CHYTRIDIALES 45 



envelops itself in a thin membrane and is then said to be encysted. 

 Some writers term it a cystospore. In the Plasmodiophoraceae 

 and Sjmchytriaceae a membrane is not formed, and in these 

 groups, composed wholly of parasites, the zoospore is capable of 

 passing directly through the host cell wall into the cell cavity. In 

 the other families the zoospore assumes a membrane, and germina- 

 tion is accomplished by the passage of the protoplast through a 

 short exit tube or germ tube. In parasitic species this exit 

 tube functions as an infection thread and penetrates the host 

 cell wall. In the Olpidiaceae and Woroninaceae the protoplast 

 of the cystospore then passes through the tube into the host and 

 there develops into the mature thallus. In the typical epibiotic 

 species (Rhizidiaccae) the germ tube of the cystospore acts as 

 a holdfast and often develops into a system of delicate endo- 

 biotic rhizoids, while the spore itself enlarges and forms an 

 epibiotic sporangium or resting spore. In the Cladochytriaceae 

 epibiotic sporangia are sometimes formed in this way, but more 

 often the protoplast of the swarmspore passes through the germ 

 tube into the host and develops there into an extensive mycelium. 

 Endobiotic sporangia and resting spores then develop from inter- 

 calary or terminal enlargements of this mycelium. 



Sexual reproduction probably takes place in many of the 

 genera of the order, but its presence has been satisfactorily 

 demonstrated in relatively few species. Probably, subsequent 

 investigations will show that the resting spores throughout the 

 group are in most cases at least sexually produced. They are 

 commonly termed resting spores, rather than oospores or zygo- 

 spores, because the method of their formation is usually in doubt. 

 Even in cases in which the details of the sexual process are com- 

 pletely known the terms oospore and zygospore are avoided 

 since in these primitive forms sexuality is often intermediate in 

 type between typical oomycetous and typical zygomycetous 

 reproduction. Considerable variation in the sexual process is 

 evident in the group, the general situation having been sum- 

 marized in the preceding chapter (p. 34). The resting spore 

 in some species {e.g., Olpidium viciae and Synchytrium endo- 

 bioticum) results from planogamic fusion of cihate isogametes. 

 In other cases {Monochytrium) gametes fuse in the amoeboid 

 condition. Probably many species form their resting spores 

 in this manner, the fusions not yet having been observed due 

 to the difficulty of following the activities of such small motile 



