OEDER PERONOSPORALES 127 



hyphae of quite different nature as occurs in many of the Saprolegniaceae 

 and in the Rhipidiaceae. The mycehum usually shows a cellulose reaction 

 upon application of a solution of chloriodide of zinc. The hyphae are 

 mostly much more slender than in the above-mentioned families but, like 

 them, are normally coenocytic and nonseptate except to set off repro- 

 ductive organs or injured portions or sometimes empty portions of the 

 mycelium. 



Following Fitzpatrick (1930) the 400-500 species are divided into 

 three families as follows: 



Pythiaceae: saprophytic or parasitic, mycelium intracellular, less often inter- 

 cellular with haustoria. Zoosporangia filamentous to ovoid or spherical, in 

 some cases the ovoid forms becoming separable "conidia," if aerial. These 

 conidia are borne singly, rarely in chains, at the tips of unbranched coni- 

 diophores or at the tips of the branches of a sympodially branched coni- 

 diophore. They germinate by the formation of zoospores or by a germ tube. 

 The oospore germinates by becoming a zoosporangium or by forming a short 

 hypha terminated by a conidium or by producing mycelium. 



Albuginaceae: strictly parasitic in herbaceous Anthophyta (Angiospermae). 

 Mycelium intercellular with globular haustoria. Conidiophores clavate, 

 clustered in extensive sori under the epidermis of the host which is ruptured 

 by the pressure of the conidia which are produced successively in chains at 

 the apex of each conidiophore. Conidia germinating by the formation 

 of zoospores or of germ tubes. Oospore germinating by the formation of 

 zoospores. 



Peronosporaceae: strictly parasitic in herbaceous, and, more rarely, woody 

 Anthophyta (Angiospermae). Mycelium intercellular with globular or fila- 

 mentous haustoria. Conidiophores emerging through the stomata singly or 

 two or three together, unbranched or monopodially much branched, bearing 

 the conidia singly at the tips of the branches or on short sterigmata on the 

 top of the unbranched conidiophore. Conidia germinating by the formation 

 of zoospores or by a stout germ tube. Oospore germinating by a germ tube 

 or by a short unbranched conidiophore. 



Family Pythiaceae. The Pythiaceae have been variously divided 

 into from 4 to 14 genera. Of these, Pythium (in its wider delimitation) and 

 Phytophthora contain the greater number of species and are best known. 



The species of Pythium are mostly soil or water inhabitants, probably 

 living most of the time as saprophytes. Some of these soil species, how- 

 ever, are capable of becoming destructive parasites upon plants, causing 

 rotting of the tissues or damping-off. A few are parasitic on other species 

 of the genus. Some species (e.g., P. proliferum de Bary, which can be 

 caught by hanging fruits in bodies of water) are saprophytic upon various 

 vegetable objects submerged in water. Of the 66 species recognized by 

 Middleton (1943) three are saprophytic and one parasitic on animal 

 matter, nine parasitic in fresh-water algae, and one saprophytic in a 

 marine red seaweed, about nine in soil and vegetable debris and not 

 known to be parasites, and forty or so capable of parasitism in higher 



