Car bon- Assimilation of Marine Algae . 675 
assimilation. There is little doubt, however, that this faculty 
is only attained by a process of gradual and natural accommo- 
dation, the absence of which in the experiments recorded 
here, I think, fully explains why the Algae did not flourish. 
As to the exact cause of the lowering of the C 0 2 -assimila- 
tion by the presence of these nitrates, it is impossible to offer 
more than suggestions. It is, however, certain that the result 
here is not due in any way to plasmolysis. Taking the 
amount of common salt in sea water as 2*5 per cent., the 
amount of KNO s , which is the isotonic equivalent of 2*5 per 
cent. NaCl, is about 4-3 per cent. KN 0 3 , a larger percentage 
than was used, and which even then would not have caused 
plasmolysis. Since plants absorb all substances, essential or 
injurious, presented to them in a soluble form, it follows 
that, seeing there is no plasmolysis, the nitrate, with other 
salts absorbed from sea water, penetrates within the proto- 
plast. This being so, it seems to me that there can be 
either a physical, or a chemical cause for the effect produced 
by these nitrates. It is known that many soluble crystalline 
substances cannot diffuse through certain plasmatic mem- 
branes. If this is the case here, the diosmosis of such salts 
as NaCl, which in Ulva would seem to have a peculiar value 
as absorption-products, would be checked, and consequently 
an inhibition of the C 0 2 -assimilation might take place. On 
the other hand, this inhibition may be due to a chemical 
change, which takes place within the plant on the absorption 
of a considerable quantity of these nitrates, and which may 
be directly injurious to the protoplast, unless a process of long 
accommodation has taken place. 
With regard to the first of these alternatives, there is no 
evidence, and I am inclined to favour the chemical theory, 
to which certain facts would seem to point. Pfeffer 1 says 
that * potassium nitrate, when absorbed, is gradually con- 
verted into a salt of an organic acid, the traces of nitric acid 
set free being immediately absorbed by the protoplast.’ It 
is possible that, when a considerable quantity of a nitrate is 
1 Pfeffer (’ 00 ), p, 131. 
Yy 
