CHYTRIDIALES 35 



The present systematic classification of the Chytridiales is only 

 provisional. At present a large number of very old, slightly related forms 

 are known. As it would be inconvenient to divide them into monotypic 

 families, they have been grouped into larger families but it must be 

 remembered that only a few of them are natural. As only in Central 

 Europe and Eastern United States have they been extensively collected, 

 it is probable that many of the present gaps will be filled. 



The above diagram follows essentially that given for the Mycochy- 

 tridineae (A. Fischer, 1892; Minden, 1915). According to the height of 

 development of the thallus, the Chytridiales are divided into three fami- 

 lies; the Rhizidiaceae, Hyphochytriaceae and the Cladochytriaceae. The 

 Rhizidiaceae are holocarpic; their thallus consists of processes (rhizoids) 

 without nuclei and therefore always dependent on the central body which 

 develops into a sporangium. The rhizoids penetrate the substrate like 

 haustoria: they correspond approximately to the pseudopodia of the 

 Rhizopoda. The Hyphochytriaceae and Cladochytriaceae are eucarpic: 

 their thallus is formed as a true mycelium which is divided into principal 

 and secondary axes. The latter possess peculiar swellings with still 

 unknown functions, the turbinate cells. 



Whether and how these three families are connected is still obscure. 

 It is possible that the Entophlycteae, Chytrideae and Rhizidieae, 

 may be derived from Rhizophidieae-like forms and perhaps the Clado- 

 chytriaceae and Hyphochytriaceae from the Rhizidaceae. Opinion is also 

 divided on their relation to the four families of the Archimycetes ; thus 

 Minden (1915) considers it possible that the Rhizidiaceae may be derived 

 from the Olpidiaceae, while Serbinov (1907) considers both families 

 phylogenetically distinct. The Archimycetes seem to be foreign to true 

 fungi and connected with the Myxomycetes and Protozoa. Gaumann 

 would derive the Chytridiales from the Chlorophyceae and consider them 

 as a phylogenetic starting point for the greater part of the fungi. 



In spite of these obscurities and in order to give a preliminary exposi- 

 tion of the system of the Chytridiales on page 34, these three families 

 have been juxtaposed on the basis of their morphological similarities. 

 Obviously this scheme in no way corresponds to the natural phylogenetic 

 relationships. 



Rhizidiaceae. — This family includes chiefly parasitic forms. There 

 are two distinct lines of development; either a species limits itself to one 

 host individual (monophagy) and by reduction of the extramatrical parts, 

 penetrates far into the host, i.e., passes from ecto- to endoparasitism, or 

 the extramatrical part of the thallus develops as much as possible and 

 infects a large number of hosts (polyphagy) thus increasing the amount 

 of available nourishment. These latter forms are ectoparasitic. Tran- 

 sitional forms are shown between the Rhizophidieae and Entophlycteae, 

 where some forms of the Rhizideae show tendencies to polyphagy. 



