CHYTRIDIALES 67 



ous materials are probably of first importance. Under conditions nec- 

 essary for observation, from three to four hours is usually a maxi- 

 mum period of swarming, although this is not always true. In one re- 

 markable instance (Braun, 1856a) the zoospores of Rhizophydium decip- 

 iens continued swarming within the sporangium for one hundred and 

 eight hours. 



Not all chytrids possess zoospores capable of pronounced and pro- 

 longed motility. Couch (1935a) has reported the flagellated spores of 

 Phlyctidium anatropum to be incapable of active swimming. After dis- 

 charge they merely creep about amoeboidly over objects in the medium, 

 and the flagellum never becomes active. A somewhat similar condition 

 has been noted by Karling (1938c) in Chytridium aggregation. In this 

 fungus the failure of the zoospores to assume motility after emergence 

 from the sporangium results in the characteristic development of clus- 

 ters of thalli on the surface of the substratum. In Amoebochytrium, 

 Sporophlyctis rostrata, and Sporophlyctidium the spore apparently fails 

 to form a flagellum. In Amoebochytrium it shows pronounced amoeboid 

 motion, but in the others it either germinates directly in the sporangium 

 (Sporophlyctis) or after discharge floats about in the medium (Sporo- 

 phlyctidium) and is termed an "aplanospore." 



Tactic response of the chytrid zoospores to certain external stimuli 

 has been observed. In Rhizidium vorax (Strasburger, 1878) and Poly- 

 phagias euglenae (Nowakowski, in Strasburger, op. cit.\ Wager, 1913) 

 they are positively phototactic. Since both of these fungi are parasitic 

 upon motile green algae, the value of this physiological adaptation is 

 readily apparent. The mechanism of the response is not as yet under- 

 stood. It has been suggested by Wager that in Polyphagus the close 

 proximity of the golden globule of the zoospore to the nucleus may in- 

 dicate that it is a photoreceptive organ. He conjectures that light rays 

 absorbed by it could cause local changes which, perhaps through the 

 mediation of the nucleus, might exert a directive influence on the free- 

 swimming zoospore. This may possibly be the explanation in Poly- 

 phagus and certain terrestrial chytrids (Kusano, 1930a), especially since 

 photoreceptive organs are ordinarily pigmented. It would not explain, 

 however, the phototactic response of the zoospore of Rhizidium vorax, 

 which has a colorless globule. Furthermore, there are certain well- 



