MIDDAY DEPRESSION 875 



kell (1928^) found that detached cherry laurel leaves, illuminated with con- 

 stant light for 24 hours, showed a deep depression of photos>Tithesis during 

 the night hours; thus, not only the "midday nap," but also the "night 

 sleep" appears to be influenced by internal factors. 



Geiger (1927), Maskell (19281-2) and St^lfelt (1935) considered the 

 closure of the stomata as the immediate cause of the middaj^ depression. 

 Maskell (1928^) observed that the nightly depression of photosynthetic 

 activity of steadilj^ illuminated leaves can be avoided by increasing the 

 jmrtial pressure of carbon dioxide ("pressing carbon dioxide through half- 

 closed stomata"), and that steadily illuminated leaves of Hydrangea 

 (the stomata of which are almost rigid) showed only a slight decline of 

 photosynthesis during the night hours. Both Maskell (1928^) and St§,lfelt 

 (1935) found a parallelism between the average aperture of the stomata and 

 the rate of photosynthesis (of. chapter 27, page 910). 



It thus seems plausible that the diurnal rhythm of photosynthesis of 

 the higher land plants is to a large extent conditioned by stomatal move- 

 ments. The question remains, however, what causes the stomata to close 

 at certain times of the day, even though the illumination and the carbon 

 dioxide supply are kept constant? 



One "internal factor" that has been much discussed in connection with 

 this problem is the accumulation of (soluble or insoluble) carbohydrates. 

 (Concerning the effect of excess carbohydrates on the rate of photosynthe- 

 sis, see chapter 13, Vol. I.) The midday depression may be a pause during 

 which these materials are translocated or partially combusted. This ex- 

 planation, first accepted by Kostychev, Kudriavtseva, Moisejeva and 

 Smirnova (1926) and Kostychev, Bazyrina and Chesnokov (1928), was 

 later rejected by Chesnokov and Bazyrina (1930^), w^ho found that plants 

 with entirely different diurnal course of translocation may nevertheless 

 show the same diurnal course of photosynthesis. It was on the basis of 

 results such as this that Kostychev (1931) finally reached his extreme con- 

 clusion concerning the purely physiological regulation of photos\Tithesis. 



Against these findings of Kostychev and co-workers, Kursanov (1933), 

 von Guttenberg and Buhr (1935) and Monch (1937) confirmed the existence 

 of a relation between the accumulation of sugars and starch and the diurnal 

 rhythm of photosynthesis. However, according to von Guttenberg and 

 Buhr (1935) and Neuwohner (1938), no smgle ex-planation can be made to 

 fit all cases of midday depression. In some cases (e. g., in young leaves in 

 spring) it is clearly traceable to the choking of the photosynthetic ap- 

 paratus with carbohydrates. In other cases {e. g., in summer leaves on hot 

 days) the loss of water and the ensuing closure of stomata provide the most 

 plausible explanation. Accumulation of half-oxidized products which 

 "narcotize" the photosynthetic apparatus, as suggested in Franck's theory 



