36 



COMPARATIVE MORPHOLOGY OF FUNGI 



In order to classify the various genera they have been divided into 

 subfamilies of which five will be discussed here. 



The Rhizophideae include a series of forms whose sporangia and 

 hypnospores arise directly from the stronger zoospores. After a swarm 

 period zoospores cling fast to the substrate, surround themselves with 

 a membrane and push a process into the substrate and themselves swell 

 to sporangia. 



In Phlyctidium brevipes on Spirogyra in North America this process is 

 still short and scarcely able to penetrate the wall of the host cell. The 

 zoospores disappear through an emission papilla at the top; if after a 

 certain time they have not found a suitable substrate, they may shed 

 their membrane (Atkinson, 1909). 



Rhizophidium pollinis (Zopf, 1887) is easily found on Pinus pollen in 

 stagnant water (Fig. 19a) and oospores of the Peronosporaceae (Melhus, 

 1914). In the interior of the pollen grain the germ tube branches to a 

 small rhizoid fascicle. The sporangium discharges its zoospores through 



Fig. 19. — Rhizophidium pollinis. A, zoosporangium on a pollen grain; B, Hypnospores 



and zoosporangia. {After Zopf, 1887.) 



several sharply limited, punctate germ pores whose membranes dissolve 

 at maturity. 



In Rhizidiomyces apophysatus (Zopf, 1885), parasitic on the oogonia 

 of many Saprolegniae, the germ tube on the inside of the oogonial wall 

 swells up to an apophysis, out of which the rhizoids pass (Fig. 20, 1). 

 The significance of this subsporangial sac is not yet clear. Directly before 

 zoospore formation, the sporangia form a long emission collar which swells 

 up like a sac at the tip (Fig. 20, 2). Into this sac the zoospore initials 

 pass singly as small bits of protoplasm and are then liberated by the solu- 

 tion of the sac (Fig. 20, 3). Hypnospores are unknown. 



Zygorhizidium Willei (Loewenthal, 1905) corresponding in the type of 

 sexuality to the Rhizideae and in the manner of opening its zoosporangia 

 to the Chytrideae, is parasitic on Cylindrocystis Brebissonii of the Meso- 

 taeniaceae in Norway. The structure of its thallus corresponds to that 

 of Rhizidiomyces, although its zoospores are not liberated by an emission 

 collar but by the dehiscence of a lid (Fig. 21, 5). 



