CHYTRIDIALES 597 



branches. The "mycelium" ultimately produced varies greatly in diam- 

 eter , is markedly undulate, and shows in the younger stages of develop- 

 ment a tendency to follow the region of juncture of the host cell walls. 

 The latter fact would seem to suggest some type of pectin relationship. 

 Further growth and branching are profuse. The contents of the thallus 

 are finely granular and refractive, with large vacuoles, and are occasion- 

 ally separated by narrow cross walls. The ultimate branches are extreme- 

 ly refractive, often fusing laterally with one another in a very charac- 

 teristic manner. It is particularly noteworthy that, in contrast to other 

 chytridiaceous forms having a well-developed thallus, such as Nowa- 

 kowskiella, the vegetative system never becomes rhizoidal, that is, 

 strongly tapering. In heavily parasitized leaves it is endobiotic as well 

 as epibiotic, although the method whereby the fungus gains entrance to 

 the host has not been ascertained. On the stouter portions of the threads 

 large broadly fusiform or irregular swellings delimited by cross walls 

 are produced (Fig. 37 A). These may develop either into sporangia or 

 resting spores. The latter may germinate after little or no resting period. 

 In this process the wall is partly assimilated and a sporangium is formed 

 on the outside of the resting spore (Fig. 37 B). Zoospores arise in the 

 usual manner. The ordinary sporangia originate as somewhat pyriform 

 terminal enlargements of the filaments. When mature they are variable 

 in shape and sometimes have a slightly inflated apophysis. Proliferation 

 has been noted in a few instances. 



Megachytrium westonii Sparrow 



Occ. Papers Boston Soc. Nat. Hist., 8:9. 1931 ; Amer. J. Bot., 20: 73, pi. 3, 



figs. A-E. 1933 



Thallus at first entirely epibiotic, later also endobiotic, consisting of 

 a profusely branched extensive tubular undulating hypha-like vegetative 

 system whose main axes are up to 5-7 \x in diameter and whose smaller 

 ones are about 3 ;jl, and of numerous terminal or intercalary swellings; 

 sporangia spherical or clavate but more often irregular, with or without 

 a single short discharge tube, varying greatly in size, usually about 

 15-50 [jl long by 10-30 [i wide, sometimes apophysate, rarely prolifer- 

 ating, wall thin, smooth, colorless; zoospores spherical, 5 u. in diameter, 



