298 BOTANICAL GAZETTE [APRIL 
possibly low temperatures might also cause filaments of this alga to take 
the palmella form. Experiments were devised to test this point, and their 
results are here given. 
Cultures of the filamentous form were made in small glass dishes with 
loosely fitting covers. The medium employed was the modification of 
Knop’s solution previously described, and had an osmotic pressure of 
60™™ of mercury. The culture dishes were placed in weighted beakers 
which floated about three-fourths immersed in ice-water contained in a 
galvanized iron tank. Ice was added from time to time as melting took 
place, and the superfluous water was drawn off. The tank was covered 
with glass and stood in the conservatory, so that the plants were supplied 
with the necessary light for growth. The cultures were shaded from direct 
sunlight. 
In the medium employed the alga grows rapidly as filaments at labora- 
tory and conservatory temperatures. Zoospores are produced and germi- 
nate to form new filaments. Such normal filaments are shown in jig. 2- 
The figures are all from camera drawings and are magnified about 300 
diameters. In the cold cultures, whose temperature rarely rose above 
6° C.,5 the growth of the original filaments was checked, but the production 
of zoospores continued at about the normal rate. At the end of fifteen 
days the old filaments had completely changed to the palmella form, in the 
manner already described for solutions of high osmotic pressure. Z0o- 
spores fail to germinate normally in the cold; most of them simply lie quies- 
cent on the bottom of the dish, having assumed the spherical form, while 
a few enlarge slowly and divide into new palmella cells. Growth of the 
palmella form is comparatively very slow at ordinary temperatures. It Ls 
still more so in the cold, and this retardation is here even more marked in 
resting zoospores than in cells produced by the breaking up of the original 
filaments. Palmella cells from one of the cold cultures are shown in 7g: 7- 
They are seen to be exactly similar to those produced by high pressure = 
toxic cations. Several resting zoospores and empty sporangia are also 
figured. That the plant was not permanently injured by the low tempera- 
ture was shown conclusively by continuing the cultures in the conservatory 
after they had been taken from the cold bath. They all responded to te 
return to normal temperature, by producing typical filaments in from win 
to fourteen days. A portion of one of these cultures after the filamentov 
form had been assumed again is shown in jig. 2. Germinating ee 
and empty sporangia are also shown. 
4 Loc. cit. (4), p. 4. ; 
During the period of the experiment, 20 days, the temperature was u 
nwittingly 
allowed to approach ro° C. several times, for periods of a few hours only. 
