ANCYLISTALES 



125 



all the cells of a single thallus function alike, as vegetative cells, 

 as male cells, or as female cells. This species is consequently 

 heterothallic, two thalli lying side by side functioning as male 

 and female thalh respectively, and all the resting spores being 

 formed in one thallus. The male gametangium puts out a short 

 fertilization tube which fuses with the wall of the nearest female 

 cell and the content of the male cell then passes over. A definite 

 oosphere has not been demonstrated, but the contents of the 



Fig. 47. — Ancylistcs dostcrii Pfitzer in cells of Clostcrium. (a) Cells of thallus 

 germinating to form infection threads, {h) Oospores, (c, d) Cells of neigh- 

 boring thalli united by conjugation tubes preceding oospore formation. {After 

 Pfitzer 1872.) 



fertilized cell contract to form an oospore-like resting spore. 

 Germination of the spore is by germ tube. Another species, A. 

 pfeifferi de Beck, also occurring in Closterium, has been incor- 

 porated in the genus but is imperfectly understood. In it the 

 male and female gametangia appear to alternate in the same 

 thallus, but this has not been clearly demonstrated. Some 

 thalli function vegetatively as in the above species. The method 

 of resting spore germination is unknown. The absence of swarm- 

 spores gives the genus particular interest, and the formation of 



