52 AQUATIC PHYCOMYCETES 



(Fig. 3 C) or it is bulbous, coralloid. or composed of short stubby 

 digitations. Such structures are often referred to as "haustoria." In 

 other chytrids. as in Coenomyces and Megachytrium, the vegetative 

 part is distinctly hypha-like, resembling in this respect the mycelium 

 of the higher fungi. All of these, however, are exceptions to the pre- 

 vailing rule that the eucarpic chytrids are rhizoidal. 



The polycentric thallus 1 is usually far more extensive than the mono- 

 centric and is characterized by the formation on it of new centers of 

 thallus organization (Karling, 1932). They are either swellings, which 

 sometimes develop into reproductive structures, or fusiform or ellip- 

 soidal, often septate, purely vegetative "turbinate cells" or "spindle 

 organs" (Fig. 301, p. 474). Various conjectures regarding their function 

 have been proposed. It is possible that they serve either to increase 

 the active absorptive surface or, if they become converted wholly or 

 in part into a reproductive structure, to collect materials for the re- 

 productive rudiment. In Physoderma (Urophlyctis). an obligate parasite 

 in the tissues of certain phanerogams, similar (but not identical) tur- 

 binate cells have been definitely shown to function as vegetative centers 

 from which, after tangential septation, new parts of the extensive thallus 

 originate (Jones and Drechsler, 1920). A like role is assigned them by 

 Karling (1931b. 1932). who points out that, in Cladochytrium, as the 

 thallus extends its growth, these enlargements serve as successive centers 

 for reduplication or replication of the growing vegetative system. 



In most genera of both monocentric and polycentric chytrids the 

 rhizoids are profusely branched. Elongation and further branching 

 may give rise to an extensive vegetative system with an enormous ab- 

 sorptive surface. Branching of the main axes is usually in dichotomous 

 fashion, the diverticula frequently emerging at wide angles. This di- 

 chotomy is usually retained in the secondary branches, although there 

 is a marked tendency here toward unequal development of the resulting 

 elements. Thus one secondary branch may remain relatively broad and 

 continue as a main axis, while the other may taper suddenly and termi- 

 nate its growth. This inequality often brings about the characteristic 

 zigzagging course taken by the main axis as it continues to grow. The 



1 Karling (1932) called the polycentric thallus a "rhizomycelium," a term which, 

 though useful, carries too great a phylogenetic implication. 



