Carbon Dioxide Supply. 85 



In order to obtain a greater assimilation, either the light or the 

 temperature would have to be increased. In this particular case 

 it can be shown by increasing either light or temperature that it is 

 ligiit which is the limiting factor. Some earlier experiments of 

 Treboux (1903) and Pantanelli (1903) also show clearly the 

 proportionality between carbon dioxide supply and assimilation until 

 light becomes a limiting factor. 



Another noteworthy point is that where carbon dioxide supply 

 is the limiting factor, Fontinalis assimilates only about half as much 

 carbon dioxide as Elodea for any particular concentration of carbon 

 dioxide. It suggests that the difference is due to there being less 

 obstacle to the diffusion of carbon dioxide up to the chloroplasts in 

 Elodea than in Fontinalis. Moreover, as values obtained with 

 Ceratophylluin and Potamogeton are of the same order as those 

 obtained with Elodea, it appears to be a class distinction between 

 Bryophytes and Phanerogams. 



It is to be observed that no depression of assimilation occurs 

 even with such a high concentration of carbon dioxide as 0'0536%. 

 This is 33-92% of saturation, and constitutes an environment as 

 rich in carbon dioxide as is an atmosphere containing 30% of the 

 gas. With higher concentrations, however, the 'real' assimilation 

 becomes much depressed. This is not to be regarded as evidence 

 that there is ' a primary optimal amount ' of carbon dioxide for 

 assimilation. Blackman regards the depression of assimilation in 

 high concentrations of carbon dioxide as due to a narcotic effect of 

 the strong carbon dioxide upon protoplasm, which has been 

 previously shown by many workers (cf. Chapin, 1902). The 

 depression has no direct relation to carbon assimilation. Blackman 

 thus concludes that " in the curve expressing, in any given light, 

 the relation of assimilation to the whole range of COj-concentrations 

 from zero to saturation, we may separate off the falling end-part of 

 the curve as an effect of narcotic poisoning. The third and last 

 phase thus contrasts with the first two phases, which are specific 

 assimilation effects, the first rising in a straight line where the CO^ 

 is limiting and the assimilation proportional to it, and the second 

 a horizontal line where the assimilation is limited by the light (or the 

 temperature) and is independent of increase of the COj-supply." 



