CHAPTER IV 



MOLDS BELONGING TO THE PHYCOMYCETES 



No Archimycetes are likely to be of interest to the bacteriologist. 

 Probably the only genus of Oomycetes that he will ever come in 

 contact with is Pythium, some species of which are pathogenic to 

 plants but others of which are soil saprophytes. The mycelium is 

 ordinarily coenocytic, but septa 

 may occasionally be seen in older 

 mycelium. The zoosporangia are 

 usually numerous and rather small, 

 and are mostly borne on the ends 

 of the mycelial strands, although 

 they may be found in the myce- 

 lium, resembling chlamydospores. 

 On germination the contents of 

 the zoosporangia are extruded into 

 a thin-walled vesicle, after which 

 they are differentiated into several 

 motile zoospores (swarm spores) 

 and these are set free. Sexual 

 reproduction has not been seen in 

 some soil species, but most species 

 are heterogamous and homothallic. 



The Zygomycetes are subdi- 

 vided into two orders, the differ- 

 entiation being based largely upon 

 the structure of the non-sexual 

 spores, the Entomophthorales mul- 

 tiplying by conidia, and the Mu- 

 corales reproducing mostly by sporangiospores. The so-called co- 

 nidia of these phycomycetous fungi are not true exogenous conidia as 

 are found in the Ascomycetes, for in some cases at least careful micro- 

 scopic examination shows that they are really small sporangia con- 

 taining only one or a few sporangiospores. They are sometimes re- 

 ferred to as sporangiola. It is also rather difficult to divide the two 

 orders sharply on the basis described above, as some of the Muco- 



81 



Fig. 31. Pythium (1, 2, and 5, un- 

 identified species from soil; 3 and 4, 

 from various authors) : 1, mycelium 

 and sporangia; 2, intercalary spo- 

 rangium; 3, zoospore formation; 4, 

 zoospores; 5, oogonium and anther- 

 idia. 



