CHYTRIDIALES 55 



The size of the thallus and, in particular, of the reproductive rudi- 

 ment formed on it, though falling within certain well-defined limits, 

 is undoubtedly modified somewhat by prevailing external conditions. 

 Of these the most important are probably the availability and nature 

 of food and, in endophytic forms, restrictions of space imposed by the 

 substratum. Competition between individuals of the same or other 

 species may result in the cutting down of both space and nutriment. 



REPRODUCTION 



Nonsexual Reproduction 



In practically all chytrids nonsexual reproduction is accomplished 

 by means of posteriorly uniflagellate zoospores produced in a sporan- 

 gium which is formed in most instances from the reproductive rudi- 

 ment of the thallus. 



Cleavage of the zoospores. — After the accumulation of protoplasm 

 in the reproductive rudiment and after the sequence of changes pre- 

 viously described the zoospores are delimited. These are usually com- 

 pletely formed, apparently always within the sporangium, by cleavage 

 of the contents into uninucleated segments of similar size. Although 

 delimitation of the spore origins has been described almost uniformly 

 as simultaneous, certain observations show beyond question that it 

 is progressive, at least in some species (see "Cytology," p. 85). In 

 Septolpidium lineare (Sparrow, 1936a), a parasite of diatoms which 

 forms a tubular thallus, progressive cleavage has been observed in 

 living material, not only in the division of the contents into zoospores, 

 but in the formation of the sporangia themselves. Cytological studies 

 of zoospore formation in the polycentric endobiotic Cladochytrium 

 replicatum (Karling, 1937b) also reveal a progressive rather than a 

 simultaneous division into zoospores. In sporangia of this fungus, in 

 which the contents are homogeneously distributed throughout, cleav- 

 age furrows first appear on the periphery of the mass and extend in- 

 ward as radial, often somewhat curved arms. The furrows for the 

 most part delimit uninucleate segments. Occasionally, however, bi- 

 and trinucleate segments may first be formed. These, by the subsequent 

 production of secondary furrows, are ultimately divided into uni- 



