1062 AQUATIC PHYCOMYCETES 



Development and Morphology 

 the thallus 



The vegetative body in Ancylistes is a relatively broad tubular struc- 

 ture bearing occasional short lateral branches, the whole crowded 

 within the confines of the host (desmid) cell. Both Dangeard (1886a) 

 and Couch (1949) describe the young hyphae as lacking a definite wall; 

 presumably only a membrane is present. When young, according to 

 both Dangeard (1903b) and Couch (op. cit.), the thallus is multi- 

 nucleate, the nuclei being regularly spaced in linear series. The contents 

 are granular with numerous conspicuous vacuoles. The following some- 

 what modified account of the establishment and development of the 

 thallus in A. netrii is taken from Couch (loc. cit.). 



Soon after entry, the parasite is carried into contact with the host 

 nucleus where it forms a short tube. The hypha then grows in opposite 

 directions toward the ends of the desmid, usually with one branch in 

 each groove between the plates of the chloroplast. Not infrequently a 

 hypha may encircle the chloroplast, returning on the other side. Mature 

 hyphae are over twice as wide as the young threads and appear to gain 

 this increase after longitudinal growth ceases. Septation then takes place 

 and is progressive from region to region. Each of the segments has 

 several nuclei, as do the conidia, gametangia, and zygotes, thus confirm- 

 ing Dangeard's (1903b) observations. The mature thallus consists of 

 about thirty to fifty multinucleate cells, each of which may effect repro- 

 duction in one of three ways: a cell may give rise to an extramatrical 

 hypha, which contacts another desmid, forms a distal appressorium and 

 infects it; after forming an extramatrical hypha this may become a 

 conidiophore bearing a conidium; it may become a gametangium which 

 will either fuse with another within the host to form a zygospore or, 

 without fusion, develop into an azygospore. 



Reproduction 



Nonsexual Reproduction 



In nonsexual reproduction external hyphae from infected host plants 

 which float near the surface function as conidiophores and may arise 

 above the water. Potentially, each cell of the endobiotic mycelium is 



