RATE FLUCTUATIONS 877 



several minutes) are associated with similar fluctuations of the carbon 

 dioxide concentration in the air. 



Scarth, Loewy and Shaw (1948) observed that the photosynthesis of 

 detached leaves, determined by measuring the infrared absorption of 

 carbon dioxide, occasionally showed unexplained, regular fluctuations 

 (with a period of the order of 1 hour) . 



When plants are investigated under natural conditions, the apparently 

 erratic behavior can be attributed to the difiiculty of controlling all the 

 relevant factors. However, considerable doubt has also been expressed 

 as to the capacity of plants to carry out photosynthesis at a constant rate 

 under controlled conditions in the laborator>\ Experiments with lower 

 plants, e. g., unicellular algae, such as Chlorella, have given comparatively 

 satisfactory results: If certain prescriptions concerning culture and treat- 

 ment were adhered to, these algae could be relied upon to maintain a con- 

 stant and reproducible rate of oxygen production for several hours (leav- 

 ing aside the short time induction phenomena to be discussed in chapter 

 33). According to Pratt (1943^), when alkaline buffers are used, the con- 

 stancy of the rate depends on the nature of the cation present: Thus, in 

 0.1 M NaHCOs, the rate declined during the first 10 hours and then be- 

 came steady; in M KHCO3, it increased during the first 10 hours, remained 

 steady for the next 5 hours and then declined rapidly; in 0.065 M Na- 

 HCOs + 0.035 M KHCO3, the rate remained steady for the first 15 hours 

 and then began to decline (see fig. 25.1). 



Noddack and Eichhoff (1939) stated that for a given suspension of 

 Chlorella, the rate of photosynthesis is reproducible within ±10%, and 

 that these variations can be further reduced by preliminary adaptation of 

 the cells to the light intensity in which they are to be studied. 



Experiments with higher land plants or aquatic plants have been con- 

 tradictory, and at first rather discouraging. Tnie, Willstatter and StoU 

 (1918) found that detached leaves, properly supplied with water and car- 

 bon dioxide, maintain a constant rate of photosynthesis (within a few per 

 cent) for 4 or 6 hours, even in strong light (40,000 lux) ; but Harder (1930, 

 1933), Arnold (1931) and Jaccard and Jaag (1932) asserted that strong 

 trends as well as irregular changes develop in the photosynthesis of aquatic 

 plants kept imder constant external conditions. Arnold obsei^ved, e. g., 

 that in moderately strong light (18,000 lux) the rate of photosynthesis of 

 Elodea dropped, in 2 or 3 hours, to one fifth or one tenth of its origmal 

 value; at 4000-6000 lux, it increased for the first 2 or 3 hours and then 

 decreased slowly ; only at 2000-3000 lux did it remain approximately con- 

 stant for several hours. Harder (1930, 1933) made similar observations 

 and stated that light intensity must be measured relative to the intensity 

 to which the plants have been "acclimated" before the experiment. By 



