928 AQUATIC PHYCOMYCETES 



transformed into the secondary swarmer. Whether such significance 

 should be accorded what is after all only an interval of rest unaccom- 

 panied by encystment or change of shape of the zoospore is problematic. 

 Scherffel considers Olpidiopsis to exhibit a partial suppression of the 

 primary swarm stage, which reaches its end point in the Pythium type, 

 where only secondary zoospores appear. As supporting evidence he 

 cites the behavior of the zoospores of O. oedogoniarum, which emerge 

 sometimes as in Achlya, sometimes as in Pythium. It is important, how- 

 ever, that unquestionable proof be presented that only one species of 

 fungus is involved in such occurrences. 



Scherffel also called attention to the close resemblance of Olpidiopsis 

 to Myzocytium; he regards Olpidiopsis as providing a connecting link 

 between the Ectrogellaceae and the Lagenidiales. Indeed, in O. oedogo- 

 niarum, where, in contrast to other species, the resting spore lies loosely 

 in the receptive thallus, this resemblance is extremely close. A constant 

 difference between the two groups is to be found, according to Scherffel, 

 in the persistence of the infection tube on the thallus of the lagenidia- 

 ceous fungus, whereas in Olpidiopsis the thallus is established in monad- 

 like fashion and lies wholly free in the host cell. 



Whether or not host specialization actually exists in all species of 

 Olpidiopsis is not known with certainty. Experimental work here, as in 

 other fungi, is hampered by imperfect knowledge of optimum con- 

 ditions for resistance of the host plants. Shanor (1940) summarizes an 

 extensive study of the host range of certain species thus: 



A study of the host range of certain species of Olpidiopsis which occur as 

 parasites of the water molds has been made in which twenty-five species 

 representing eight genera of the Saprolegniaceae were used as possible hosts. 

 In all, two hundred and sixty-four single host exposures have been made. 

 Two species, Olpidiopsis varians and O. fusiformis, confine their parasitism 

 to species of Achlya. Olpidiopsis varians parasitizes Achlya flagellata and 

 A. proliferoides very vigorously and A. racemosa and A. color ata only to a 

 very slight degree. Olpidiopsis fusiformis parasitizes A. imperfecta and A. 

 klebsiana vigorously and A. racemosa only slightly. Two other species, 

 Olpidiopsis saprolegniae and O. incrassata were found to parasitize all species 

 of Saprolegnia exposed to them. Olpidiopsis saprolegniae is also capable of 

 parasitizing Isoachlya anisospora, I. unispora and 1. eccentrica while O. 

 incrassata has parasitized only /. anisospora and /. unispora. Olpidiopsis 



