402 MR. A. J. EWART ON ASSIMILATORY INHIBITION. 
cells of seedlings of Lupines only freeze at —4^ C., and that 
hence above this temperature respiration can still continue. 
At zero he finds respiration to take place with jlth to jth 
its optimal activity. Jumelle finds that with Conifers and 
Lichens respiration may not entirely cease till a temperature of 
— 10? C. is reached. This statement is rendered more probable 
by the researches of Dixon and Joly *, who find that in a plant- 
tissue all the intracellular and imbibed water may not freeze 
until the temperature is lowered to —10? C. or —12? C. As 
regards assimilation, Jumelle states that Evernia Prunastri, Picea 
excelsa, and Juniperus communis at from —30? C. to —40° ©., if 
exposed to light, evolve oxygen and absorb CO,. He supposes 
that the heat-rays which fall upon the specimens along with the 
light-rays when exposed to light may prevent all the water of the 
plant-cells being frozen even when exposed to from —30° C. to 
—40° C., and thus permit of assimilation taking place, though in 
darkness all the contained water is frozen and respiration ceases 
at —10? C. To this hypothesis two objections may be made:— 
(1) the same heating-effect would be produced upon the regis- 
tering thermometer as upon the exposed specimens; and (2) if 
such a special heating-effect were produced, then the experiments 
would be valueless, for the specimens would be exposed to a 
temperature higher than that registered by the thermometer. 
It is possible that the changes in the composition of the air 
of the receiver observed by Jumelle at these low temperatures 
may be due not to the continuance of any actual assimilation, 
but to physical diffusion, carbonic acid being slowly absorbed 
and oxygen slowly exhaled from the frozen specimens. The 
following facts make probable this assumption, thus:—(1) The 
amounts of gas absorbed and exhaled are relatively small. (2) 
The volumes of oxygen and CO, which are exhaled and absorbed 
do not at allcorrespond. (3) The gaseous interchanges from the 
first diminish in amount as the exposure continues. (4) The 
gaseous interchanges do not alter in amount within very wide 
ranges of such low temperatures, and only increase when the 
temperature is raised to a few degrees below zero. 
When the experiment is started, the plants being exposed to 
light, and at first at a temperature above zero, will continue to 
assimilate until the temperature falls below zero and the plants 
* “The Path of the Transpiration-current,” in Annals of Botany, 1895. 
