CHAPTER VII 

 CHYTRIDIALES 



In the present work, the Chytridiales, often called Chytridineae, 

 include the Mycochytridineae; they are parasitic or saprophytic, usually 

 in water, rarely on land. In the lowest forms, the thallus consists of 

 one or more spherical cells, parasitic within the host. In the higher 

 forms it is extramatrical and sends pseudopodia-like haustoria into the 

 substrate ; in the highest forms it develops a tubular coenocytic mycelium 

 which may be differentiated into a basal part serving for nourishment 

 and an apical portion serving for reproduction. 



Zoosporangia and hypnospores are known. The lowest forms are 

 holocarpic; i.e., the whole thallus forms a fructification. The higher 

 forms are eucarpic; i.e., fructifications are formed from only a portion 

 of the thallus. Both the zoosporangia and the hypnospores germinate 

 by zoospores, which are generally uniflagellate, exceptionally aflagellate; 

 their emission takes place through openings in the mother-cell wall or by 

 emission collars. In the individual details of their formation and libera- 

 tion, particularly in the varied manner of their accumulation at the 

 mouth of the emission collar, important ontogenetic problems are still 

 hidden. 



In some species, the formation of hypnospores is preceded by a 

 sexual act, which occurs between two zoospores which behave as piano- 

 gametes or between two plants or between two daughter cells of the same 

 plant which have been transformed into gametangia. 



The systematic position of the Chytridiales has been controversial. 

 By Bary (1884), Brefeld (1889), Zopf (1890) and Petersen (1910), they 

 are regarded as derivatives of the higher Phycomycetes, particularly 

 of the Zygomycetes, which as a result of their parasitic and aquatic 

 habits, have undergone considerable degeneration. By Bessey (1903), 

 they are connected with the Cladophoraceae in which they had degen- 

 erated beyond the Valoniaceae and Botrydiaceae. By Fisch (1884), 

 Dangeard (1889), A. Fischer (1892), Serbinov (1907), Atkinson (1909a), 

 Cavers (1915) and Scherffel (1925), they are considered as primitive 

 organisms which, because of the regular appearance of flagellate, often 

 amoeboid, zoospores and simple forms of sexuality, may not be explained 

 by degeneration. 



While this last conception is becoming generally accepted, it does not 

 suggest where the roots of the Chytridiales may be sought. Fisch (1884), 



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