ORDER CHYTRIDIALES 61 



The dozen or so genera of Chytridiales that are polycentric differ from 

 the eucarpic, monocentric genera in the production of many nucleate 

 expansions on the rhizoidal system, each of which may become a center 

 for the production of zoosporangia, resting spores or other rhizoidal 

 branches. As a result the fully developed individual consists of many 

 centers of varying size and shape, connected by (usually) slender, non- 

 nucleate rhizoidal strands or tubes. This system of tubes and rhizoids 

 was given the name "rhizomycelium" by Karling (1932). One of the 

 crucial points in the recognition of the affinity of these organisms is to 

 determine whether they possess a true mycelium, with nuclei along its 

 course, or a rhizomycelium. Furthermore, does this distinction necessarily 

 separate these fungi into different orders or do we have here a gradual 

 gradation within the same order from organisms scarcely distinguishable 

 from the Rhizidiaceae or Entophlyctaceae to those in which the rhizo- 

 mycelium has become truly mycelial? 



The two families tentatively accepted by the author are as follows: 



Cladochytriaceae : intra- or extramatrical, eucarpic, with many centers connected 

 by slender, branching, rhizoidal or tubular threads, with swellings here and 

 there. Sporangia thin-walled, terminal or intercalary, operculate or in- 

 operculate. Resting spores thick-walled, apparently not sexually produced. 



Physodermataceae : epibiotic, monocentric, and eucarpic at first, then by inde- 

 pendent infection endobiotic and strongly polycentric, producing resting 

 spores only. The latter eventually produce internally one to several thin- 

 walled, inoperculate zoosporangia. Probably the epibiotic stage produces 

 gametes while the resting spore sporangia produce zoospores but this has 

 not been proved. 



Family Cladochytriaceae. In the Cladochytriaceae the chief genus 

 with inoperculate sporangia is Cladochytrium, which is saprophytic and 

 forms a system of fine branching rhizoids within the tissues of the sub- 

 stratum. Here and there arise swellings which may become the round 

 or pyriform sporangia or which may become spindle-shaped, once septate 

 "turbinate organs," one of these cells becoming a sporangium and per- 

 haps later the other. Septa are formed to separate the sporangia and 

 resting cells from the remainder of the rhizomycelium. The zoosporangia 

 empty through an inoperculate exit tube which may be quite long. Some 

 of the swellings become colorless resting spores with a thickened wall. 

 These, according to Karling (1934), germinate by becoming sporangia in 

 which numerous zoospores are produced by cleavage and escape through 

 a short sporangial neck, in C. replicatum Karl. The cytology of this fungus 

 has been studied by Karling (1937). (Fig. 17A-C.) 



Physocladia (Sparrow, 1932) differs in habit from the foregoing in 

 that it is external to the matrix (pine pollen in the case of P. ohscura 

 Sparrow) which is penetrated only by the fine tips of the rhizomycelium. 

 It consists of a slender, branched thread with rhizoidal branches and 



