ORBEK CHYTRIDIALES 63 



intercalary swellings here and there and, rarely, with septate turbinate 

 cells, and with large terminal spherical zoosporangia, each with an 

 apophysis. These produce large numbers of zoospores which escape 

 through a nonoperculate opening and swim actively for some time in a 

 large vesicle before its membrane is ruptured. Large spherical, thick- 

 walled resting spores are formed terminally. Their further fate has not 

 been determined. 



Polychytrium (Ajello, 1942) differs from both the foregoing genera in 

 its coarse branched rhizomycelium which has occasional rhizoids but no 

 turbinate cells or swellings. At the ends of the branches and sometimes in 

 intercalary positions the zoosporangia arise, usually in pairs, sometimes 

 several close together. They are pyriform. In them the zoospores are 

 developed and push out through the apical exit papilla or tube surrounded 

 by a slimy matrix in which at first they lie motionless. Soon they become 

 active and break away and swim off. They have a long posterior flagellum 

 and a conspicuous lunate nuclear cap. After the discharge of the zoospores 

 new zoosporangia are formed by proliferation. In addition to these typical 

 zoosporangia, yellowish-brown very tuberculate zoosporangia may occur, 

 in pairs or in clusters. From these the zoospores escape in the same 

 manner. No resting spores have been observed. 



The principal operculate genera of this family are Nowakowskiella, 

 Septochytrium, Megachytrium, and Catenomyces. In Nowakowskiella the 

 slender rhizomycelium is nonseptate, except where the enlargements be- 

 come zoosporangia, with usually a large apophysis. Zoospores escape in a 

 mass before separating or those few remaining behind creep out one by 

 one. The zoosporangia sometimes proliferate. Resting spores are formed 

 in some species of the genus. Roberts (1948) shows that in some species 

 of Nowakowskiella there is a marked morphological differentiation be- 

 tween the vegetative (or trophic) and the reproductive portions of the 

 rhizomycelium. The former is mostly confined to the interior of the 

 substratum and produces no reproductive organs from the enlargements. 

 The nuclei are found only in the swellings but not in the isthmuses or 

 rhizoids. In the reproductive portion, which is mostly external to the 

 substratum, the nuclei occur not only in the enlargements, which may 

 eventually become the zoosporangia, but also in the intervening fila- 

 mentous structures. In Septochytrium the primary swelling becomes a 



Fig. 17. Chytridiales, Family Cladochytriaceae. (A-C) Cladochytrium tenue Now. 

 (A) General view of part of the thalhis, showing spindle organs, zoosporangia in vari- 

 ous stages of development, and rhizomycelium. (B) Resting spore which developed in 

 a spindle organ. (C) Resting spore germinating to produce external zoosporangium. 

 (D-F) Nowakowskiella macrospora Karl. (D) General view of part of the thallus, show- 

 ing rhizomycelium and swellings and operculate zoosporangia. (E) Resting spore. (F) 

 Resting spore germinating to produce external zoosporangium. (Courtesy, Karling: 

 Am. J. Botany, 32(l):29-35.) 



