50 Arber—On the Effect of Salts on the Assimilation 
‘ that from ordinary tap water containing mere traces of salts, 
plants may collect large quantities of non-volatilizable mineral 
constituents, and that, moreover, these constituents are ab- 
sorbed in proportions altogether different from those obtaining 
in the water in question V The Cambridge tap water con- 
tains about -036 per cent, of total solids, of which the largest 
components are CaC 0 3 = -on 8 per cent, and NaCl = -0032 
per cent. A typical experiment was as follows : — 
Experiment V. April n, 1900. Ulva exposed in 
tap water. 
Date . 
Days. 
Amount of Starch. 
Apr. 13 
2 
moderate 
Apr. 15 
4 
large 
This result was confirmed by four other experiments, 
although in one or two cases I failed to obtain more than 
a ‘little’ starch. I concluded, therefore, that for some time 
at least there was no marked inhibition of carbon-assimilation 
when the medium was tap water. Experiments for a period 
longer than a week were not however successful ; the Alga 
becoming unhealthy and finally dying. In none of these 
experiments did I obtain the maximum of starch-accumula- 
tion. The larger amount of starch formed in a medium of 
tap water, as compared with distilled water, can only I think 
be accounted for by the fact that the former does contain 
some nutrient substances in however small proportions ; and 
that the plant can make use of these to such a degree that 
for some time there is little inhibition of the carbon-assimila- 
tory function, or at least only a very partial inhibition. If 
this is the case, we have here a striking illustration of the 
extent of the influence of nutrient salts on the power of 
carbon-assimilation. 
These experiments would seem to throw some light on 
1 Pfeffer (’00), pp. 120-1. 
