390 Dr. F. F. Blackman and Mr. A. M. Smith.  [Dec. 19, 
ones with the more general problem of how to disentangle the action of the 
different factors of the environment in any given case. In Section IV an 
attempt will be made to clear up and interpret the results of previous workers 
from our own point of view. 
Section ].—-THE RELATION OF ASSIMILATION AND COo-SUPPLY. 
In this section we give the results of our experiments on the effect of 
varying the factor of CO2-content upon the assimilation of submerged water- 
plants. The greater part of this work has been done upon Elodea; in 
addition a series of experiments was made with the “ water moss” Fontinalis 
antipyretica and a few isolated ones with Ceratophyllum and Potamogeton. 
Our expectation in approaching this subject was that there would be only 
one direct primary relation between COo-supply and assimilation, namely 
that the assimilation should be directly proportional to the amount of CO, 
available. In some sets of conditions, however, this relation will not be 
realised, owing to the hmit imposed upon functional activity by some other 
factor. Light, temperature, or the “ time factor” may each act thus asa 
limiting factor. Working over a wide range of CO:-content, it will therefore 
make all the difference to the results obtained what temperature and what 
illumination are selected for the series of experiments. 
Our first series consists of 10 evaluations of the assimilation by Montinalis 
antipyretica in different strengths of dissolved COs. 
In Table I these experiments are arranged in order of increasing COs- 
content of the water flowing through the assimilation chamber. ach 
experiment proceeded exactly like the “typical experiment” described in 
“Assim. and Resp., VIII,” Section II (a). For most of them medium 
temperatures and a mediuin illumination were employed. The number 
expressing the veal assimilation in each experiment is the final value 
calculated in the way set out in “ Assim.and Resp., VIII,” Section II (c), and 
is fully corrected for CO, lost in gas-bubbles, and for CO, supplied by 
respiration. 
The unit of tissue to which it apphes is the standard area of 137 sq. cm. 
of illuminated leaf.* 
In the lower part of the diagram—tig. 1—the values of the real assimila- 
tion in the series of experiments with Fontinalis are plotted against the 
CO2-content of the surrounding water. This diagram presents several features 
of interest, and the broken line which represents the idealised relation 
of the successive values shows in a typical manner the anticipated effect of 
some limiting factor. 
* “ Assim, and Resp., VIII,” p. 384 supra. 
