THE FUNGI WHICH CAUSE PLANT DISEASE 67 



walled resting spores are produced. The zoospores have either 

 one or two cilia. There are over forty genera and two hundred 

 species. The majority of the species are inconspicuous parasites 

 of algae and infusoria; but some genera, hke Synchytrium and 

 Urophlyctis, produce conspicuous sori and even cause hyper- 

 trophy of land plants. 



Key to Families of Chytiidiales 



Spores all asexual, or rarely formed by the 

 union of free-swimming gametes 

 Mycelium none 



Sporangia solitary 1. Olpidiaceae, p. 67. 



Sporangia grouped into sori 2. Synchytriaces, p. 69. 



Mycelium present 

 Mycelium of delicate, evanescent haus- 

 toria-like strands 

 Mycelium limited, sporangia ter- 

 minal 3. Rhizidiaces. 



Mycelium extended, sporangia ter- 

 minal or intercalary 4. Cladochytriacese, p. 72. 



Mycelium of permanent hyphaB .5. H3rpochytriacetB. 



Spores both sexual and asexual 



Gametes hetrogamous 6. Oochytriacese, p. 73. 



Gametes isogamous 7. Zygochytriaceae. 



Four only of these families have parasitic representatives on 

 higher plants in America, the others being chiefly parasitic on 

 algae and infusoria. 



Olpidiaceae 



This family which contains the simplest members of the order 

 has no mycelium; the entire plant body consists of a single more 

 or less globular or elliptic cell which never divides, but at maturity 

 forms either a zoosporangium or an asexual resting spore which 

 after a period of rest gives rise to swarm spores. All the species 

 are endobiotic. The family contains some forty species but few 

 of which are of economic importance. 



