190 AQUATIC PHYCOMYCETES 



SYNCHYTRIACEAE 



Thallus endobiotic, holocarpic, without a specialized vegetative sys- 

 tem, at maturity converted either into a sorus of inoperculate spo- 

 rangia, a prosorus, or a resting spore; sporangia formed within or out- 

 side, always at first surrounded by a common soral membrane; zo- 

 ospores posteriorly uniflagellate, with a single globule; sexual repro- 

 duction, where known, by conjugation of isogamous planogametes, 

 the zygote forming the thick-walled endobiotic resting spore, which 

 upon germination functions either as a sporangium or a prosorus. 



Synchytrium, the largest genus of the family, is composed of species 

 all of which are obligate parasites of flowering plants (see Tobler, 

 1913; Minden, 191 5) 1 . Many of them, as S. endobioticwn, the cause 

 o\' the black-wart disease of Irish potatoes, attack hosts of economic 

 importance. This genus is included in the key but is not treated here. 

 Species of the genera Endodesmidium and Micromyces (including those 

 formerly in Micromycopsis) are parasitic on green algae and have thus 

 far been observed primarily on members of the Conjugatae. 



Well-authenticated occurrences of sexuality have been observed only 

 in Synchytrium (Curtis, 1921; Kusano, 1930a), although Couch (1931: 

 23 1 ) presents evidence that the resting spore of Micromyces may pos- 

 sibly be formed from a zygote. 



KEY TO THE GENERA OF THE SYNCHYTRIACEAE 



[Parasitic on flowering plants; thallus large, never amoeboid, forming 

 a simple sorus surrounded by a common soral membrane, pro- 

 sorus, or resting spore; prosorus never forming a discharge 



tube; zoospores freed outside the host cell Synchytrium 2 ] 



Parasitic on algae; thallus small, always forming a prosorus or a resting 

 spore; sori simple or compound 3 , sessile or at the tip of a 

 discharge tube; zoospores freed inside or outside the host cell 

 Sorus sessile on prosorus, wall not divided into segments; spo- 

 rangia small, spherical, amoeboid, occasionally uniflagellate 



Endodesmidium, p. 191 



Recently Karling < 1953 et seq.) has initiated a comprehensive series of researches 

 on Synchytrium. 



Not treated; see Tobler (1913), Minden (1915), and Karling (1953 and many 

 subsequent papers). 

 : ' See p. 193. 



