64 COMPARATIVE MORPHOLOGY OF FUNGI 



uninucleate portions (Fig. 38 A and B); these round off, acquire two 

 flagella and swarm (Trow, 1899; Davis, 1903). In many forms (as in 

 many species of Saprolegnia and Leptolegnia) the adjacent portion of the 

 hypha penetrates the empty sporangium and there swells up or, if it has 

 grown through the sporangium, it forms a second sporangium at its tip. 

 This may be repeated several times. In other genera, as in Achlya, 

 Aphano?nyces and Pythiopsis, the sporangia are renewed by lateral out- 

 growth of a portion of the hypha next the empty sporangium. There, too, 

 the process may be repeated, forming sympodially divided, verticillately or 

 spirally arranged sporangia. In still other forms as Saprolegnia torulosa, 

 and some species of Pythiopsis and Dictyuchus, the sporangia may be 

 intercalary, catenulate or occasionally intermingled with oogonia. 



The behavior of swarming zoospores is very different in different 

 genera. In Saprolegnia, they are ovoid (Fig. 4, s 2 ) with two nagella at 

 the pointed ends. After a time they come to rest and surround them- 

 selves with a cellulose wall. They do not put forth a germ tube, but 

 their naked protoplasm slips out of the wall as a reniform swarm spore 

 with two nagella in a lateral indentation. The empty sheath degener- 

 ates. Generally the second swarm stage lasts longer than the first. 

 When it ends, the zoospores are again surrounded by a wall and develop 

 to a mycelium. » 



This diplanetic basic form of the Saprolegnia type has been modified 

 in the various genera by a suppression of one or both swarm stages. In 

 Pythiopsis, the second swarm stage has disappeared, its zoospores come 

 to rest immediately after the stage with two apical nagella and germinate 

 with a hypha. In the Achlya-Thraustotheca series, as in the Ectrogella 

 Bacillariacearum and E. Dicksonii series, the first stage is gradually 

 suppressed. In Achlya (Fig. 39, 1), Plectospira (Drechsler, 1927) and 

 Aphanomyces, the generally aflagellate zoospore initials (in some species 

 they still seem to possess two terminal flagella) form a small cluster in 

 front of the sporangial opening, shed their walls and swarm as reniform 

 zoospores with lateral flagella, leaving the group of empty sheaths 

 before the opening of the sporangium. In Dictyuchus, the zoospore 

 initials no longer come out but surround themselves, while still in the 

 sporangium, by a membrane out of which the spores swarm singly 

 through an opening in the sporangial wall. The empty spore membranes 

 remaining behind in the sporangium thus form a delicate, transient, net 

 sporangium. In Thraustotheca, the zoospores, surrounded by a mem- 

 brane, are liberated through a rupture in the sporangial wall or by its 

 destruction (Fig. 39, 6). There, according to circumstances, they 

 germinate with a laterally flagellate zoospore, a mycelium or a 

 sporangium. 



In other forms, reduction has also affected the second swarm stage; 

 thus in Achlya aplanes (Maurizio, 1894) the zoospore initials come out 



