92 THE PHYCOMYCETES 



SAPROLEGNIALES 



Members of this order are commonly termed "water molds." 

 The group includes about 15 genera and approximately 120 

 species, of which a number are of rather common occurrence. All 

 members of the order are characterized by a well-developed mv- 

 ceHum, the hvphae of which have cell walls composed of cellu- 

 lose. The zoospores, produced in zoosporangia of various t\"pes, 

 are invariably biflagellate. Sexual reproduction occurs by means 

 of antheridia and oogonia containing one to several oospores; 

 periplasm is lacking. 



The characteristic habitat of the Saprolegniales includes all 

 kinds of fresh water, in which they exist as saprophytes on vari- 

 ous kinds of organic materials. Practicallv all the species mav 

 be readily grown in pure culture, either on solid media or in 

 \^ater cultures on hempseed. Because of the great variability dis- 

 played by many species, the hempseed medium has been rather 

 extensively used as a basis for taxonomic work. The most w^ork- 

 able monographs of the group are those of Coker (1923) and 

 Coker and Matthews (1937). 



When Saprolegniales are cultivated in water culture on hemp- 

 seeds, the hyphae radiate in a white mat that surrounds the seed. 

 Typically hyphal tips are delimited by cross walls, and each such 

 terminal segment becomes a zoosporangium. As observed with 

 low magnification under a microscope, young sporangia are 

 densely filled ^\•ith protoplasm, whereas the remainder of the 

 hypha seems empty. If the stale water is replaced by fresh 

 w^ater, spore formation and emergence, a fascinating spectacle, 

 may be observed. As the zoospores take shape, the apex of the 

 sporangium softens; at the critical moment it opens, and the zoo- 

 spores rapidly emerge. Within 10 to 20 seconds they may all 

 have come to the exterior and may be dispersed quickly. 



Although most Saprolegniales are aquatic, the discovery by 

 Harvey (1925) that certain of them can readily be isolated from 

 the soil has resulted in the finding of many previously known 

 forms in this habitat and in the description of a number of new 

 genera and species [Coker (1927), Harvey (1928), Cook and 

 Morgan (1934)]. 



