58 



PHTCOMYCETEAB 





Fig. 16. Chytridiales, Family Rhizidiaceae. Siphonaria petersenii Karl. (A) Mature 

 plant with zoospores escaping. (B) Union of male and female plants through a narrow- 

 tube. (C) Sexually produced thick-walled zygote with empty male plant still attached. 

 (Courtesy, KarHng: Am. J. Botany, 32(9):580-587.) 



which enter the host cells encountered. Sometimes, where the latter are 

 crowded in considerable numbers, as many as fifty may be attacked by 

 the haustoria from one parasite. The latter remains uninucleate and is 

 invested by a firm thin wall. Within the swelling (prosporangium) repre- 

 senting the original zoospore, the nucleus divides to form the nuclei of 

 the new zoospores, and the whole contents bud out into a somewhat 

 elongated thin-walled sporangium within which the division into the 

 uninucleate zoospores takes place. As many as several hundred zoospores 

 may be produced. Upon the occurrence of conditions unfavorable for 

 further asexual reproductions there may occur the conjugation of two 

 cells. A somewhat smaller cell sends out a slender process (perhaps a 

 modified haustorium), the tip of which enlarges when it comes into 

 contact with a larger cell. Into this enlarged tip the nucleus and contents 



