of Carbon Dioxide in Ulva latissima , L. 49 
in quantity. For reasons more fully expressed in Section V, 
I do not attribute this to an incomplete removal of all traces 
of sea water from the Ulva at the beginning of the experi- 
ment. These results can only, I think, be accounted for 
either by supposing that the carbon-assimilation can go on 
for a very short time in the total absence of inorganic nutrient 
salts, or that the small starch-accumulation is due to the 
presence of reserves of such salts within the plant. I am 
inclined to favour the latter hypothesis, since there is evidence 
to show that such reserves are constantly present in the plant. 
An experiment of de Saussure 1 , made more than a century 
ago, illustrates this fact. De Saussure placed some Pepper- 
mint plants with their roots in pure water, in a place exposed 
to air and light but sheltered from rain. After allowing them 
to vegetate for a period of two and a half months, he found 
that plants which originally weighed 100 parts had increased 
to 316 parts, and the total dry matter, originally 40 - 3 , had 
become 63 parts. It is obvious then that at least one-third 
of the ash of the original plants was in excess, and constituted 
a reserve which was drawn on during the two and a half 
months of culture in pure water. 
The lack of knowledge of the nature and amount of the 
various inorganic reserves available within the plant was 
a constantly present difficulty throughout this work, and in 
this instance prevents any definite conclusion. The fact that 
a small amount of starch can be formed in the absence of 
nutrient salts, in no way invalidates the conclusion that a total 
inhibition of the carbon-assimilation can be caused by the 
continued absence of all the inorganic salts obtained by means 
of absorption. 
Section III. Tap water. 
Several experiments were made in which ‘ starch-free ’ 
Ulva was exposed to light in ordinary tap water, and the 
difference between these cultures and those in distilled water 
was very striking, and forms a good illustration of the fact 
1 De Saussure; vide Johnson (’69), p. 175. 
E 
