of Carbon Dioxide in Ulva latissima , L. 65 
Experiment XIII did not seem to affect the amount of 
carbon-assimilation. Whether the plant can get on without 
it for a time if magnesium is present, or whether reserves of 
calcium exist in the plant, did not fall within the province of 
this paper to determine. Perhaps a very little calcium only 
is needed, and this may have crept in as an impurity in these 
relatively rough experiments. The conclusion drawn, how- 
ever, was that the absence of either of these salts did not 
markedly affect the power of carbon-assimilation. 
As in the case of the chlorides, experiments were made 
with various percentages of these salts alone . 
(A) Ca S 0 4 . 
Two series of experiments were made ; in one the amount 
was the same as in sea water (-14 per cent.), and the other 
was a saturated solution. In the latter case, 5 grams Ca S 0 4 , 
finely powdered, were placed in 500 c.c. distilled water, and 
constantly stirred. After allowing the solution to stand for 
some hours, the undissolved Ca S 0 4 was filtered off. The 
solution, therefore, contained considerably less than 1 per 
cent. Ca S 0 4 . Starch-free Ulva was allowed to remain 
for 8 days or more in these two solutions, but after frequent 
testings, with two series of experiments, I never obtained 
more than a ‘ trace/ while in the great majority of cases no 
starch whatever was found. The Ulva began to fragment, 
and show other signs of becoming unhealthy after a few days. 
These experiments were conducted in the latter part of June, 
when, as already explained, the conditions were unfavourable. 
The conclusion which, I think, may be provisionally drawn is 
that CaS 0 4 alone cannot take the place ofNaCl in regard 
to carbon-assimilation. 
(B) Mg S 0 4 . 
Magnesium sulphate is readily soluble in water. Solutions 
containing 0-23 per cent, of this salt in distilled water (the 
amount in sea water) gave in all cases a ‘trace’ or a ‘little’ 
F 
