6 



PHYCOMYCETEAE: PERONOSPORALES 

 AND PROTOMYCETALES 



Order Peronosporales. The fungi discussed in the previous chapters 

 were mainly inhabitants of soil and water, and largely saprophytic or 

 parasitic in habit. This order contains many genera and species that are 

 very strict parasites on Flowering Plants (Anthophyta or Angiosperms) 

 and which have hitherto resisted all attempts to grow them on artificial 

 culture media. However, most if not all of the members of the family 

 Pythiaceae respond more or less readily to efforts to bring them into cul- 

 ture upon nonliving media. Some genera of this family were placed in 

 the past among the Saprolegniales because of characters intermediate 

 between the two orders. 



In the main the asexual reproduction of the Peronosporales exhibits 

 a step in evolution above that in the Saprolegniales. In the latter the 

 zoospores are mostly produced from zoosporangia that remain attached 

 to the main mycelium when the zoospores are set free. However, it must 

 be noted that the hyphae may break up into separate rounded segments 

 called "gemmae" which eventually are capable of functioning as zoo- 

 sporangia. In the majority of Peronosporales the tips of aerial hyphae 

 enlarge and are separated from the main hypha by a cross wall and are 

 then set free and distributed, usually by air currents. These so-called 

 "conidia" show their true nature as potential zoosporangia when they 

 fall into water, as then tlioir contents may divide internally into zoo- 

 spores. The zoospores produced in this order are always of the secondary 

 type. These zoospores may in some cases encyst and become free-swim- 

 ming several times. 



Sexual reproduction occurs by the fertihzation, through a conjugation 

 tube from an antherid, of the normally single egg in an oogone to form 

 a thick-walled oospore. With a few possible exceptions the egg is sur- 

 rounded by periplasm as in the Rhipidiaceae of the order Saprolegniales 

 and Pythiella in the order Lagenidiales. 



There is no sharp distinction of root-like holdfast hyphae and external 



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