400 Dr. F. F. Blackman and Mr. A. M. Smith. [Dee. 19, 
elsewhere* been attributed to the “time factor,” which term we may continue 
to use aS a non-committal name for the collective action of certain internal 
depressant factors still obscure. 
An increase of illumination in the middle of an experiment superposed on 
a natural decline of assimilation might partly or wholly neutralise this 
decline and yet not lead to an actually increased assimilation. We have, 
indeed, shown that the progressive decline in time is less marked when the 
light intensity is increased than when it is constant, but the experiments are 
not significant enough to be given in detail here. 
Experiment D: Lemperature the Limiting Factor —The light was of 5:7 
intensity throughout and the CO2-supply 0:0230 to 0:0215. The experiment: 
started at a temperature of 7° C., which was maintained by the careful 
addition of ice to the bath at short intervals; at 4.12 p.m. the bath was. 
quickly warmed up and kept at about 21°C. from 5 to 7 P.M. (see curve of 
temperature of chamber in fig. 5). At the low temperature the assimilation 
t (SE ce ee oe 
lOc emma ea 
"O00 | 
Light =5-7 Throughout 
Fig.5 Exp.D. 
remained at 0:0115 hour after hour, not half the value that this combination 
of light and CO, permitted in 1905. It is clear then that this value marks 
* “Optima and Limiting Factors,” p. 282. 
