THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 201 



Chytridiales 



The Chvtridiales are Phycomycetes in which sexual reproduction is effected 

 by the fusion of isogametes and no trace of oogamy occurs. The vegetative 

 thallus may be either a plasmodium devoid of a cell wall or it may be 

 unicellular, while in the highest groups it consists of a multicellular or 

 a coenocytic hypha. 



The group contains the most primitive members of the Phycomycetes. A 

 few live as parasites in the tissues of higher plants, but the majority are 

 ectoparasites or saprophytes on Algae. In some the life-history is complicated 

 by special adaptation to the parasitic habit and to overwintering in the host, 

 but in many the life-history is remarkably simple and demonstrates the way 

 in which the group may have evolved. In general, it may be said to consist 

 of two stages, the first an amoeboid vegetative phase, followed by a flagellate 

 reproductive stage. As we pass up the group to the more complex types the 

 tendency appears to have been to reduce the amoeboid or plasmoidal stage 

 and to enclose the protoplast in a wall, which then becomes the wall of a 

 sporangium in which eventually the reproductive organs arise. 



We shall consider two types, the one Rhizophidiiim globosum, representing 

 the ectoparasitic type, and the other Synchytrium endobioticiim, the endo- 

 parasitic mode of nutrition. 



Rhizophidium globosum 



This little organism is by no means well known, though actually it is 

 very common. It occurs with 

 other closely allied species on the 

 filaments of Green Algae, particu- 

 larly Spirogyra and Cladophora, 

 and may be found at almost any 

 time of the year in ponds or 

 streams. 



The mature plant consists of 

 a spherical sac attached to the 

 surface of the algal filament by a 

 short rhizoid (Fig. 192). The 

 extent of development of this 

 rhizoidal system is a specific char- 

 acter ; in some species it is little 

 more than a peg which does not 

 penetrate the cell wall of the host, 

 in other species the wall is pene- 

 trated and a system of rhizoids 



develops inside which may ramify over the chloroplast of the host cell, 

 suggesting that food material is withdrawn through the rhizoids. 



Fig. 192 



Rhizophidium globosum. Zoospor- 

 angium on wall of Spirogyra filament. Note 

 disorganization of host chloroplast. 



