394 Dr. F. F. Blackman and Mr. A. M. Smith. [Dec. 19, 
Considering only the low concentrations of CO, when COz is certainly the 
limiting factor, there is, however, an interesting consistent difference between 
the assimilation-values for the two plants; one which holds for each con- 
centration of CQO», Fontinalis assimilates just about half the amount 
achieved by Elodea for any given concentration of COs, so it looks as if the 
obstacles to the diffusion of CO, up to the chloroplasts from outside were less. 
in Hlodea than in Fontinalis. 3 
This difference seems to be a class distinction between the Bryophyte 
and the Phanerogam, for values obtained with plants of Potamogeton and 
Ceratophyllum are of the same order as those for Elodea, and quite 
different from those for Fontinalis. These experiments are given at the 
bottom of Table I, and the values for the real assimilation are incorporated 
in the top part of fig. 1. The four points marked P or C fall into perfect 
line with the crosses which represent the values for Elodea. The two 
highest values for Ceratophyllum, 0°0321 and 0:0329, are too high to be 
obtainable with light of 5:7 intensity, and it will be noted that the light 
was 80 and 8°8, due to its being placed closer to the plant. Thereupon 
the light ceases to be limiting, and the proportional relation to COs-supply 
is continued further than with the weaker light of 5°7. .Ceratophyllun 
in 00014 grm. CO: solution, however, must be limited by the CO2-supply, 
and so, although the light has the higher intensity, the assimilation value 
is down near that for Elodea in a similar solution. | 
The Bryophyte then is less efficient in utilising the CO, that surrounds 
it; possibly this may be the outcome of important structural differences, 
as submerged flowering plants have a well developed system of air- 
spaces containing an “internal atmosphere,” while Fontinalis lacks this 
physiologico-anatomical characteristic. 
This internal atmosphere certainly makes for greater efficiency in gaseous 
exchange generally, and the only doubt in accepting this theory of the 
cause of the difference between these plants arises from the fact that the 
leaves of Fontinalis are only one cell thick, and would therefore be 
expected to be highly efficient in assimilation. 
A certain number of estimations of assimilation have been made with 
concentrated solutions of COs, exceeding those dealt with in the present 
paper and approaching saturation. With these concentrations there may be 
vigorous giving off of gas-bubbles; but, on analysis, the gas turns out to be 
mostly COs, and the corrected real assimilation is much depressed. 
That, ultimately, depression of the assimilation sets in when the CQO: is 
sufficiently increased is no evidence in favour of there being a primary 
