56 Arber .• — On the Effect of Salts on the Assimilation 
Experiment VIII. April 15, 1900. Ulva in a solution 
of Na Cl in tap water. 
Date. 
Days. 
2.5% NaCl. 
5°/ 0 Na Cl. 
7'5% NaCl. 
Apr. 1 6 
1 
moderate 
little 
trace 
Apr. 18 
3 
large 
moderate 
little 
Apr. 20 
5 
large 
large 
moderate 
Any direct attempt to estimate the effect of the total absence 
of sodium chloride , and more especially of the element sodium, 
would necessitate an exceedingly difficult and laborious piece 
of research. For such, the most pedantically accurate experi- 
ments, as well as the most stringent precautions, would alone 
suffice. In the first place it would be exceedingly difficult to 
obtain Ulva free from starch and in good condition, without 
any trace of the sodium chloride from the sea water in which 
it was darkened. Glass vessels could not be used, and the 
water could not be distilled from glass retorts. Lastly, 
perhaps the most insuperable difficulty of all would be the 
air, which always contains sodium in the shape of dust. The 
complete absence of sodium would therefore be very difficult 
to insure. Pfeffer 1 says that ‘ seaweeds have not up to the 
present time 2 been cultivated in the absence of sodium 
chloride/ nor ‘ has it been found that plants frequenting saline 
habitats can exist without sodium.’ 
It was no part of my object to solve the problem of the 
complete absence of sodium chloride. I am able, however, to 
offer indirect evidence from experiments in which the amount 
of sodium chloride was apparently only very small, if any at 
all was present. I may here anticipate the results obtained 
from the experiments in which a single one of the principal 
salts in sea water, other than Na Cl, was alone present in the 
medium. A fuller account of these experiments is contained 
1 Pfeffer (’00), p. 41 1. 
2 1897. 
