58 AQUATIC PHYCOMYCETES 



and forms the papilla at its tip. Thus a passageway is made available 

 through which the zoospores are conveyed to the outside medium. 

 This tube may be short, as in Olpidium vampyreUae (Scherffel, 1926a) 

 and Diplophlyctis laevis (Sparrow, 1939a), or may attain a considerable 

 length, particularly if it is not so oriented as to gain immediate contact 

 with the wall of the substratum. Its length may vary strikingly in a 

 single species. Usually, endophytic types produce only one. Pleotrachel- 

 us fid gens (Zopf, 1884) offers a notable exception, however, in that it 

 typically forms many such structures (Fig. 12 E, p. 124). Indeed, it is 

 on this basis that the genus is separated from the predominantly one- 

 tubed Olpidium, which it otherwise resembles. 



The discharge papilla is characterized by its contents and its strongly 

 arched contour. The contents are ordinarily clear and highly refractive 

 and of an apparently viscid material. At the moment of discharge they 

 may frequently be observed to ooze out into the medium ahead of the 

 emerging zoospores. Just beneath this peripheral zone a clear, less re- 

 fractive region may often be seen, the concave base of which is in con- 

 tact with the plasma of the zoospores. At the moment of discharge, 

 however, these two zones appear to become quickly confluent. Whether 

 the materials of the two regions are derived from the wall or from the 

 protoplasm of the sporangium it is difficult to say. In species in which 

 a pronounced zonation is apparent, it is possible that the outer, re- 

 fractive layer is derived from the wall, the inner, from the contents. 

 Upon dissolution of the papilla a discharge pore is formed through 

 which the zoospores emerge (Fig. 23 E-H, p. 385). 



Couch (1932) observed in Rhizophydium couchii a somewhat different 

 method of discharge-pore formation. In this multiporous species there 

 are formed on the wall from one to five small rounded thin areas. The 

 internal pressure of the expanding spore mass within the sporangium 

 causes the wall to bulge out at these thin spots, which results in the 

 development of blunt papillae. A similar situation has been described 

 by Zopf (1887) as well as by Gaertner (1954c) and many others. 



In some forms (Rhizosiphon crassum Scherffel, 1926b; Rhizophlyctis 

 spp.) the wall material of the apex of the tube disappears before dis- 

 charge, leaving the contents separated from the outside medium by a 

 thin membrane, which may thicken. In certain sporangia of Dip/o- 



