CHYTRIDIALES 53 



individual components of the rhizoidal system may be slightly undulate 

 or unusually straight for long distances. When passing through the 

 walls of the substratum they are sometimes strongly constricted. 



Certain chytrids have developed special types of vegetative systems. 

 some of which have been previously mentioned. One of the most curious 

 is the highly flexible extramatrical stalk formed by the diatom parasite 

 Chytridium versatile. By its flexibility this needle-like structure which 

 elevates the sporangium above the frustule of the host enables the ob- 

 pyriform sporangium to bend back as the diatom goes between or 

 pushes against bits of debris commonly present in its environment. 

 After the obstruction is passed the sporangium snaps back to its orig- 

 inal upright position (Scherffel. 1926a: Sparrow. 1933a). In genera 

 such as Rhizosiphon, Aphanistis. Megachytrium, Zygochytriwn, Tetra- 

 chytrium, and Coenomyces the vegetative part of the thallus is more 

 or less isodiametric and blunt-tipped, assuming as a result a distinctly 

 hypha-like aspect (Fig. 37 A. p. 598). In Aphanistis and Coenomyces 

 transverse septa are formed. Although these hypha-like filaments 

 strongly resemble those of the higher fungi it is probable that they 

 have no phylogenetic significance and are simply similar vegetative 

 devices occurring in wholly unrelated organisms. Thus they have been 

 repeated in the posteriorly uniflagellate series (Megachytrium and 

 others), the anteriorly uniflagellate series (Hyphochytrium). and the 

 nonchytridiaceous laterally biflagellate series (Lagenidium. Pyihium. and 

 the like). 



The walls of the chytrid thallus rarely give a \\ell-marked cellulose 

 reaction with ordinary reagents. Cellulose has. however, been demon- 

 strated in certain species, notably in Rhizophydium by Zopf (l ss i. 

 Pringsheimiella by Couch (1939b). and Rhizovhlyctis by Ward (1939). 

 Following methods used by Xabel (1939). Ajello (1948a) demonstrated 

 the presence of chitosan in the walls of Poiychytrium. as had the former 

 investigator in several other chytrids. Haskins (1948), working with 

 Rhizophlyctis. found that the outer and inner walls of zoosporangia 

 differed in their reaction to zinc chloriodide. the outer were yellowish 

 or unstained, the inner, mauve-purple, indicating cellulose. If cellulose 

 is universally present in the chytrids. either the usual methods of dem- 

 onstrating it are not effective or it is masked, as in the Monoblepha- 



