28 AQUATIC PHYCOMYCETES 



normal zoospore emergence, and tend to reduce the rate of multiplication 

 of bacterial contaminants. 



A striking illustration of the importance of watching for toxic metabolic 

 products was revealed in the work of Emerson & Cantino [1948] on Blasto- 

 cladia. This organism produces acidic products so rapidly from glucose that 

 the pH of the medium close to the colonies drops quickly below the lower 

 limit of tolerance, and transfers have to be made every few days in order to 

 maintain viable material on agar. That these observations are of more than 

 isolated significance became clear recently when the writer found that Rhipid- 

 ium and Sapromyces are also strong acid producers which require frequent 

 transfer when they are cultured on agar media. Cantino [1949b] has just 

 reported that Pythiogeton too forms acids when carbohydrates are metab- 

 olized. 



While the development of methods for the isolation and cultivation 

 of chytrids has been receiving increasing attention, it should be noted 

 that Zopf (1887) long ago pointed out that some chytrids could be iso- 

 lated from water on fern spores or pollen grains. The bait could be drop- 

 ped into a large sample of pond water (one liter or more) and watched 

 for fungi. By transferring to sterile water, with a sterile needle, a pollen 

 grain having a single ripe sporangium and adding new pollen grains 

 a unifungal culture could be established. Since this sporangium was 

 formed from a single zoospore, the culture started from it, if no other 

 plants were present on the pollen grain, was the equivalent of a single- 

 spore culture. This method has been used with success by Couch (1939a) 

 and his students in developing unifungal cultures of a number of chy- 

 trids. Couch has found pollen of Liquidambar a particularly favorable 

 substratum for this work. Unifungal cultures have been reported by 

 other investigators. Minden (1902) briefly records the isolation and cul- 

 tivation on plum (or prune) gelatine of an unnamed member of the 

 Rhizidiaceae, but he does not indicate whether or not the cultures were 

 free from bacteria. Sparrow (1931c) isolated filaments of Spirogyra 

 infected with Cladochytrium replication and after washing them thor- 

 oughly in sterile water, planted them in dishes of maize agar. The 

 fungus spread out from the algal thread into the medium. Because of 

 the chytrid's slow rate of growth, however, it could not be freed of 

 bacteria. Other chytrids have been secured in unifungal (but not always 

 pure) culture by various investigators, notably Karling (1937a, et seq.), 



