1910.] © On Vegetable Assimilation and Respiration. 398 
the water in the chamber to the assimilating cells is much more sluggish 
than if the chamber were filled with a gaseous mixture containing 30 per 
cent. COs, so that in the latter the chloroplasts would probably be bathed in 
stronger CO, than in the former medium. No doubt we should then find 
a narcotic effect of the COz on the protoplasm, though an aqueous medium 
containing 30 vols. per cent. dissolved CO, does not produce this effect. 
Of the effect of subjecting the plants to still stronger solutions of COQ2 we 
shall speak briefly presently. 
In the middle of Table I are found the results of a long series of 
experiments with Elodea and these data are represented by small crosses in 
the upper part of fig. 1. With Elodea the same form of curve is revealed 
as with Fontinalis; the limiting value is here slightly higher, 0°0237 (average 
of last eight values),* and until it is reached increase of COQ2 causes a 
proportional increase of assimilation. 
With Elodea the assimilation values along the horizontal part of the curve 
are not so regular as with Fontinalis, but their agreement is perhaps as close 
as might be expected when it is noticed that the experiments range over 
three years and that identity for this value in successive experiments 
depends on repeating identically intense illumination of an identical area of 
leafy tissue. 
The intensity of illumination varies with the square of the distance of the 
chamber from the hght, and this had to be adjusted for each experiment but 
could not be measured directly, as the bath-window and _ cooling-screen 
intervened. An error of a few millimetres may have occurred and the light 
must have varied from changes in efficiency of the mantles. The uniformity 
of the area illuminated, 137 sq. cm., depended upon careful packing and 
distribution of the green shoots upon the silver grid; this was always 
covered as completely as possible, and the error from this cause is probably 
not great. 
The difference of about 5 per cent. between the limiting values for the two 
plants has possibly no real significance at all and we may well hold that 
equal areas of different water-plants equally illuminated produce the same 
amount of assimilation, provided of course that light is the limiting factor. It 
may be recalled that this law was demonstrated in a previous papery for 
a variety of types of land-plants and in that case also the agreement found 
experimentally was within 5 per cent. 
* The sixth value, 0:0276, is excluded from this average because a more intense light, 
= 8'1, was then employed, which would give too high a value should light 5°7 be limiting. 
+ Blackman and Matthaei, “Assim. and Resp., IV.,” ‘Roy. Soc. Proc.,’ 1905, B, vol. 76, 
p. 444. 
