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MR. A. J. EWART ON ASSIMILATORY INHIBITION. 403 
are completely frozen. The intracellular and extracellular gases 
which the frozen tissues of the specimens now hold will contain a 
much larger percentage of oxygen, and a much smaller percentage 
of CO,, than the assimilatory mixture of air and CO, contained 
in the receiver in which the plant is lying. Owing to the frozen 
condition of the tissues, this inequality is only slowly adjusted by 
diffusion to and from the surrounding air contained in the 
receiver, the result being that the air of the receiver will lose 
CO, and gain oxygen just as if slight assimilation had taken 
place. 
Besides the above, there are very weighty à priori reasons for 
doubting that any assimilation could possibly take place at a 
temperature of from —30° C. to —40° C. As we have seen 
throughout, assimilation seems to be a function which remains 
active through much narrower ranges of temperature than respi- 
ration does. Thus the minimal temperature for respiration is 
apparently, as a general rule, several degrees lower than that for 
assimilation, whilst the maximal temperature for respiration is from 
5° C. to 15° C. above that for assimilation, and the same general 
differences appear to hold good for all plants, a difference in the 
minimal and maximal points for respiration being accompanied 
by a corresponding difference in those for assimilation. 
Effect of Irrespirable Gases upon Assimilation. 
Direct observations were first made under the microscope in a 
gas-chamber with a hanging drop and Bacterium Termo, mosses 
and water-plants being examined (Bryum cespititium, Ortho- 
trichum affine, Dicranum scoparium, Elodea canadensis, and Chara 
fragilis). In an atmosphere of H, the preparations continue to 
evolve oxygen for as long as they are exposed to light (10 to 12 
hours), the necessary CO, for assimilation being supplied by the 
Bacteria in the hanging drop. At the end of this time the 
evolution of oxygen is with moss-leaves still quite active, but with 
Chara and Elodea has, as a general rule, become distinctly less 
active than it was at first. 
If preparations of Chara or Elodea are kept in a stream of 
H, either in light or in darkness, they finally in from a few 
to many hours lose the power of assimilating and evolving 
oxygen, though they may still continue to show rotation. Even 
if kept exposed to light in a current of an assimilatory miature 
