62 AQUATIC PHYCOMYCETES 



minutes, during which the spores continue to swarm violently and with 

 incredible speed, the shape of the mass changes and becomes broadly 

 sagittate. Finally a few spores dart from the apex of one of the lobes 

 and are soon followed by the remainder, which escape by the same 

 route or from the tip of the opposite lobe. In this type of discharge the 

 spores appear to be beating against a vesicular structure that confines 

 them until they have undergone a period of preliminary "test" swarming. 



The significance of this collective swarming, which has been noted 

 in other chytrids as well, notably in Chytriomyces, is not clear unless 

 it is connected with the final fashioning of the flagella. It is possible 

 that these are only partly formed at the time of zoospore emergence 

 and may require further maturation before the spore can cope success- 

 fully with the outside medium. 



True operculate chytrids differ from the inoperculate in possessing 

 a more or less convex cap which surmounts the discharge papilla. The 

 operculum is a definite and constant morphological structure in species 

 in which it is formed. For example, all sporangia on the polycentric 

 thallus of Nowakowskiella are operculate, and plants grown from 

 zoospores of this plant will have sporangia which in turn are operculate. 

 There has not as yet been discovered a species of chytrid in which spo- 

 rangial discharge is at one time inoperculate, at another time opercu- 

 late. 



The operculum is formed of wall material and in epibiotic and inter- 

 biotic species is from a portion of the wall of the zoospore cyst (Haskins, 

 1948; Berdan, 1942; and others). At the moment of discharge it is 

 dehisced and either thrown aside or carried up by the mass of emerging 

 zoospores (Fig. 35 B-D, p. 583). Often, as in Endochytrium ramosum 

 and Chytridium perniciosum (Sparrow, 1933a, c), it appears to be at- 

 tached hingelike to the orifice. It may be smoothly contoured like a 

 watch glass, with varying degrees of convexity, or may have on its 

 outer surface a pronounced apiculus or umbo. Although in some 

 species (for example, C. sphaerocarpum) it is so thin-walled as almost 

 to escape detection, in others it is large and thick-walled, often ap- 

 pearing solid and very conspicuous (as in C. perniciosum, C. olid). 

 The presence or the absence of an operculum is of great taxonomic 

 importance. Since, unfortunately, one cannot foretell with certainty 



