LEPTOMITALES 



The establishment of the Leptomitales as an order coequal with the 

 Saprolegniales (Kanouse, 1927) has proved eminently feasible and 

 its division in the first edition of the Aquatic Fhycomycetes into two fam- 

 ilies, the Leptomitaceae and Rhipidiaceae, has received general accept- 

 ance. Members of these two families are all fresh-water saprophytes 

 which usually form tangled or flocculent mats of hyphae or small whit- 

 ish pustules on the substratum. They occur primarily on vegetable 

 debris, particularly twigs and fruits. Some species seem to prefer cool 

 clear water, whereas others grow under exceedingly foul environmental 

 conditions. While the pustules are occasionally composed of a single 

 species, more often they consist of a mixture with other Phycomycetes 

 such as Blastocladia and Gonapodya. Most of them appear, at least from 

 published records, to be of infrequent occurrence. 



The Leptomitales constitute a rather closely related group having in 

 common well-marked characters. Generally speaking, these are the 

 presence of a more or less well-defined basal cell, segmentation of the 

 thallus, cellulose walls, pedicellate reproductive structures, and oogamous 

 sexual reproduction. When Kanouse (1927) separated the order from 

 the Saprolegniales, she pointed out that the members of the Leptomi- 

 tales show definite affinities with the Saprolegniales on the one hand and 

 with the Peronosporales on the other. Like the Saprolegniales they lead 

 an aquatic life, have the same general thallus structure, and form 

 numerous zoosporangia which produce biflagellate zoospores. Further- 

 more, as is true in the Saprolegniales, the zoospores of two genera of 

 the Leptomitales (Leptomitus and Apodachlya) exhibit diplanetism 

 (dimorphism) or show evidences of it (Zopf, 1888; Coker, 1923). The 

 Leptomitales resemble the Peronosporales in the nature of their 

 reproductive organs and processes, but are unlike them in habitat and 

 in their lack of parasitic tendencies. As in the Peronosporales, the 

 members of the Leptomitales (with the exception of Apodachlyel/a 



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