874 



EXTERNAL AND INTERNAL FACTORS 



CHAP. 26 



pattern. Thoday (1910) discovered that the production of organic material 

 by leaves may show a temporary decHne in the middle of the day; the 

 plant takes an "afternoon nap" (c/. fig. 26.7). McLean (1920) observed 

 that this decline may even result in the release of carbon dioxide. 



The gas exchange measurements of Kostychev and co-workers (1926- 

 1931) (cf. also Chesnokov, Bazyrina and co-workers 1932), carried out 

 under a large variety of climatic conditions, from Central Asia to the Arctic 

 Sea, and with algae as well as with land plants, showed that the phenom- 

 enon of "midday rest" is widespread in the plant world, but that it assumes 



8 II 13 14 17 



TIME AFTER MIDNIGHT, hr. 



20 



Fig. 26.7. Diurnal course of photosynthesis of two leaves of Erio- 

 botrya japonica under natural conditions (after Kursanov 1933): 

 solid line, May 30; broken line, June 6. 



the extreme form of a reversal of photosynthesis and evolution of carbon 

 dioxide only under special conditions, particularly in hot climates. 



The diurnal course of photosynthesis has received the attention also of numerous 

 other investigators, among whom we may mention Geiger (1927), Montfort and Neydel 

 (1928), Maskell (1928i), Hiramatsu (1932), Harder, Filzer and Lorenz (1932), Bosian 

 (1933), Kursanov (1933), Stalfelt (1935), von Guttenberg and Buhr (1935), Monch 

 (1937), Filzer (1938), Neuwohner (1938), Neubauer (1938), B. S. Meyer (1939) and 

 Bohning (1949). 



Harder (1930) and co-workers, Schoder (1932) and Drautz (1935), as 

 well as Neuwohner (1938), have attempted to explain the diurnal curves by 

 combined variations of several external factors (light intensity, humidity 

 and temperature) . They succeeded only partially, and had to admit that a 

 considerable part of the observed variations remained unexplained and had 

 to be attributed to unknown "plasmatic" factors. This is particularly 

 clearly demonstrated by the observations of Filzer (1938), who found that 

 leaves, picked from trees at different times of the day and then investigated 

 under constant conditions in the laboratory, showed the same periodic 

 changes in photosynthetic production as did leaves left attached to the 

 plant and exposed to the natural change of night and day. Similarly, Mas- 



