Carbon Assimilation. 
160 
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 
light 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 
Ceratophyllum 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 C0 2 -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 C0 2 -supply.” 
To be continued. 
