of Carbon Dioxide in Ulva latissima , L. 51 
the biology of Algae. It is well known for instance that 
certain Algae flourish in the estuaries of large tidal rivers, 
in localities which at low tide are practically freshwater. 
These results with tap water would seem to show that even 
at low tide the carbon-assimilation is not inhibited but that 
it can go on even in very dilute solutions, until on the return 
of the tide the water becomes strongly saline again. It has 
been long known that some marine Algae can accommodate 
themselves to brackish water and ultimately to fresh water, 
if the process is gradual and prolonged. On the other hand, 
other Algae are obligate Halophytes, and will not thrive 
for any length of time in dilute solutions of nutrient salts. 
To which class Ulva belongs I am unable to speak posi- 
tively or definitely, as I had not the time at my disposal for 
the long accommodatory experiments necessary to answer 
this question. Whatever evidence these experiments afford, 
seems rather in favour of the view that Ulva is more or 
less obligate, and in that case these experiments with tap 
water show that a habitat, which is exposed to short periods 
of almost fresh water conditions, would cause a very slight, 
if any, inhibition of the power of carbon-assimilation, even to 
obligate Algae. 
Section IV. Sea Water. 
Many cultures of Ulva were made in sea water, chiefly 
as controls to other experiments, and as a rule there was 
no difficulty in obtaining the maximum of starch accumula- 
tion. A ‘ moderate ’ amount of starch was usually formed 
in a couple of days or even less of bright weather, whereas 
in the case of Experiment III, in mid-winter, the period was 
much longer. The chief use of such controls, apart from 
comparison with other cultures as to the amount of starch 
formed, was to make sure that at the beginning of the ex- 
periment the material was in a healthy condition. 
Sea water being the natural medium in which such 
Algae live, contains all the salts necessary for its life and 
E 3 
