PERONOSPORALES 1013 



unnamed one reported as the cause of rotting of Porphyra (Arasaki, 

 1947). Doubtless, there are others that live in salt water. Hohnk (1953a) 

 records ten species of Pythium, several considered new taxa, from 

 brackish habitats. Pythiogeton is, morphologically speaking, closely 

 related to Pythium, and occurs chiefly on submerged twigs and fruits, 

 often in very foul water. It has been investigated physiologically by 

 Cantino (1949b; 1951b). The species of Phytophthora x that were for- 

 merly placed in Pythiomorpha occupy similar substrata to those of 

 Pythiogeton but usually live under cleaner conditions offering a better 

 oxygen supply. Zoophagus is frequently found ramifying between 

 filaments of algae and between plants of Chara or Nitella. Its members 

 are of unusual interest because of their predaceous habit of capturing 

 and parasitizing actively moving microscopic animals. Most of the 

 Pythiaceae have been successfully cultivated on artificial media, a recent 

 instance being that of Zoophagus insidious (Prowse, 1954b). The mono- 

 typic genus Diasporangium (Hohnk, 1936), although recovered as free- 

 living in the soil, is evidently a facultative parasite, since it is capable 

 of causing root rot in various phanerogams. 



Development and Morphology 



the thallus 



The vegetative body in the Pythiaceae is a well-developed, usually 

 much-branched, slender mycelium, which in actively growing parts is 

 nonseptate except where reproductive organs are delimited. With age, 

 however, occasional septations may separate empty from viable parts 

 of the hyphae. Demonstration of a cellulose reaction in the walls of the 

 hyphae is difficult to achieve (Butler, 1907). In aquatic forms the 

 mycelium runs from cell to cell in the host; in terrestrial parasitic ones, 

 such as certain species of Phytophthora, it is also intercellular as well, 

 haustoria being formed which absorb materials from the host cells. 

 Gemmae are produced in some species of Pythium (Butler, 1907: 20), 

 in Zoophagus (Arnaudow, 1921, 1925; Karling, 1936c), and in Phy- 

 tophthora (Ito and Nagai, 1931; Hohnk, 1936). Ward (1883), Butler 

 (op. cit.) and later investigators found irregular toruloid swellings or 



1 See Blackwell (1956). 



