimental to photosynthesis in white pine. 

 Klugh (51) observed no photosynthesis in Enteromorpha Lima with infra- 

 red radiation. 



Table 3. — Photosynthetic Efficiencies in Different Spectral Regions as 



Determined by Briggs 



