CHAPTER III 

 CHYTRIDIALES 



The members of this order are characterized by extreme 

 simphcity of form and structure, and are regarded as the lowest 

 of all the fungi. They are commonly termed chytrids. 



Mycelium is either wholly wanting or only sHghtly developed. 

 When present it consists usually of delicate threads, the terminal 

 branches of which taper to sharp-pointed, almost invisible tips. 

 In only a few genera are the threads of broad diameter and 

 definitely cylindrical as in most higher fungi. Frequently, the 

 thallus is so Umited in extent that its branches are confined to 

 a single host cell. It is then said to be monophagus. In other 

 cases, the branches enter several or many host cells, the term 

 polyphagus being appHed. In species in which mycehum is 

 wholly absent the thallus functions in its entirety at maturity 

 in reproduction. A single thallus may be transformed into a 

 sporangium, a group (sorus) of sporangia, a resting spore, or 

 a group (sorus) of resting spores. Two thalh may conjugate, 

 one functioning as the male cell, the other as the female. In 

 species possessing mycelium the reproductive cells are far more 

 prominent than the vegetative threads. 



The thallus, in the chytrids, has its beginning in almost all 

 cases as a motile spore, zoospore, which comes to rest, enlarges, 

 and finally attains the form and size characteristic of the mature 

 thallus. Zoospores are amoeboid or ciliate. In the latter case, 

 they are often called swarmspores. 



In asexual reproduction zoospores are borne in indefinite 

 number in a unicellular sac, called the sporangium, which may 

 or may not, as indicated above, compose the entire thallus. 

 During the summer in temperate regions several generations 

 of the organism usually follow one another rapidly as long as 

 a food supply is available. The sporangia produced at this 

 period are typically thin-walled and may be termed summer 

 sporangia, thin-walled sporangia, swarmsporangia or merely 

 sporangia. Especially at the close of the season or in periods 



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