5S THE PHYCOMYCETES 



Amone Phvcomvcetes of the simplest structure, those pre- 

 sumed to be the most primitive, the entire coenocxte is the en- 

 larged bodv of the spK:>re, which mav at marurit\- become trans- 

 formed into a single sporangium, as occurs in the Olpidiaceae. 

 In the S\-nch\-triaceae the thallus fragments become a sorus or 

 cluster of sporangia. Organisms in which the entire thallus is 

 modified into a fructification, as it is in the Olpidiaceae and 

 S\-nch\-triaceae, are spoken of as holocarpic. If. on the other 

 l^d. the thallus is differentiated into sterile and fertile portions, 

 it is eucarpic. Eucarpic thalli with a single reproductive rudi- 

 ment are monocentric; w"ith more than one. polycentric. The 

 Rhizidiaceae possess discoid, -bulbous, or tenuously branched 

 rhizoids bv means of which nutrients are absorbed. Among the 

 aadoch\Triaceae the assimilator\- portion is mycelioid at first 

 but is quite evanescent and is eventually transformed into repro- 

 ductive structures. All the higher Phvcomvcetes, on the other 

 hand, possess richlv branched thalli that course over or through- 

 out the substrata. The parasitic species among them are topically 

 intracellular or else are intercellular, absorption of food in inter- 

 cellular species beinor accomplished by haustoria, as in the Albur 

 oinaceae and the Peronosporaceae. 



The sporaxgium. Asexual reproduction among most Phy- 

 comvcetes is accomplished bv sporangiospores that are borne 

 endosenouslv w-ithin sporangia. They are formed by cleavage 

 of the contents of the sporangium and may be either motile or 

 non-motile, depending mainly upon the species. Motile spores 

 are commonlv called zoospores or planospores; non-motile spores, 

 aplanospores. 



In aquatic species the zoospores are emitted and swim away in 

 the water. In terrestrial species, such as those of the Perono- 

 sporaceae and Albueinaceae. the sporangia are detachable and 

 are dispersed in toto, mainly by air currents; whereas in other 

 species, for example, those of the Mucorales. the sporangia burst 

 and the sporangiospores are themselves disseminated. 



Shape of sporangia is rather generally employed as a generic 

 character in this group. In some genera, such as Leptolegnia, the 

 sporangia are little different from the vegetative hyphae. In 

 Saprolegnia, Achlva, and Dict\-uchus they are broadly clavate; in 

 . Olpidium, Mucor. Thamnidium, and Choanephora, spherical; in 

 Ph\-tophthora, Pxthium, and P\-thiopsis, pyriform. 



