42 Arber. — On the Effect of Salts on the Assimilation 
in old specimens), and the thallus would fragmentate into 
small pieces, or the solution become cloudy. Side by side 
with this the chlorophyll would disappear ; although this 
sometimes took place before fragmentation. 
The period of darkening required to obtain the Ulva 
starch-free was much greater than was at first anticipated. 
In the winter and spring months a period of five weeks or 
longer had to be allowed. After a month, however, most of 
the starch had disappeared. In May and June the period 
was shorter ; no doubt as a result of the higher mean tem- 
perature. In three weeks to a month all the starch had 
disappeared. The Alga was tested from time to time, and 
if after two or three testings no starch was found, it was at 
once used for exposure experiments. 
Experiment I. Ulva darkened Feb. 24, 1900. 
Date. 
Days. 
Amount of Starch. 
Feb. 26 
2 
maximum 
Mar. 10 
14 
large 
Mar. 18 
22 
moderate 
Apr. 7 
4 2 
a trace 
Apr. 9 
44 
none or the slightest trace 
Apr. 11 
46 
none 
Apr. 15 
50 
none 
Even after such prolonged darkening it was sometimes 
found that a small part of the thallus still retained a trace. 
A fact which was noticed throughout, and which I am unable 
to explain, was that when only a trace of starch was present, 
either as a residuum of darkening or as a minimum of carbon- 
assimilation, such starch was almost invariably to be found 
only at the edges of the thallus, and in this position there 
was in nearly all cases more starch than in the centre even 
when a moderate amount was present. 
Another point observed in connexion with the darkening 
was the change in colour of the thallus when exposed to light 
