80 ] The Classification of Lower Organisms 



Order 2. Peronosporina [Peronosporinae] Fischer in Rabenhorst Kryptog.-Fl. 

 Deutschlandl,Abt.4: 383 (1892). 

 Suborder Peronosporineae Engler in Engler and Prantl Nat. Pflanzenfam. I Teil, 



Abt. 1: iv (1897). 

 Order Peronosporineae Campbell Univ. Textb. Bot. 155 (1902). 

 Order Peronosporales Auctt. 

 Mostly parasites on terrestrial plants, but including also aquatic parasites and a 

 few saprophytes, the bodies filamentous, reproducing sexually by fertilization, the 

 eggs solitary in the oogonia, reproducing asexually chiefly by conidia, that is, by air- 

 born cells cut off from the ends of the filaments. The conidia are homologous with 

 the sporangia of the Saprolegnina : they germinate in most examples by release of 

 zoospores (which show no signs of diplanetism), but in the more highly evolved 

 examples they give rise to filaments. Ferris (1954) found the zoospores of Phytoph- 

 thora to bear the paired flagella, respectively pantoneme and acroneme, which are 

 typical of Phaeophyta. 



In the multinucleate oogonia of most members of the group, single flares of mitoses 

 occur. The sharp-pointed spindles, described in some accounts as ending in centro- 

 somes, are formed within the persistent nuclear membrane, which undergoes con- 

 striction during the final stages of mitosis. A coenocentrum appears (this structure 

 was first described as occurring in Albugo, by Stevens, 1899); in general, one nucleus 

 becomes associated with it, and is thus selected as the egg nucleus, the remaining 

 nuclei being cast out to undergo disolution in a body of periplasm. The antheridium 

 develops in contact with the oogonium, and fertilization is accomplished by the 

 growth of a fertilization tube through the periplasm to the egg (Davis, 1900; Stevens, 

 1899,1901,1902). 



In Albugo Bliti and A. Tragopogonis, Stevens observed two flares of simultaneous 

 mitoses in the oogonium and antheridium. If this phenomenon were general in the 

 group one would confidently identify it as meiosis. The single coenocentrum attracts 

 many nuclei; the fertilization tube delivers a large number of sperm nuclei; thus 

 multiple karyogamy occurs within a single cell. The further history of the resulting 

 peculiar zygote, containing many nuclei which are not by any evident necessity 

 genetically uniform, is unknown. 



This order is evidently a specialized offshoot of the preceding. The family Pythiacea 

 is a good example of a transition group; many authorities have assigned it to the pre- 

 ceding order. 



1. Producing solitary globular sporangia or 

 conidia at the ends of scarcely specialized 



filaments; mostly aquatic Family 1. Pythiacea. 



1. Producing conidia usually in clusters at the 

 ends of specialized filaments (conidio- 

 phores) ; parasites on land plants. 



2. Conidiophores brief, unbranched, the 



conidia in chains Family 2. Albuginacea. 



2. Conidiophores elongate, usually branch- 

 ed, the conidia solitary or clustered, not 



in chains Family 3. Peronosporacea. 



Family 1. Pythiacea [Pythiaccae] Schroter in Engler and Prantl Nat. Pflanzenfam. 

 I Teil, Abt. 1: 104 (1893). Aquatic parasites and saprophytes releasing zoospores 

 from globular reproductive structures terminal on the filaments, together with para- 



