1910.| On Vegetable Assimilation and Respiration. 395 
optimal amount of CO, for assimilation; rather it is a sign of a general 
narcotic effect of the strong CO. upon protoplasm, and has no specific 
relation to assimilation. It is quite a general phenomenon, and it has 
been shown by previous workers that many vital processes are depressed 
by an atmosphere containing 20 to 25 per cent. of CO, Chapin* has 
given an account of previous work on CQ,-narcosis, and has himself shown 
that the growth of shoots and roots of higher plants is thus affected. He 
finds that the growth of bean-roots is retarded by 5 per cent. COs, and 
inhibited by 25 per cent.; with the hypocotyl of Sinapis, 15 per cent. 
retards and 25 per cent. suspends growth. 
We conclude then that in the curve expressing, in any given light, the 
relation of assimilation to the whole range of COs2-concentrations from 
zero to saturation, we may separate off the falling end-part of the curve 
as an effect of narcotic poisoning. This 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 ight (cr the temperature) and is independent of increase 
of the COz-supply. 
Section IJ.—SuB-NORMAL ASSIMILATION OF UNHEALTHY PLANTS. 
We must now point out that the results just dealt with are not the 
whole body of experiments done, but are selected results, and this 
selection has to be justified. 
Many observers have noted that water-plants are very sensitive to . 
‘ unfavourable conditions of environment, and we have found marked 
depression of vigour and assimilation-power brought about by keeping plants 
in vessels of water in the laboratory. Even plants freshly collected from 
a natural habitat do not always show the same uniform vigorous activity : 
these sickly plants give low assimilation values and have been excluded 
from our tables and figures, which contain all the high normal results 
obtained under each experimental condition. 
We may now give some examples of this sub-normal assimilation. For 
instance, on July 29th, 1905, as a test experiment, pieces of Elodea were 
gathered of quite dull and unhealthy appearance, lacking the bright green 
colour of the normal plants. The assimilation of this material was measured 
in the highest intensity of light (8°8) and in water containing 0°0416 erm. 
COz per cent. In these conditions normal material would have given a 
* ‘Flora,’ 1902, Erginzungsband, p. 348. 
