of Carbon Dioxide in Ulva latissima, L. 57 
in the two following sections. In these it is possible that in 
all cases a small amount of NaCl may have been present as 
a residuum of the original sea water. This amount, however, 
seems to have been quite negligible, for the following reasons. 
As already stated, I found that Ulva in 0-005 per cent. 
NaCl in distilled water gave a ‘moderate’ amount of starch 
in a week at the end of April. In experiments with K Cl 
alone (vide section 6), in which the percentage in distilled 
water was 0-07 (the amount in sea water), I was unable to 
obtain more than a ‘ trace ’ of starch. Now, if the impurity of 
Na Cl present were as large as -005 per cent., there would 
seem to be no reason why a ‘ moderate ' amount of starch 
would not have been formed, despite the presence of -07 per 
cent. K Cl ; for the element potassium is absolutely essential 
to the plant. Further, in no case in which a single one of the 
salts MgCl 2 , MgS 0 4 , CaSO,,, and KC 1 was used alone, in 
any percentage that I experimented with, did I obtain a 
c moderate ’ amount of starch. The small amount of starch 
found might be accounted for in some cases through the 
medium being too dilute, but if KC1 were as favourable to 
carbon-assimilation as NaCl, a solution of 2 per cent. KC 1 , 
which is equivalent to about i-6 per cent. NaCl, should 
have given a ‘ large ’ amount of starch within a week, as was 
found with this percentage of the latter salt ; whereas only 
a ‘trace’ was obtained. It would therefore seem that, if 
these results with chlorides and sulphates of potassium, 
magnesium, and calcium hold good, the amount of Na Cl 
as an impurity is so small as to be negligible, in fact, that 
it may be considered as absent altogether. The lack of even 
a moderate amount of starch-accumulation, which was charac- 
teristic of experiments with solutions of these salts, cannot, 
I think, be in any way due to the starch being used up for 
growth, as in nearly all cases, especially in solutions of Ca S 0 4 
and KC1, the plants become obviously unhealthy after a 
short time. 
