1174 Wisconsin Academy of Sciences, Arts and Letters. 
make about a dozen cultures on glucose agar. As a rule, about 
half of the cultures are uncontaminated at the end of two weeks. 
In earlier experiments the third inoculation was made into a 
medium containing agar and a mineral solution only, but the 
development was very much slower and it was more difficult to 
separate the cultures that contained bacteria and fungi. The 
purity of cultures was determined at first by plating a sample 
of the culture in a nutrient medium which was known to favor 
the development of bacteria, to see whether or not there were 
any bacteria present. Later this was found unnecessary, since 
all infected glucose-agar cultures are readily detected. 
Having obtained pure cultures, they can be preserved on the 
agar surface or in a liquid medium. I have preferred to keep 
iny cultures running in 200 cc. Erlenmeyer flasks containing 
about 50 cc. of the mineral solution to which 0.2 per cent of 
glucose has been added. This small proportion of glucose is 
better, since, as Artari has pointed out (3), the algae are apt 
to degenerate in stronger concentrations of glucose. 
The algae used in this investigation, which were grown in 
pure culture were Scenedesmus acutas Meyen, Scenedesmus 
guadricauda (Turp.) Breb., Dactylococcus infusionem Haeg., 
and Tetradesnvus unsconsinensis Smith. These algae were all 
collected from. Murphy’s creek where it flows past the Dane 
county fair grounds near Madison, Wisconsin. The unialgal 
cultures of these forms were obtained in August 1911, and the 
pure cultures in September of the same year. 
The observationsi on Pediastrum Boryanum (Turp.) Mengh. 
and Coelastrum microporum ISTaeg. were made on material in 
unialgal culture. These were obtained in unialgal culture in 
January 1913. 
The observations on Pcdiastrum tetras (Ehr.) Balfs, and 
Tetracoccus botryoides West were made on these organisms as 
they occurred mixed with other algae in nature; as yet I 
have not succeeded in obtaining them in unialgal or pure cul¬ 
ture. The material was collected from Murphy's Creek, and 
the Dane county fair grounds, in the summer and fall of 1912, 
