18 
for 2 years or longer. The length of time that the young Ulva 
plant remains attached by its own disc, and the method of propaga- 
tion in these vast beds has not yet been studied in detail. Kylin 
remarks that he has never observed the large floating fronds ina 
fruiting condition. Schiller, commenting on the quantity of loose 
Ulva near Triest, states that it is always sterile. 
In being able to flourish in stagnant and polluted water Ulva 
latissima is unlike other marine algae. Quiet water is usually poor 
in species and localities contaminated by sewage are always avoided 
by the algologist. Most marine algae prefer turbulent conditions, 
or situations where a vigorous current or tide ensures thoroug 
aeration and constant renewal of the necessary salts. 
The remarkable power possessed by Ulva for assimilating nitro- 
genous compounds, may possibly explain its excessive growth in 
contaminated water. The supply of oxygen, which in such water is 
exceedingly poor, is possibly derived from that evolved from 
the plant itself. The i f ammonia and nitrates would 
stimulate growth and hence also the assimilation of carbon dioxide, 
and this in turn would be attended by a corresponding liberation of 
oxygen which in quiet water would not be swept away but be 
available for respiration by the plants. Letts has pointed out that 
Ulva may be regarded as an instrument designed by nature for the 
final purification of polluted sea water. Bacteria first convert the 
nitrogen of the albuminoids and other organic compounds into 
ammonia and nitrates, the Ulva follows absorbing these substances 
and using the carbon dioxide which has also been supplied in 
abundance by the action of micro-organisms, the final result being 
the oxygenation of the water. This suggests an enquiry as to wh 
it is that other algae cannot flourish under similar conditions. 
any species are very sensitive to a lack of the normal amount 0 
ium chloride, and it is probable that they are equally susceptible 
to the presence of impnrities, It would be of special interest to 
