46 Arber. — On the Effect of Salts on the Assimilation 
every day or at shorter intervals as would have been the case 
with such an object in view. The time required to reach any 
special point in the scale must obviously depend on external 
conditions, such as continued brightness of illumination, tem- 
perature, &c., as well as on the vitality of the plant itself. 
The effect of these is seen in a comparison between Experi- 
ments II and III, the latter being performed in the depth of 
winter and the former at the end of April. The object of such 
ah experiment as No. Ill was to show that Ulva in sea water 
can assimilate freely, and that a maximum of assimilation is 
possible. This was the only kind of inference intended to be 
drawn from such experiments, and for this purpose the starch- 
scale given above was found to be quite sufficient. 
The work was carried on during the past year in the 
University Botanical Laboratory at Cambridge. I may here 
take the opportunity of expressing my great indebtedness 
to Mr. Francis Darwin, who first suggested the subject to 
me, and who has helped me throughout by suggestions as 
to scope and methods. I am also particularly indebted to 
him for help with a somewhat scattered literature. 
Section II. Distilled Water. 
Experiments with distilled water were especially interest- 
ing, as in such a medium there is of course a complete 
absence of nutrient salts. I found that it was only possible 
to keep Ulva alive for a short time in distilled water. 
In the majority of cases, a certain amount of starch was 
formed, although the amount was never ‘ moderate.’ Out 
of nine series of experiments, five showed a ‘ little ’ starch. 
In two others only a c trace ’ of starch, while in the remaining 
two, no starch could be detected even after several testings. 
The amount of starch in the control in sea water on the 
seventh day was { large/ showing that the Alga was in good 
condition originally. The Alga used was darkened on 
March 24, 1900, nearly two months before the experiment, 
and had been proved starch-free by repeated testings. 
