OOMYCETES. 



103 



single zoosporangium. whose zoospores have generally one cilium. 

 The resting-spores arise either directly from the zoosporangium, 

 which, instead of forming zoospores, surrounds itself by a thick 

 cell-wall; or they are formed by the conjugation of two cells (in 

 which case they are spoken of as oospores). Microscopic Fungi, 

 parasitic on water plants (especially Algae) or small aquatic ani- 

 mals, seldom on land plants. 



Order 1. Olpidiaceae. Without mycelium. Swarmspores 

 and resting-spores. 



Iu the Olpidieie, the swarmspores, probably, most frequently form themselves 

 into a plasmodium (naked mass of protoplasm) which may become a single 

 zoosporangium or a resting sporangium. Olpidium trifolii occurs in Trifoliuni 

 repens. In the Synchytriece the plasmodium emerging from the swarmspores 

 breaks up either at once, or after a period of rest, into smaller plasmodia, each 

 of which will become a zoosporaugium. Synchytrium anemones is found on 

 Anemone nemorosa ; 8. mercuriulis on Mercurialis perennis ; S. aureum on many 

 plants, particularly Lysimachia nummularia. 



FIG. 86. Ghytridium lagcnuln. Zoo- 

 sporangium a before, b after the libera- 

 tion of the swarmspores. 



FIG. 87. Obelidium mucronatum, : 

 m mycelium ; s swarmspores. 



Order 2. Rhizidiaceae. Mycelium present. Zoospores and 

 resting-spores. 



Chytridium (Fig. 86). Obelidium (Fig. 87) is bicellular; the one cell is the 

 mycelium, the other the zoosporangium ; found on insects. The species of 

 Cladochytrium are intercellular parasites on marsh plants. Physoderma. 



Order 3. Zygochytriaceae. Mycelium present. Zoospores 

 and oospores. The latter are the product of the conjugation of 

 two cells (Fig. 88). 



Polyphayus euglmuc on Euglena viridis. Urophlyctis pulposa on species of 

 Chenopodiuiii. 



