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 Algee) or small aquatic ani- 

 mals, seldom on land plants. 



Order 1. Olpidiaceae. Without mycelium. Swarmspores 

 and resting-spores. 



In the Olpidiez, 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 Trifolium 

 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 zoosporangium. Synchytrium anemones is found on 

 Anemone nemorosa ; S. mercurialis on Mercurialis perennis ; S. aureum on many 

 plants, particularly Lysintachia nunimularia. 



FIG. 86. Chytridium lagenula. Zoo- FIG. 87. Obelidium mucronatum : 



sporangium a before, b after the libera- m mycelium ; s swarmspores. 



tion of the 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 

 Cladochytriiim, are intercellular parasites on marsh plants. Physoderma. 



Order 3. Zygochytriacese. Mycelium present. Zoospores 

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

 two cells (Fig. 88). 



Polyphagus euglence on Euglena viridis. Uroplilyctis pulposa on species of 

 Chenopodinm. (Edomyces leproides on the Beet-rooc. 



