1914) 
FOSTER—SOURCE ОҒ COMBINED NITROGEN FOR ULVA 231 
saprophytie conditions, as in solutions containing amino acids 
and glucose in the absence of light and carbon dioxide. In all 
these cases, however, growth is more rapid under so-called 
mixotrophie conditions, i. e., with both organie nitrogen and 
carbon present in addition to sunlight and carbon dioxide. 
Artari takes up the question of the relative value of different 
nitrogenous compounds, and shows that they vary greatly 
with different algee,—some preferring peptones, others, amino 
acids and ammonium salts, and a few, nitrates. On the whole, 
the majority of forms investigated grow best in the presence 
of amino nitrogen. Many alge, especially of this last class, 
are often found in water polluted with sewage or decaying 
organic matter. 
Among the marine alge, there is a more or less definite flora 
characteristic of sewage-polluted waters. Most conspicuous 
among the plants of this group are the species of Ulva. Letts 
and Richards (11), in their reports on sewage in British har- 
bors, state that Ulva latissima grows in excessive quantities 
in polluted waters, and they find that the nitrogen content of 
this seaweed varies with the degree of pollution of the water. 
Cultural experiments conducted by Letts and Richards showed 
that Ulva latissima grows more rapidly in a mixture of sewage 
and sea-water than in pure sea-water alone. 
EXPERIMENTAL 
Preliminary experiments were made at the Woods Hole 
Laboratory to determine the sources of available nitrogen for 
Ulva lactuca. The algal material used in the experiments was 
collected at the mouth of an inlet where the water was at all 
times highly polluted with sewage. The cultures were main- 
tained in the laboratory in glass tumblers containing 150 сс. 
of solution. When brought in, the fronds of Ulva were well 
rinsed in clean sea-water and cut into strips exactly 3 cm. in 
length and about 2 cm. wide. Three such strips were placed in 
each vessel, and the cultures kept at a temperature of 21°C. 
by placing the vessels in a tray of running water. In each 
case the solution was renewed at the end of 5 days. After 10 
days the strips were again measured and the increase in length 
recorded. 
