132 PHYCOMYCETEAE 



able host plants. Other species are usually found as parasites on higher 

 plants, although even these are capable of cultivation on nonliving cul- 

 ture media. The zoosporangia of the soil-inhabiting types may be sub- 

 merged and then remain attached to the mycelium, or may emerge into 

 the air in which case they may become detached and wind-distributed. 

 In the more strictly parasitic forms the conidiophores emerge through 

 the epidermis of the host, piercing it or passing through the stomata. 

 They are simple or may branch sympodially. In all species the zoospo- 

 rangium or the conidium when it falls into water may produce zoospores 

 which emerge singly or pass into a vesicle which soon ruptures so as to 

 allow them to escape. They are of the secondary type and resemble those 

 of Pythium. They swim for a while and then encyst. They germinate by 

 a germ tube. Instead of producing zoospores the conidia may produce 

 short conidiophores upon which one or more small conidia arise or may 

 germinate by a long germ tube. Sometimes the conidia germinate in situ 

 without becoming detached. In P. infestans (Mont.) de Bary the my- 

 celium overwinters in infected tubers which then give rise to diseased, 

 spore-bearing shoots which serve as centers of infection. (Fig. 41.) 



As in Pythium the mycelium of many species is intracellular, directly 

 killing the invaded cells, but in some species it also grows intercellularly, 

 sending haustoria into the adjacent cells (Cooper and Porter, 1928; 

 Szymanek, 1927; Klebahn, 1909; Butler, 1910). In general the species 

 act as destructive parasites, killing the tissues very rapidly. 



Sexual reproduction is essentially like that in Pythium, the single egg 

 surrounded by periplasm being fertilized by a male nucleus which passes 

 from the adhering antherid into the egg through a conjugation tube. 

 The antherid may arise as a separate branch and become attached to the 

 oogone at any point. In some species the antherid appears to surround 

 the base of the oogone. This has been interpreted as an antherid coiled 

 around the hypha bearing the oogone (Shanor, 1938), but Pethybridge 

 (1913) claims that development is as follows: On the end of a hypha an 

 antherid is formed and through this another hypha grows, piercing the 

 antherid completely and swelling then to form the oogone above it. B. D. 

 Mundkur (1949) confirmed the occurrence of this amphigynous type 

 of antherid in another species (Phytophthora himalayensis Dastur). In 

 some species oogones may be found with these basal (amphigynous) 

 antherids and others with antherids in the lateral position (paragynous). 

 After fertilization the thick-walled oospore rests for some time and then 

 germinates to form mycelium or a short conidiophore. Infection of the 

 host may occur by the zoospores produced in the conidia or by the germ 

 tubes from the oospores. With the soil-inhabiting species the mycelium 

 may directly penetrate the subterranean portions of the host. Narasinhan 



(1930) reports that P. arecae (Golem.) Pethyb. is heterothallic but Tucker 



(1931) believes that this needs further study. (Fig. 42.) 



