876 EXTERNAL AND INTERNAL FACTORS CHAP. 26 



of induction (chapter 33) , is another type of mechanism which must be taken 

 into consideration. Drop of the CO2 content of the air (Bohning 1949) and 

 enhanced CO2 supply through the roots (p. 910) also have been blamed for 

 the midday depression. 



The phenomenon of midday depression was found by Montfort and 

 Neydel (1928) also in stomata-free ferns, and by Kostychev and Soldaten- 

 kov (1926), Kursanov (1933) and Neubauer (1938) in algae. 



Gessner (1938) found no pronounced midday decline of photosynthesis 

 in the higher aquatic plants (Elodea, Potomageton etc.), except with shade- 

 adapted species or individual plants in which it could be interpreted as 

 "inhibition by excess light" {cf. Volume I, page 535). Although minor 

 fluctuations of the rate remained unexplained, the rate of photosynthesis in 

 Gessner's submerged plants generally followed the changes in the intensity 

 of illumination. The maximum of photosynthesis was often found in the 

 early afternoon rather than at noon, but this could be explained as a tem- 

 perature effect. 



Chesnokov, Giechikhina and Jermolayeva (1932) found that respiration, too, has a 

 complicated diurnal rhythm. Because of this, the true rate of photosynthesis cannot be 

 obtained by applying a uniform respiration correction to the net rate of oxygen liberation 

 measured at different times of the day. They also found that the respiration of leaves 

 often is much stronger than was generally assumed before. In young leaves, in particu- 

 lar, the rate of respiration may approach that of photosynthesis. This explains why 

 the rate of carbon dioxide liberation by some plants during the midday depression was 

 found to be almost as large as the rate of the carbon dioxide consumption by photosyn- 

 thesis before and after this rest period. 



However interesting the phenomena of the diurnal rhythm of photo- 

 synthesis, and similar "physiological" effects (such as aging, fatigue, etc.) 

 may be, the primary question for a kinetic study of photosynthesis is not 

 whether these variations can be explained, but whether they can be elimi- 

 7iatcd, and photosynthesis made to proceed at an even and reproducible 

 rate. It is difficult to realize such steadiness in field experiments. Boysen- 

 Jensen and Miiller (1929), Boysen-Jensen (1933) and Mitchell (1936) said 

 that, if conditions are reasonably constant, rate of photosynthesis in natural 

 surroundings remains steady ; but Maximov and Krasnosselskaja-Maximova 

 (1928) and Waugh (1939) observed that the photosynthetic production of 

 leaves on the tree fluctuated, under constant external conditions, in suc- 

 cessive four minute periods, by as much as =*= 100% of the hourly average. 

 Kostychev (1931) concluded from these observations that measurements 

 of photosynthesis over short periods have no meaning, and that the 

 minimum time over which photosynthesis should be measured is a whole 

 day! 



Stocker, Rehm and Paetzold (1938) found that rapid changes of the 

 rate of jjhotosynthesis under natural conditions (fluctuation period: 



