146 DR. A. J. EWART ON THE EVOLUTION OF 
oxygen from an assimilatory cell or tissue ceases in the darkness 
generaly in a few seconds, and at the outside in a minute or 
two. If, however, an actively assimilating cell be suddenly 
killed, it may continue for as long as an hour after death (Bacte- 
rium Termo method) to show a very faint evolution of oxygen. 
If a quantity of green alge be killed by HgCl, whilst a current of 
hydrogen is being passed through, and the tube then soon sealed 
and heated to 90? C.-100? C., the dead alge may apparently 
evolve per gramme of moist weight 0:005 to 0:007 c.c. of oxygen, 
an amount, however, which is almost within the limits of error. 
If these experiments are repeated with alge kept in darkness 
during the whole time, or if the current of hydrogen is continued 
for 15-20 minutes after the algæ have been killed, no perceptible 
trace of oxygen is found. It is possible that a minute trace of 
oxygen is evolved from an assimilating cell which is suddenly 
killed in this manner, but there is no reason to suppose that the 
oxygen was anything else but dissolved oxygen derived from 
the previously active assimilation and permeating the cell-sap 
and protoplasm. 
An observation, mentioned in a previous paper *,is interesting 
in this connexion, namely, Chara-filaments in a current of 
hydrogen cease to show rotation in one-third the time that they 
do if simply kept in perfectly oxygenless water in a closed cell. 
In the former case it has been shown (J. c.) that an assimilating 
cell brought into an atmosphere of hydrogen evolves oxygen for 
a short time more rapidly than normally. Hence it is possible 
that the above difference is due to the fact that the cell-proto- 
plasm holds oxygen in two forms, partly in a loose, and partly 
in a strong combination ; the former, in the absence of an external 
supply of oxygen, being used up first and the latter later. Inà 
eurrent of hydrogen the loosely-held oxygen would be evolved 
and lost, and hence rotation would cease sooner than if the 
Chara-filament were simply immersed in a closed cell in oxyge™ 
less water, where all the supply of combined stored oxygen 18 
available for intramolecular respiration. Just as feasible an 
explanation, however, is that in the current of hydrogen all the 
oxygen which the cell contains, held merely in solution by the 
cell-sap &c., is rapidly evolved and lost, whereas in the oxyge™ 
less water practically none of this dissolved oxygen is lost; but 
* “On Assimilatory Inhibition,” in Journ. Linn. Soc. (Bot.) xxxi. (1896) 
p. 386, 
