OOMYCETES 69 



with the female nucleus. The mature oospore is uninucleate. After a 

 resting period of 6 months, it develops a germ tube. 



If we consider the life cycle of the Saprolegniaceae from the point of 

 view of nuclear phase, we get the following picture: 



T 1 



y^oospores 

 /Zoosporangia^ PC R 



/ Antheridia— >-Male nuclei 

 Mycelium s^^ [-^Oospores— >Zoosporangia — ^Zoospores 



\)ogonia— >egg cells J 



Diagram VII. 



This scheme corresponds essentially with that of Polyphagia Euglenae 

 (p. 44) if one ignores the complication caused by heterothallism in the 

 latter. The differentiations caused by the development of the mycelium 

 in form and function of the haplont, have been discussed in the intro- 

 duction to the Oomycetes (p. 51). 



Saprolegnia is saprophytic on animal cadavers in all sorts of water, 

 occasionally, parasitic on living fish, whose eggs it may infect and thus 

 cause extensive epidemics. As far as is known, the infection is preceded 

 by a weakening or injury of the individuals infected. The genus includes 

 about 30 species which are chiefly distinguished by the structure and 

 arrangement of the sexual organs; the value of these specific characters 

 should be more thoroughly studied as its habit is extensively influenced 

 by nutritive conditions. The same species occur in Europe and North 

 America, as S. dioica, S. monoica, S. mixta and S. ferax (S. Thureti) 

 (Humphrey, 1892; Coker, 1923). Some of them appear in the Alps and 

 in Lapland beyond the borders of the snow (Tiesenhausen, 1912; Gau- 

 mann, 1918). S. anisospora forms two kinds of zoospores like Gonapodya, 

 a smaller typical form and a larger, more than double the size of the 

 smaller, with dark-brown protoplasm. There are transitional types; 

 but one sporangium contains spores of one type only. Whether this 

 phenomenon has a special significance is still unknown. 



Achlya polyandra of Europe and A. racemosa of North America behave 

 like Saprolegnia. In Protoachlya we have a transition from Saprolegnia 

 to Achlya, as in P. paradoxa whose form and sporangial arrangement 

 agree with Achlya and whose diplanetism resembles that in Saprolegnia 

 (Coker, 1914; 1923). Isoachlya is similar but all the zoospores are 

 motile, the oogonia are often in chains and the antheridia are rare 

 (Kauffman, 1921; Coker, 1923). 



In Aphanomyces, which differs from Achlya in its long fusiform 

 sporangia, A. laevis causes a root blight of sugar beets and A. euteiches 

 (Jones and Drechsler, 1925) a root rot of peas. Plectospira myriandra 

 produces injury of tomato rootlets. 



