484 AQUATIC PHYCOMYCETES 



elements of a turbinate cell or as an outgrowth from it, often bearing 

 pores through which protrude antler-like bushy projections, upon 

 germination either cracking open or dehiscing circumscissily and form- 

 ing a saclike, elongate endosporangium with a broad apical papilla 

 that deliquesces to form a pore through which the relatively large 

 zoospores emerge. 



Obligate parasites of pteridophytes and angiosperms. 



This is the oldest genus of chytrids. It was established by Wallroth 

 (1833) on the basis of the resting-spore stage nearly twenty years before 

 Braun founded Chytridium. Many years were to elapse, however, before 

 Physoderma was unquestionably recognized (Schroeter, 1883) as a 

 member of the Chytridiales. 



The relationship of Physoderma to other polycentric chytrids and to 

 Urophlyctis and a resume of older authors' disposition of species of 

 the genus are discussed in the previous edition of this book (1943) and 

 need not be repeated here. 



Whether or not Karling's (1950) implementation of Sparrow's (1943) 

 suggestion that Urophlyctis be merged with Physoderma will prove a 

 wise course, must await renewed investigations on species which have 

 in the past been assigned to these taxa. 



In Physoderma the extensive polycentric endobiotic thallus has never 

 been observed to give rise to any type of reproductive organ save the 

 thick-walled, flattened, elliptical, usually colored resting spores. 1 Each 

 of these upon germination cracks open and produces a sporangium as 

 a saclike extrusion. The zoospores from this sporangium come to rest 

 on the surface of the host cell and form epibiotic sporangia, each of 

 which has a bushy monophagous rhizoidal system. This phase is 

 definitely epibiotic, monocentric, and on one host cell as contrasted 

 with the strongly polycentric, and polyphagous phase within the host 

 in which the resting spores are formed. What connection exists between 

 these two thalli is not at present known. The "sporangia" may in 

 reality be gametangia 2 , the "zoospores," gametes, and the endobiotic 



1 According to Karling ( 1950), endobiotic, thin-walled sporangia have been found 

 by Gopalkrishnan in Physoderma graminis, but no account of these has seemingly 

 been published. 



2 This suggestion of the gametic nature of the zoospores from ephemeral spo- 

 rangia has been unquestionably proven true in Physoderma lycopi and by Y. Lingappa 

 in the case of P. pulposa. 



