130 PHYCOMYCETEAE 



mainder of the mycelium by a septum (subgenus Nematosporangium of 

 Fischer, 1892) or the septum may be lacking (subgenus Aphragmium of 

 Fischer). (Fig. 40 A, B.) 



2. The more or less filamentous zoosporangium is somewhat thickened 

 and lobed, often only near the base, to form the so-called toruloid struc- 

 ture, the plasmatoonkosis of Sideris (1931). This is a storage organ ena- 

 bling the zoosporangium to produce larger numbers of zoospores in the 

 vesicles at the tips of the emission tubes. (Fig. 40 E.) 



3. The zoosporangium is spherical, ovoid or limoniform, terminal or 

 intercalary, single or in short chains {Sphaerosporangium of Fischer). 

 (Fig. 40 F.) 



Schroeter (1897) and Sideris (1931) included types 1 and 2 in a genus 

 which they called Nematosporangium, applying the name Pythium to 

 Fischer's subgenus Sphaerosporangium. Inasmuch as the type species of 

 the genus, P. monospermum Pringsheim (1858), forms zoosporangia of 

 type 1, the name Pythium must be retained for Schroeter's Nematospo- 

 rangium, if the genus is divided. The later monographers of the genus 

 (Butler, 1907; Miss Matthews, 1931; and Middleton, 1943) do not make 

 this division but include all three types of sporangia in one genus. 



In some submerged species with spherical or ovoid sporangia new 

 zoosporangia arise within the emptied ones by proliferation as in Sapro- 

 legnia, or a branch arises just below the point of attachment of the zoo- 

 sporangium and is terminated by a new one, sympodial development 

 proceeding as in Achlya. In some species the zoosporangia may be pro- 

 duced on aerial hyphae in which case they may become detached and 

 distributed by currents of air, germinating when they fall into water 

 either in the usual manner or by a germ tube. These wind-carried de- 

 tached zoosporangia are called conidia. Usually the submerged zoo- 

 sporangia and the aerial conidia are alike in the same species. Pythium 

 deharyanum Hesse, with its zoosporangia representing potential conidia, 

 is common in soil as a damping-off parasite of seedlings when the soil is 

 too moist or the seedlings too much crowded. It can often be obtained by 

 placing a little soil in a dish with some cooled boiled water and placing 

 in the latter a few boiled hemp seeds (not too many or the bacteria will 

 become numerous). On these seeds Pythium and various Saprolegniales 

 will appear in a few days. Pythium aphanidermatum (Edson) Fitzpatrick, 

 the cause of black-root disease of radishes (especially noticeable in the 

 White Icicle variety) and rotting of sugar-beet seedlings and roots of 

 many other plants, is a species with the second type of sporangia. It was 

 originally described as the type of a new genus, Rheosporangium, by 

 Edson (1915). 



The genus Phytophthora contains 15 or 20 species some of which live 

 as saprophytes in the soil, developing as parasites in the presence of suit- 



