44 AQUATIC PHYCOMYCETES 



Europe and eastern North America, the regions where most of the in- 

 vestigations on these fungi have been undertaken. 



The most striking characteristics of the chytrids are their simple 

 body plan, their vegetative system of strongly tapering rhizoids, and 

 their posteriorly uniflagellate zoospores, which often exhibit a hopping 

 motion and which contain a highly refractive oil globule. The purely 

 vegetative part of the thallus, if formed, is ordinarily inconspicuous 

 because of its tenuity, but nonetheless it may be exceedingly wide- 

 spread and profusely branched. Only one reproductive structure is 

 produced on the thallus in the majority of these fungi; its rudiment 

 is for the most part clearly differentiated from the less obvious vege- 

 tative system, and when reproductive activity is culminated the whole 

 plant body usually disintegrates. In certain chytrids, however, more 

 than one center of thallus organization is formed, some or all of which 

 may ultimately be converted into reproductive organs. 



Development and Morphology 



the thallus 

 Establishment and Development 



Considerable differences exist among the chytrids with respect to 

 their relation to the substratum. In some the thallus may be formed 

 completely within the substratum (endobiotic). In others it is partly 

 on the outer surface (epibiotic) and partly within, the two components 

 being separated by the wall of the substratum. In this situation the outer 

 part (which frequently becomes the reproductive organ) and the inner 

 (the nutrient-gathering system) are joined by a narrow tube formed 

 from the penetration tube of the infecting zoospore. In a third type 

 of relationship the reproductive rudiment and the bulk of the rhizoidal 

 system radiating from it lie free in the water among the sources of nu- 

 trition, only the tips of the rhizoids penetrating the nutrient material 

 (interbiotic). 



It is difficult to make any generalizations in regard to the establish- 

 ment and development of the thallus in the Chytridiales, since its 

 members often differ so widely in these respects. Taken as a whole, 

 however, certain principal types may for practical purposes be recog- 



