INTRODUCTION 27 



isolated by ordinary methods and cultivated on a variety of prepared 

 solidified media. Particularly is this true of members of the Pythiaceae, 

 Saprolegniaceae, and Blastocladiales. By far the greater number, how- 

 ever, have not been isolated in unifungal culture or induced to grow 

 on artificial media. The evidence that has slowly accumulated shows 

 very definitely that many of them are capable of living under conditions 

 of the laboratory. Opportunity exists, therefore, for wider investigation 

 of their nutritional requirements and the like. Meanwhile, knowledge 

 is being steadily added to this field; see Emerson (1950) and Cantino 

 (1950, 1955) for a resume of the newer literature on the subject. 



If one is dealing with the rarer water molds, Emerson's (op. cit.) 

 comprehensive summary of methods to use in obtaining pure cultures 

 and maintaining them should be consulted. An excerpt from his paper 

 is given below. 



At least four important points ought to be borne in mind in making 

 initial isolations of water fungi. The organism may tolerate only a rather 

 narrow range of hydrogen ion concentration; aside from mineral constit- 

 uents, it will require a supply of nutrients including a source of nitrogen, a 

 source of carbon, and very possibly a source of growth factors; its zoospores 

 may easily be injured or killed if the total osmotic concentration of the medium 

 is high; and, finally, it may itself produce metabolic products which accu- 

 mulate and rapidly become toxic, thus inhibiting growth and necessitating 

 early transfer .... 



In those instances where no specific information is available regarding the 

 nutritional requirements of a fungus, the various natural media, which have 

 so long been used by mycologists, are still extremely useful, since they can 

 be expected to supply adequate amounts of all the organic nutrients which 

 might ordinarily be required. Such standardized products as the corn meal 

 agar and prune agar of the Difco Laboratories give transparent preparations 

 that permit detailed microscopic observation of zoospores and germlings. 

 Semisynthetic media, provided with yeast extract, beef extract, or peptone, 

 and some generally available carbohydrate like glucose, also serve well . . . 

 When gross enrichment cultures, like those previously mentioned, have 

 given definite indication of some of the nutritional characteristics of an 

 organism, more selective media can often be used to advantage. Thus, Stanier 

 [1942] and WhifTen [1941b] both employed mineral-glucose or mineral- 

 cellulose media in isolating cellulolytic chytrids. 



Some of the water fungi, as might be expected, seem unable to tolerate 

 high osmotic concentrations, and media containing relatively small amounts 

 of salts and nutrients are certainly advisable for initial isolations. Dilute 

 media also induce more rapid spreading of the filamentous forms, often allow 



