of Carbon Dioxide in Ulva laiissima , L. 67 
most of the other salts in sea water are present, none of these 
salts could be found to replace sodium chloride in this respect 1 . 
In most of the experiments in which an inhibition was caused 
by the presence or absence of a certain salt, such an inhibition 
was rarely absolute at first. Even in experiments with dis- 
tilled water, a very small amount of starch was detected, and 
this may have been due to reserves of inorganic material 
within the plant. If these conditions were continued, not only 
did the inhibition become absolute, but such conditions were 
generally found to be fatal to the plant. 
Ulva is a member of a special biological group of plants, 
the Halophytes, and probably an obligate Halophyte. In 
such no doubt there are special requirements which may have 
in part arisen as adaptations to the environment, and which 
are largely or totally absent in other groups of plants. Con- 
sequently such conclusions as are here drawn are not general, 
but only applicable to such plants as are Halophytes, and 
perhaps only to obligate Halophytes. 
The following is a short summary of the chief results 
obtained : — 
1. Distilled water for a short time allows of a very small 
amount of carbon-assimilation, but is quickly fatal through 
the absence of all essential inorganic salts. 
2. Tap water, containing only a small percentage of salts, 
will permit of a comparatively large amount of carbon- 
assimilation, but not the maximum. 
3. Sea water, being the natural medium of these plants, 
allows of the maximum of carbon-assimilation within the 
shortest time. 
4. Sodium chloride seems to be an absolutely indispensable 
salt to the medium for even a moderate amount of carbon- 
assimilation. From indirect evidence, in which this salt was 
as far as possible absent, in rough experiments, there was 
found to be a marked inhibition. 
1 It is possible that by a process of accommodation MgCl 2 , or some other 
salt, could be made to take the place of Na Cl, but this is a point which it did not 
fall within the province of this work to determine. 
