130 IQl ITIC PHYCOMYCETES 



namely, O. viciae and O. trifolii. Cytological evidence for its occurrence 

 in O. agrostidis has also been presented by Miss Sampson (1932). 

 It has not been convincingly described in any of the purely aquatic 

 species. 



In Olpidium viciae, K.usano observed that the swarmers copulated in 

 pairs ( Fig. 4 A, p. 72.) the zygote then infecting the host and producing 

 within it a resting spore. Swarmers which did not copulate eventually 

 penetrated the host, where each formed a sporangium. A more critical 

 investigation by Kusano o\' sexuality as it exists in O. trifolii seems to 

 indicate that sister gametes from the same gametangium are usually 

 incapable of fusing with one another to form a planozygote. He con- 

 cludes, therefore, that the fungus is dioecious but has a tendency toward 

 monoecism. Another feature of the behavior of the gametes in O. trifolii 

 was their aggregation at certain places in the medium. This aggregation 

 was found to have a definite influence in promoting conjugation. Ku- 

 sano suggests that the groups of nearly stationary gametes, undergoing 

 amoeboid changes of shape, exert an attractive action upon free- 

 swimming ones and thus induce fusion. In both species gametes dis- 

 charged from recently formed gametangia were not so apt to conjugate 

 as those from somewhat older ones. 



In the following analytical key to the species considerable use is 

 made o\' differences in substrata. In all probability this is not sound, 

 and future work is very likely to reveal that a single species may occur 

 in a variety of substrata. Since the basic morphological plan of an 

 Olpidium is very simple there exists little chance for wide variation 

 in this respect. Features such as the length of the discharge tube and 

 the relative position o\' the fungus and the plasma of the substratum 

 impressed the earlier investigators, but these are now known to be 

 o\' little taxonomic value. 



klYTOIIII SPECIES OF OLPIDIUM 1 

 In plants 

 In Chlorophyceae; sporangium variable in shape; 

 zoospores formed in the sporangium 

 Sporangium ellipsoidal, spherical, subspherical, 

 pyriform, or ovoid 



Note that this key is not completely dichotomous. 



