CHAPTER VIII 

 PERONOSPORALES 



The highest point of development attained in the Oomycetes 

 is reached in this order. The aquatic habit typical of the 

 Saprolegniales and other lower forms has been largely abandoned, 

 and most of the higher members of the order lead a terrestrial 

 existence as parasites in the tissues of higher plants. Correlated 

 with this change in habit has occurred a pronounced alteration 

 in the character of the asexual stage. The sporangia are typically 

 deciduous, and in the highest genera of the order germinate 

 directly by germ tubes instead of by swarmspores. In general 

 they are borne in the air on specialized hyphae termed spo- 

 rangiophores, and are commonly disseminated by the wind. In 

 the lower members of the group they usually germinate by 

 swarmspores, though in some cases the type of germination is 

 dependent on environmental factors. In a few of the most 

 primitive forms, they develop under aquatic conditions and 

 remain attached. When swarmspores occur they are of the 

 reniform, laterally biciliate type. 



The term conidium is applied by many writers to the sporan- 

 gium in this group, but usage is extremely varied. Some authors 

 prefer the term sporangium in all cases in which germination is 

 by swarmspores. Others use conidium wherever the cell is 

 deciduous, regardless of the method of germination. It is clear 

 that from the morphological and phylogenetic points of view only 

 a single structure is present, and the writer has found it desirable 

 for the sake of clarity and uniformity of usage to avoid the 

 apphcation of the term conidium in the Oomycetes. 



Mycelium is profusely developed throughout the entire order, 

 and consists of more or less cylindrical, repeatedly branched 

 hyphae of relatively small diameter as compared with those of 

 the Saprolegniales. Septa are usually absent but may form in 

 old hyphae. The mycelium of parasitic species is usually inter- 

 cellular, the host cells being entered only by short lateral branches 

 termed haustoria. These differ in form in the various genera, 



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