THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 227 



almost indefinitely. The thick wall of the oospore bursts, and the contents, 

 surrounded by a thin membrane, may either grow out directly into a hypha, 

 which immediately forms a fresh mycelium, or, under other conditions, this 

 hypha may bear an asexual sporangium. Occasionally, however, the develop- 

 ment of the hypha is suppressed, and zoospores similar to those which would 

 have been borne in an asexual sporangium are developed directly within the 

 oospore. These swim about in the water surrounding the host, until they 

 find fresh tissues to penetrate, when they form a germ tube which develops 

 into a hypha. 



The life-cycle oi Pythium is graphically represented thus (Fig. 219): — 



■ Oospore 



Zoospores 



Fertilization 

 tube 



Antheridium 



■Pythiurn plant ' 

 Fig. 219. — Life-cycle oi Pythiurn. 



It is important to realize that a large number of species of Pythiurn occur 

 in this country, many of them in the soil, and that while some are destructive 

 parasites of higher plants others may play an equally important part in the 

 decomposition of plant remains in the soil. These species show a consider- 

 able variation from the type described above. For example, in some the 

 oogonium may be beset with spines, while in others the chief differences are 

 in the formation of the zoosporangia and the germination of the zoospores. 

 In fact it is possible to trace certain evolutionary sequences indicating a 

 possible gradual evolution from a typically aquatic mode of life to one adapted 

 to terrestrial conditions among the various species of the genus. A similar 

 sequence is found in the genus Peronospora, and we will postpone our account 

 of these matters till after we have described the life-history of this organism. 



Peronospora (The Downy Mildew) 



There are a number of species of Peronospora which occur on higher 

 plants, all of which cause diseases known as the Downy Mildews. Most of 



