CHYTRIDIALES 47 



in the majority of eucarpic types, only a single reproductive rudiment 

 is developed on the thallus, the thallus is said to be "monocentric," 

 whereas if more than one is formed (as in Cladochytrium), it is termed 

 "polycentric" (Karling, 1932). The rhizoids of all eucarpic thalli are 

 probably conveying materials back to the enlarging reproductive rudi- 

 ment at all times during their elongation into the substratum, except 

 when they are first being established. When extension of the rhizoids 

 ceases the residue of materials within them is soon drained into the 

 future reproductive body, which is then delimited by septa from the 

 now empty vegetative system. 



Variations and elaborations of the Entophlyctis type may be found 

 among both operculate and inoperculate chytrids. Indeed, the Chytri- 

 dium type itself may be regarded as one of these variations. Since it 

 occurs with great frequency, however, it is kept separate here. No 

 attempt is made to decide whether the endobiotic or the epibiotic 

 center of thallus organization is the more primitive, or whether one 

 has been derived from the other. 



3. Chytridium type. — The early stages of the Chytridium type (Fig. 

 3 C, p. 48) resemble those of the Entophlyctis type. After establish- 

 ment within the substratum, however, the rhizoids continue to elongate 

 and soon convey materials back through the penetration tube to the 

 persistent epibiotic cyst of the infecting zoospore. As a result, the cyst 

 expands and ultimately becomes the rudiment of the reproductive 

 structure within which all the contents of the vegetative system have 

 been concentrated. This system is then separated from the fertile portion 

 by a cross wall. Although the contents of the cyst are probably partly 

 depleted during the early stages of establishment and growth of the 

 rhizoids, materials soon begin to flow in the opposite direction, and 

 cause the expansion of the reproductive rudiment. 



The type of development that occurs in Phlyctochytrium and apoph- 

 ysate species of Chytridium might be considered here a variation of 

 the Chytridium type, although, as has been mentioned, there is equal 

 reason for supposing it to be allied to that found in Entophlyctis. In 

 Phlyctochytrium spp. and Chytridium lagenaria, for example, an endo- 

 biotic subsporangial apophysis is formed which probably always arises 

 as a secondary enlargement of the proximal portion of the already 



