juiyis, I9IS Rheosporangiuvi Aphanidermatus 291 



Rheospdrangium, new genus. 



Mycelium aerial or aquatic, well-developed, nonseptate, branched. Reproduction 

 by zoospores under aquatic conditions and by oospores. Terminal, enlarged, more or 

 less distorted mycelial-like prezoosporangia cut off from the ends of hyphae. Sporangia 

 thin-walled, normally escaping from the presporangium through a terminal rupture, 

 cleaving into zoospores. 



Rheosporangium aphanidermatus, n. sp. 



Vegetative mycelium white, in water hyaline, nonseptate except in fructification, 

 branched, finely granular, frequently exhibiting pronounced protoplasmic streaming. 

 Young hyphse varying in width from 2.8 to 7.3/i, averaging from 4 to 6/1, but frequently 

 becoming wider as fructification approaches. Presporangia developing by the enlarge- 

 ment of terminal portions of hyphae, unbranched or irregularly clavately to normally 

 branched, length varying from less than 50 to more than 1,000//, width from 4 to 20/i, 

 distal portion usually unbranched and tapering to a rounded end. Sporangia with 

 nearly invisible, flexible, membranous walls escaping from the distal end of the pre- 

 sporangia or, rarely, from that of one of the branches, becoming spherical on release, 

 varying in diameter according to the presporangia, at once cleaving into zoospores. 

 Zoospores escaping by the rupture of the sporangia, plano-convex, with a single central 

 vacuole, and on the flattened side a sinus, from the bottom of which the two cilia of 

 unequal length arise. Average size, 12 by 7.5/j. Oogonia terminal, spherical, 22 to 

 2jfi in diameter. Antheridia terminal or intercalarj^ suborbicular, becoming cylindric 

 or broadly clavate, average dimensions, 9 to 11 by 10 to 14/1. Oospores single, smooth 

 or contoured, average diameter, 17 to ig/f. 



The following illustrations are reproduced from camera-lucida drawings 

 of fixed and stained material, except when otherwise stated. 



