of Carbon Dioxide in Ulva latissima , L. 45 
1. Maximum : thallus a uniform deep black. 
2. Large : thallus black but starch not uniformly distri- 
buted, i. e. darker in some places than others. 
3. Moderate : thallus bluish-black at the edges. Centre 
of the thallus blue, with less starch. 
4. Little : thallus bluish at the edges with hardly a trace 
in the centre. 
5. Trace : thallus only very faintly blue. 
The following experiment, and the inferences drawn from 
it, may serve to illustrate the use of the starch-scale, and its 
limitations in regard to reliable evidence. 
Experiment III. Ulva (starch-free) in sea water, 
January 19, 1900. 
Date. 
Days 
'Exposure. 
Amount of Starch. 
Jan. 20 
1 
a trace 
Jan. 23 
4 
a little 
Jan. 29 
10 
moderate 
Feb. 2 
M 
large 
Feb. 6 
18 
maximum 
It will be noticed that the amount of starch increased for 
some time until the maximum was reached. Such a gradual 
increase was in greater or less degree common to all experi- 
ments under conditions where carbon-assimilation could take 
place freely, and is readily explained. At the end of the first 
day’s illumination the amount of starch in the cells is that 
formed during the day. But after n days’ exposure the 
amount of starch in the thallus is that formed during the 
n th day, and what remains of the starch formed during n - 1 
days. Consequently the iodine reaction after n days gives 
a much deeper coloration, the ‘ maximum ’ of the starch-scale 
representing the maximum of starch-accumulation. I wish to 
specially emphasize the fact that Experiment III was not 
made with the intention of finding the time necessary for the 
formation of a c moderate ’ or any other degree of starch. The 
testing was only performed on the days indicated, and not 
