404 Dr. F. F. Blackman and Mr. A. M. Smith. [Dee 19, 
caused generally a proportional increase in the rate of bubbling, but further 
increase of light, even up to 64/1 sunlight, gave no further increase of 
bubbling, but a maintenance of the same uniform rate as at 1/2 sunlight. 
To us this is a perfect example of the action of COs-supply as a limiting 
factor; the temperatures were between 20° C. and 28° C., and therefore too 
high to be limiting with this small (though indeed unknown) CO>-supply. 
Reinke naturally, at that time, regarded the long level part of the curve 
as being in direct relation to the intensity of light and as being a prolonged 
optimal effect. 
It should be mentioned here that Pfeffer, in 1897,* suggested that possibly 
the photo synthetic work of the Elodea in Reinke’s experiments was limited 
by the inadequate CO.-supply, and that if adequate CO, were supplied, an 
optimal relation between light and assimilation might become evident. This: 
first suggestion of a specific factor limiting the activity of assimilation was 
not developed further. 
Treboux, in Pfeffer’s laboratory, worked chiefly at the effect of drugs, acids, 
and salts upon the bubbling of Elodea, using the light of one or two Auer 
burners at a temperature of about 16° C.in a dark room; he has, however, 
in addition a section upon the influence of the quantity of dissolved COz. 
His procedure was to change, every fifteen minutes or so, the water in which 
the Elodea was bubbling for a water containing twice the previous quantity 
of COs All the COs-concentrations were low, beginning at 0:1 vol. CQ2 © 
per cent., and doubling up to 12°8 vols. COz per cent. His series of 
bubble-countings show that at first the rate doubled with the doubled CO:- 
supply, but that abruptly the increase ceased,and further concentrations of 
COz gave continually only the same value. 
On carrying out a similar experiment with a doubled light throughout 
(Experiment X VIII), the progressive doubling of assimilation was continued 
one stage longer, and then a maximal value was reached—at about 3 vols. 
CO: per cent.—and maintained unaltered. 
Treboux concludes (1) that assimilation is proportional to COs- Seale and 
(2) that with weak lights the CO.-optimum shifts proportionally with the 
light-intensity. Treboux’s observations are to us also a clear case of the 
CO.-supply at first being a limiting factor to the assimilation, and then the 
feeble light in its turn acting as one. ‘These results are just the converse 
of Reinke’s, and both are according to expectation. : . 
The work of Pantanelli, also finished in Pfeffer’s laboratory, must be con- 
sidered in more detail. It is the latest and fullest work which deals with 
the effect of the factors of light-intensity and CO2-supply upon assimilation 
* ‘Physiology,’ vol. 1, sect. 59. 
