910 



CONCENTRATION FACTORS 



CHAP. 27 



recent discussions of this question, we refer to Bergamaschi (1929), Livingston and Beall 

 (1934), Suessenguth (1937), Overkott (1938, 1939) and Hartel (1938). The experiments 

 of the two last-named authors, in particular, have confirmed unambiguously that a cer- 

 tain amount of carbon dioxide can be conveyed from roots to leaves by convection (and 

 to a smaller extent by diffusion), and that this supply can be utilized by leaves for the 

 synthesis of carbohydrates. It was even suggested that this "invisible" CO2 supply, 

 brought about by increased transpiration during the hot hours of the day, may be 

 the cause of the decline of the carbon dioxide absorption from the air, which is often ob- 

 served at midday (c/. page 873). Whether this is a valid hypothesis cannot be judged 

 without quantitative investigations; but in any case, it cannot explain all the aspects 

 of the so-called "midday depression," (o) because these also include a decline of oxygen 

 liberation, (6) because they have been observed not only in the higher land plants but 

 also in aquatics. 



6. Role of the Stomata 



It was stated above that, in experiments with the leaves of the higher 

 land plants, a special problem is posed by carbon dioxide passage through 



Fig. 27.5. Diagram of a section through stoma and substomatal cavity of a 

 leaf to show direction of diffusion of gases in photosynthesis (after Robbing and 

 Rickett). Arrows with black balls represent carbon dio.xide; those with tri- 

 angles, oxygen. 



the stomata and air channels, through which it has to flow in order to reach 

 the photosj^nthesizing cells of the palisade tissue and of the spongy paren- 

 chyma. 



A controversy as to whether the carbon dioxide enters the leaf only through the 

 stomata or also through the cuticle was decided by Blackman (1895). He proved, by 



