THE FUNGI: ARCHIMYCETES AND PHYCOMYCETES 211 



Finally, in the genus Aplanes, a less common one, there is no motile 

 stage at all, and the spores germinate within the zoosporangium by means of 

 germ tubes which penetrate the wall of the sporangium and reach the exterior. 



As will be seen in subsequent examples, there appears to be a progressive 

 tendency among these groups of Phycomycetes to change from a condition 

 with apicallv flagellate pyriform zoospores to one of reniform laterally 

 flagellate zoospores. This is brought about by the interpolation of an 

 encysted condition. In the more advanced types the production of the first 

 type of zoospore disappears completely, and the second stage takes place within 

 the sporangium, or in a vesicle formed as an outgrowth from it, so that the 

 only motile stage is the third. Finally we see that this motility is lost and 

 the spores germinate by a germ tube within the sporangium. 



In Saprolegnia, after the liberation of the zoospores from the zoo- 

 sporangium, secondary sporangia may be developed. This may be efi^ected, 

 either by the development of a second sporangium from the base within the 

 primary one, or by the production of a hypha from the same point, which 

 grows up through the opening in the tip of the primary sporangium to form 

 a secondary sporangium, sometimes some way above the first. This is termed 

 proliferation. Several successive generations of zoosporangia may be formed 

 in this way (Fig. 203). 



Zoospores 



Zoosporangium 

 (secondary) 



Antheridium 



Oogonium 

 Oosphere 



A B 



Fig. 203. — Saprolegnia diclina. A, Zoosporangium showing the de\e]opment of a second 

 zoosporangium inside the old one. Zoospores are escaping from the young zoospor- 

 angium. B, Sexual reproduction. Oogonia in chains with antheridia originating 

 from different mycelium from the oogonia ; the diclinous condition. 



