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MR. A. J. EWART ON ASSIMILATORY INHIBITION. 367 
Bacteria move much more actively inside it, to and fro from the 
partition wall, than they do elsewhere. This is, however, because 
a certain amount of nutritious substance is present in such cases, 
and the Bacteria move more actively in the presence of dissolved 
nutritious substance and oxygen than in the presence of oxygen 
in water only. In many cases, on exposure to strong light the 
chlorophyll grains accumulate for the most part on the partition 
walls of the hair-cells, and in such separated end cells the 
evolution of oxygeu is naturally more marked from the end 
wall than from the side walls. Epidermal hairs of Cucurbita 
Pepo and epidermal cells from Allium and Aspidium gave similar 
results, the chlorophyll grains being rather more numerous and 
the evolution of oxygen correspondingly more active. In the 
young leaves of Cucurbita, where the hairs are very abundant 
and large, the total amount of material assimilated by them must 
be fairly great, and is most active at a time when the assimilatory 
powers of the leaf are weak and undeveloped, but when it never- 
theless demands stores of plastie material in greatest abundance. 
It must be remembered that the conditions for the evolution 
of oxygen gas from a cell in water are not quite the same as 
they are in air. ln the first case, the gas simply diffuses from a 
liquid in which it is abundant, through a damp membrane, to a 
fluid in which it is less abundant ; in the second case, it is evolved 
from the surface of the cell because its partial pressure in solution 
is greater than in the mixture of gases, 7. e. the atmosphere, by 
which the cellis surrouuded. In ordinary aerified water, owing 
to the fact that CO, is much more soluble in water than oxygen 
is, the evolution of the former gas will take place much more 
rapidly and readily as compared with the rates in air, from a cell 
in water, than that of the latter gas will. In the closed cell 
with Bacteria, on the other hand, since the partial pressure in 
solution of the dissolved CO, is very high, whilst that of oxygen 
is very low, the latter gas will be evolved more rapidly and 
readily than the former. The difference in the rate at which 
gas is evolved persists only a very short time after transference 
from the one medium to the other, and ceases as soon as the 
amounts of gas dissolved in the cell-sap and permeating the cell 
are adjusted to their new levels, when the rate at which gas is 
evolved becomes, provided that the rate of production remains 
unaltered, the same as it was previously. A living cell or tissue 
kept in a closed cell-chamber with Bacteria will reach a condition of 
