302 THE SAPROLEGNIACE^ OF THE UNITED STATES 



the material has made it necessary to refer to European descriptions for certain 

 details. In all such cases the borrowed matter has been indicated by quotation marks. 

 For species not yet known to be American a brief informal statement of the chief diag- 

 nostic characters is given to aid the student who may meet with them, since many of 

 them are likely to be found with us. 



Artificial dichotomous keys to all the intelligibly described species are prefixed 

 to the detailed accounts, in all but the smallest genera, as a practical encouragement 

 to their study ; but they should never be relied on alone for the determination of 

 species. 



Sapeolegniace^ Pringsh. ('57). 



Aquatic fungi, living as saprophytes or facultative parasites, with usually 

 branched mycelium ; the hyphse in a few species constricted at intervals, but remain- 

 ing unseptate except in the formation of reproductive organs. The latter of two 

 kinds, non-sexual and sexual, both formed from the hyphse and separated from their 

 vegetative portions by transverse walls. !N^on-sexual propagation by means of bicili- 

 ate, often diplanetic, zoospores, produced in cylindrical or swollen s^^orangia ; or very 

 rarely by homologous non-motile bodies ; occasionally also by chlamydospores. Mor- 

 phologically sexual reproduction by oJspores developed in typically globular oijgonia, 

 one or more fi'om the entire protoplasm of each oogonium ; antheridia on branches of 

 androgynous or diclinous origin, very rarely on the oogonial branch, uniting with all, 

 or with only a part of the oogonia, or in several species wholly absent; when present, 

 usually producing fertilization tubes which remain closed, at least in some species. 



Key to Genera. 



A. Vegetative filaments with their walls unconstricled. . . . , , {Saprolegniece) B. 



Vegetative filaments deeply constricted at intervals (Leptomitecs) I. 



B. Zoospores normally leaving the sporangium by a common mouth C. 



Zoospores not leaving the sporangium by a common moulh O. 



G. Zoospores swarming separately on escaping from the sporangium D. 



Zoospores collecting in a hollow sphere at the moulh, on escaping F. 



D. Zoospores diplanetic ; new sporangia growing through the empty ones E. 



Zoospores monoplanetic ; sporangia renewed by cymose branching Ptthiopsis. 



E. Oospore single, wholly filling the oogonium Leptolegnia. 



Oospores one or more, not wholly filling the oogonium Saprolegnia. 



F. Sporangia usually broader than the vegetative hyphte ; zoospores irregularly arranged Achlya. 



Sporangia equaling the vegetative hyphae in breadth ; zoospores in a single file Aphanomyces. 



0. Zoospores encysting within the sporangium, afterwards swarming H. 



Zoospores encysting and germinating within the sporangium, never swarming Aplanes 



II. Zoospores set free by the breaking up of the sporangial wall Tkraustotheca. 



Zoospores escaping each by a separate perforation of the sporangial wall, leaving a "net." Dictyuchus. 



1. Zoospores swarming separately on escaping Leptomitls. 



Zoospores collecting at the mouth of the sporangium, on escaping Apodachlta. 



