40 Arber. — On the Effect of Salts on the Assimilation 
case was taken as a measure of the carbon-assimilation during 
the period of exposure. 
The Ulva I used was partly obtained from the Welsh 
coast at Bangor through the kindness of Professor Phillips, 
and partly from Hunstanton on the Norfolk coast. I found 
that the Alga succeeded best when gathered as near the low 
tide limit as possible. Another point of great importance 
was that it should be free from other kinds of seaweed. 
The Ulva was placed in sea water in large earthenware 
vessels, in a cool dark room in the laboratory. The 
temperature of this room was at the end of April about 
i6°-i8° C. at midday ; at the end of June between 1 8°-20° C., 
or even higher. The sea water was supplied by the Great 
Eastern Railway from the coast at Lowestoft. The specific 
gravity of two samples was taken and found to be (i) 1026 ; 
(2) 1-0254. The average specific gravity of sea water is 
about 1-027. The samples were therefore slightly lower than 
they should have been, but the difference is so small as to be 
negligible. 
The chief difficulty experienced in this work was to keep 
the Alga in good condition during the prolonged period of 
darkening that was necessary to get rid of all accumulated 
starch. During the winter and spring this difficulty was not 
so great, but in May and June it was found to be almost 
impossible to preserve the Algae in a healthy state, although 
the period of darkening was then shorter. This difficulty 
was undoubtedly one of temperature. Modern work 1 has 
shown that these Algae require a comparatively low tempera- 
ture ; in fact, as cool a situation as possible. I had no 
means at my command for keeping down the temperature in 
1 During the last decade considerable attention has been directed to the best 
physical conditions and mediums in which to cultivate marine and freshwater 
Algae. Attention may be called to the following works : — Klebs (’96), Oltmanns 
(’92 and ’95), Noll (’92), Ward (’99), Molisch (’95). References to the works 
quoted will be found in the Bibliography at p. 69, at the end of this paper. The 
numbers in brackets after the authors’ names indicate the year of publication of 
the work referred to in the Bibliography. Thus Noll (’92) means that the paper 
was published in 1892. 
