150 THE LOWER FUNGI—PHYCOMYCETES 



minate by a germ tube which develops directly into mycelium. 

 In Saprolegnia, Isoachlya, and Leptolegnia the zoospores escape 

 separately into the water and swim away instead of encysting 

 at the mouth of the sporangium. They are ovoid or pit-shaped, 

 and swim by means of two cilia attached at the forward more 

 pointed end. After a period of motility they encyst separately 

 and, later, escaping from the membrane, swarm again. In the 

 second period of motility they are reniform with two lateral 

 cilia. At the close of the second swarming period they encyst 

 and germinate by germ tube. In Protoachlya some of the spores 

 escape immediately as in Saprolegnia while the remainder 

 encyst at the mouth of the sporangium as in Achlya. In these 

 genera the zoospores are termed diplanetic on account of the two 

 periods of swarming, the phenomenon of diplanetism exhibited 

 here contrasting with monoplanetism in those genera in which 

 only a single swarming occurs, and with aplanetism in Aplanes 

 and Geolegnia. The spores of Achlya and Aphanomyces are 

 regarded as diplanetic but the first swarming period is very brief, 

 being confined to the sporangium, and serves only to carry the 

 spores through the terminal pore. In Achlya the spores during 

 this period correspond to those of the first type in Saprolegnia, 

 and this is probably also the case in Aphanomyces, although the 

 narrow diameter of the sporangium in this genus makes demon- 

 stration of the fact difficult. In Thraustotheca the spores are 

 of the second type and definitely monoplanetic. In Pythiopsis 

 they are monoplanetic, but strangely enough are of the first type. 

 The spores of Dictyuchus have been regarded as monoplanetic, 

 but in recent years a peculiar condition has been shown (Weston 

 1919) to exist in this genus. The spores which escape from the 

 sporangium are of the reniform type. After swarming they 

 encyst and later under favorable conditions may swarm again, 

 though germination by a tube often occurs. The spores in the 

 second swarming period are reniform and laterally biciliate as 

 in the first period. Following the second swarming, encystment 

 and germ tube formation occur. This condition of "repeated 

 zoospore emergence" in Dictyuchus is perhaps present also in 

 other genera, having been merely overlooked. It seems to be 

 present in Pythiopsis intermedia where the zoospore is of the first 

 type. The two swarming periods in such cases are not to be 

 confused with true diplanetism in which zoospores of two dis- 

 tinct types follow each other always in the same order. The 



