984 AQUATIC PHYCOMYCETES 



(1944g) both have observed their encystment at the orifice as in Achlya. 

 Certain species resemble in complexity of thallus development and 

 sexual reproduction an Aphragmium type of Pythium. Thus, in Lage- 

 nidium giganteum (Couch, 1935b) the branched threads may extend as 

 much as 1-2 mm. from the host and the segments which will form the 

 zoosporangia may be 50-200 \x long; similar sizes are found in L. 

 humanum. Although in L. marchalianum these sizes are not attained, in 

 this species, too, the thallus is extensive and ramifies throughout the 

 algal host in a strikingly Pythium-like manner (Fig. 81 B, p. 1002). 

 Further resemblance to Pythium is evident in the sexual organs, which 

 are more differentiated in Lagenidium than in Myzocytium. The anther- 

 idium in particular is often more specialized and generally produces a 

 fertilization tube rather than a simple pore. It is still ordinarily not 

 separated, however, from the antheridial branch by a cross wall as in 

 the more highly developed species of Pythium. The assumption that 

 the female gametangium lacks periplasm appears to rest entirely on 

 morphological observations. 



KEY TO THE SPECIES OF LAGENIDIUM ** 2 



Thallus never septate, forming a single gametangium or zoosporangium 

 In fresh-water algae 

 In diatoms 



Sporangium rounded or more or less flask-shaped, discharge 



tube short, blunt, thin-walled throughout . . L. cyclotellae, p. 986 

 Sporangium tubular, unbranched, or occasionally with finger- 

 like branches 

 Discharge tube very short, with a thick-walled "forcing appa- 

 ratus" ; resting spore lying loosely in the gametangium 



L. brachystomum, p. 988 

 Discharge tube prolonged somewhat extramatrically ; rest- 

 ing spore filling the gametangium L. enecans, p. 989 



In Oedogonium L. oedogonii, p. 990 



In other Phycomycetes 

 Sporangium spherical or pyriform; resting spores single, sexually 



formed ; in Pythium L. pythii, p. 991 



1 See also Lagenidium muenscheri Cutter (1943), parasitic in root hairs of Pota- 

 mogeton spirillus, a vascular plant. 



2 Note that key is not strictly dichotomous. 



