Card on- Assimilation of Marine Algae . 677 
few, owing to their insolubility or acidity ; or because some 
(sodium phosphate) precipitate in sea water. Four experi- 
ments were made in which 1 per cent, potassium -phosphate 
was added to sea water, and in these starch-free Ulva was 
exposed to light, with a control in sea water under the same 
conditions. After six days the amount of starch obtained 
was, in three experiments, 4 a trace/ while, in the fourth experi- 
ment, a 4 little 5 was obtained. The Alga became unhealthy 
in a short time. The control continued in good condition, 
and gave a 4 moderate ’ amount of starch in the same time. 
The conclusion was that the presence of an appreciable 
quantity of potassium phosphate in sea water inhibited the 
C0 2 -assimilation. Schimper 1 has shown that the fate of 
phosphates is altogether different from that of nitrates and 
sulphates. But in the formation of calcium oxalate, which 
according to Schimper takes place as follows, — potassium 
phosphate in the presence of oxalic acid forms potassium 
oxalate, and then potassium oxalate in the presence of 
an inorganic calcium salt forms calcium oxalate, — : it is 
clear that, in the first step at least, phosphoric acid is set 
free. Hence from the standpoint adopted here, the amount 
of free acid, the result with phosphates should be similar 
to that with nitrates and sulphates, as indeed it was found 
to be. 
Phosphor US) Iodine , Iron . 
I have now concluded experiments on what I have termed 
the principal salts of sea water, using that term only in the 
sense of forming the greater percentage of that substance. 
Other elements, such as iron and phosphorus, are equally 
essential to the plant, but are remarkable as occurring only in 
the minutest traces in sea water. Although no experiments, 
with the exception of those on potassium phosphate, were 
made with these bodies, it may be of interest briefly to point 
out a fact too generally overlooked, namely, how minute the 
traces of these elements in sea water really are. 
1 Schimper (’90). 
Y y 3 
