EXTERNAL SUPPLY AND EXHAUSTION EFFECTS 903 



(at 1 P.M., in June), 0.13% in a forest, 0.10% in grassland, and 0.18% in a 

 river bottom. The concentration declined steeply with height above 

 ground in all these habitats, dropping to near average (~0.04%) 8 or 10 

 cm. above ground. 



The decline of photosynthesis at excessively high concentrations of carbon 

 dioxide {e. g., 10 volumes per cent CO2 or more, corresponding to over 300 

 X 10-^ M), which, before Blackman, was considered a confirmation of the 

 "optimum theory," was reinterpreted by Blackman as an inhibition effect, 

 alien to the intrinsic kinetic mechanism of photosynthesis. It was dis- 

 cussed as such in chapter 13 (Vol. I) which dealt with various inhibitors 

 and stimulants. Referring the reader to this chapter, we merely repeat 

 here references given there to the work of de Saussure (1804) (who dis- 

 covered the effect), Boussaingault (1865), Bohm (1873), Ewart (1896), 

 Chapin (1902), Pantanelli (1903), Jaccard and Jaag (1932) and Livingston 

 and Franck (1940). A recent study by Ballard (1941) with leaves of 

 Ligustrum can be added to the list. It showed that, at 17° C, inhibition 

 occurred (at 35,000 lux) at [CO.] = 2%, while at low temperatures (6° C.) 

 no inhibition was noticeable up to 5%. We recall that Chapman, Cook 

 and Thompson (1924) found that high carbon dioxide concentration induces 

 closure of the stomata; it was therefore suggested in chapter 13 that sto- 

 mata may account for some of the observed carbon dioxide inhibition ef- 

 fects. Other phenomena, which also may contribute to the inhibiting 

 influence of excess carbon dioxide, are its adsorption on catalytic surfaces 

 ("narcotization"), and possibly also acidification of the cell fluids (shift 

 of intercellular buffer equilibria). 



That the closure of stomata is not the only reason for carbon dioxide 

 inhibition is illustrated by the observation of Osterlind (1949) that it also 

 occurs with algae such as Scenedesmus quadricauda. An inhibition of the 

 growth of this alga became noticeable at 2 X 10"^ mole/1., and reached 

 50%, at 10 X 10-3 mole/1. CO2. 



5. External Supply and Exhaustion Effects 



In commenting on Table 27.1, we noted wide variations in the numerical 

 values of the saturating carbon dioxide concentration' and suggested that 

 these variations may be due largely to the exhaustion of carbon dioxide in 

 the immediate neighborhood of the plants. We will now consider this as- 

 pect of the problem more closely. 



The experimental results fall roughly into three classes. One group, 

 which includes the results of Blackman and Smith (1911) and Singh and 

 Kumar (1935), the somewhat less extreme data of James (1928) and the 

 figures given by Steemann-Nielsen (1946) for Fontinalis, and by Wassink 



