THALLOPHYTA: FUNGI 157 



Phycomycetes. Plant body typically a nonseptate multinucleate 

 mycelium. Asexual reproduction by spores formed by cleavage and 

 borne in indefinite numbers in sporangia. Lower members with zoo- 

 spores, higher members with aerial spores. Sexual reproduction isoga- 

 mous or heterogamous. Heterogamous forms with well-developed sex 

 organs. 



Ascomycetes. Plant body typically a septate mycelium. Spores 

 borne usually in groups of eight in a sac-like structure, the ascus, their 

 nuclei arising by three successive divisions of a fusion nucleus. Zoospores 

 wanting. Sex organs reduced, obscure, or entirely absent. 



Basidiomycetes. Plant body a septate mycelium. Spores borne 

 usually in groups of four on a club-like structure, the basidium, their 

 nuclei arising b}^ two successive divisions of a fusion nucleus. Zoospores 

 wanting. Sex organs not present. 



GENERAL CONCLUSIONS 



The fungi are a heterogeneous assemblage of thallophytes of diverse 

 origin held together by a physiological character — the absence of chloro- 

 phyll. Two classes, the Schizomycetes and Myxomycetes, stand apart 

 from each other and from the three classes of "true fungi" (Eumycetes). 

 In their unicellular organization, cell structure, and reproduction the 

 Schizomycetes resemble the Cyanophyceae much more closely than they 

 resemble any of the other fungi. The Myxomycetes, with their naked 

 Plasmodia, highly developed sporangia, and amoeboid isogametes, exhibit 

 similarities to some of the Protozoa, on the one hand, and to some of the 

 lower Phycomycetes (Plasmodiophorales) on the other. 



Some botanists believe that the "true fungi" are a monophyletic group 

 that have arisen from colorless flagellates and have subsequently differen- 

 tiated into the three existing classes of Phycomycetes, Ascomycetes, and 

 Basidiomycetes. According to this theory, no direct relationship exists 

 between the algae and fungi, their resemblances being a result of parallel 

 evolution along two independent lines. Other botanists believe that the 

 "true fungi" have been derived from the algae through loss of chlorophyll, 

 their origin having been either monophyletic or polyphyletic. According 

 to this theory, the Phycomycetes have evolved from the Chlorophyceae, 

 the Ascomycetes from either the Phycomycetes or the Rhodophyceae, 

 and the Basidiomycetes from the Ascomycetes. 



Vegetative Body. The characteristic plant body of the fungi' is a 

 mycelium, made up of branching hyphae that may be either nonseptate 

 and coenocytic (Phycomycetes) or septate (Ascomycetes and Basidiomy- 

 cetes) . Only a few forms are unicellular. The hyphae elongate by apical 



1 In the following discussion the term fungi will be limited to the three classes of 

 "true fungi." 



