INTRODUCTION 33 



In order to make perfectly sure of the purity of cultures, zoospores from 

 the first plate showing no bacterial colonies were streaked once more as pre- 

 viously described, and only from these latter plates was material taken for 

 pure culture slants. In addition, the purity of stock cultures was checked at 

 frequent intervals by microscopic examination. 



The synthetic medium on which Stanier (1942) cultivated Rhizo- 

 phlyctis rosea was composed of a mineral base (l.Ogm. K 2 HP0 4 , 

 1.0 gm. (NH 4 ) 2 S0 4 , 0.2 gm. MgS0 4 , 0.1 gm. NaCl, 0.1 gm. CaCl 2 . 

 0.02 gm. FeCI 3 , 1000 ml. tap water, pH adjusted to 7.0-7.2) to which 

 cellulose in the form of lens paper, filter paper, or precipitated filter 

 paper and 0.5 per cent glucose were added as carbon sources. This 

 medium could be solidified by the addition of 1.5 per cent agar. Stock 

 cultures were maintained on mineral-glucose agar slants or in tubes 

 of liquid mineral base with strips of filter paper partly immersed. 

 Transfers were accomplished simply by flooding agar slants less than 

 a week old with water and streaking the resultant zoospore suspension 

 on agar. If older cultures or cellulose media were used, pieces of agar 

 or cellulose were removed and placed in sterile water blanks. 



According to Emerson (1950) sixteen different chytrids have been 

 cultivated in pure culture, and more have been reported since his 

 paper (Crasemann, 1954; Gaertner, 1954a-d; Koch, 1957) appeared. 



Pure cultures 1 of several chytrids were obtained by Whiffen (1941b). 

 She separated sporangia growing in unifungal culture on various nat- 

 ural substrata and transferred them by means of a capillary pipette 

 to 3 per cent plain agar. Here they were freed of remaining debris with 

 glass dissecting needles and carried to a plate of 0.5 per cent plain 

 agar. After zoospore discharge the spores swam about in the weak 

 agar, from which they (with any remaining bacteria) were picked up 

 in a fine-pointed dilution tube containing 1 cc. of sterile water. Drops 

 of this mixture were then allowed to run over the surface of a plate 

 of 0.5 per cent agar, upon which some spores free of bacteria subse- 

 quently formed young plants. These could then be transferred to nu- 

 trient media. 



Whiffen cultivated her material on the following nutrient agar media: 

 (1) 600 mg. maltose, 40 mg. meat peptone, 3 gm. agar, and 500 cc. 



1 See Koch (1957) for a method of securing pure cultures of chytrids. 



