132 



THE SAPROLEGNIACE^ OF THE UNITED STATES, 



or by sympodial branching, or in both ways on the same hypha. Zoospores encysting 

 within the sporangium at once after their formation, polyhedral from mutual pressure, 

 the membrane of each face united with that with which it is in contact to form an 

 apparently single wall ; after a time escaping, each by a separate opening through 

 the outer wall, and swarming in the laterally biciliate form ; finally encysting again 

 and germinating. Oogonia terminal or intercalary, one- or several-spored. Anthe- 

 ridia usually present. -e 



Key to the known Species. 



a. Plants diclinous ; oospores single h. 



Plants androgynous ; oogonia several-spored D. polyspoms. 



b. Oogonia under dOfi in diameter, encircled by the antheridia -D. monosporus. 



Oogonia over 30// in diameter, not encircled by the antheridia D. Magndsii. 



DicTYUCHUs Magnusii Lindst. ('72). 



111.: Lindstedt, '72, PI. I, Figs. 1-15. 

 PL XX. Figs, 112-1 14. 



Hyphse rather large. Zoosporangia cylindric or fusiform. Oogonia terminal on 

 slender branches, globular, smooth-walled, unpitted. Antheridia cylindric or slightly 

 clavate, on all oogonia, borne on slender branches of diclinous origin. Oospores 

 single, centric, about 25//. in diameter. 



Massachusetts — Cambridge, Trelease. Europe. 



Our knowledge of the occurrence of this species in America rests on the notes 

 and preparations of Prof. Trelease, who obtained it in 1881 from water in the 

 Botanic G-arden, at Cambridge. It can be confounded only with D. monospai-us, 

 from which it differs in its somewhat larger oogonia and less coiled antheridial 

 branches. 



Lindstedt states that it is only in this species that the sporangia are formed from 

 the hyphse in basipetal succession, but it seems doubtful if this is strictly true, in 

 view of certain observations to be mentioned later. 



The definition of the present genus by its author was less restricted than that 

 above adopted, which is essentially that -of Lindstedt ('72), but seems, for reasons 

 suggested in the discussion of the genus Thraustotheca, to be a more accurate and 

 philosophical one. 



