202 THE LOWER FUNGI— PHYCOMYCETES 



The species included in Phytophthora arc more typically para- 

 sitic in their natural habitat than those embraced in Pythium, 

 but nearly all of them will grow on artificial culture media. In 

 the host the hyphae are intra- as well as intercellular, and in the 

 latter case bear haustoria. Usually on the host the branches 

 which bear sporangia (Fig. 72) protrude from the stomates, but 

 in culture they are developed indefinitely over the surface of the 

 medium. Though these branches are termed sporangiophores 

 they differ little if any from the vegetative hyphae. Their 

 branching is sympodial in type, terminally produced sporangia 

 coming to occupy a lateral position due to further apical growth 

 of the hypha. The production of sporangia is thus successive 

 and differs essentially from the simultaneous maturation of 

 sporangia at the tips of a specialized sporangiophore as seen 

 in the Peronosporaceae. In several species (Phy. infestans, 

 Phy. phaseoli, Phy. thalictri) a slight thickening of the hypha 

 occurs immediately below the point of attachment of the spo- 

 rangium, and gives it a characteristic aspect, but this is absent 

 in other species. Germination of the sporangium by means of 

 one or more germ tubes may occur, and in some cases secondary 

 and even tertiary sporangia may be formed on these tubes. 

 When germination by swarmspores takes place a part or all of the 

 spores may round up while still within the sporangium and ger- 

 minate in situ by germ tubes. The sporangia developed in the air 

 in nature fall away and are wind borne, but those developed in 

 culture usually germinate while still attached to the sporangio- 

 phore. The term conidium is frequently encountered in the 

 literature of this genus and has been applied to the sporangium 

 without reference to its method of germination. Spherical, 

 smooth, thick-walled, terminal or intercalary spores, usually 

 yellowish or brownish in color, are sometimes found and have been 

 termed chlamydospores. Investigators have interpreted them 

 in different ways : as resting conidia, as parthenogenetic oospores, 

 etc. 



The sexual process in the genus is in one respect remarkable. 

 Pethybridge (1913 h) demonstrated that in some of the species, 

 designated by him the infestans group, the antheridium exists 

 at maturity as a collar around the base of the oogonium. The 

 oogonium is initiated beneath the antheridium, perforates its 

 wall, passes into its interior, grows entirely through it, emerges 

 at its summit, and swells out there to form the spherical portion in 



