16 THE LOWER FUNGI— PHYCOMYCETES 



especially in higher groups, the sporangium germinates directly 

 by one or more germ tubes, functioning thus in its entirety as a 

 spore. It is then called by many writers a conidium. The 

 most characteristic structures of the group as a whole are the 

 sporangium, coenocytic mycehum, and zoospores. 



In the Ascomycetes the mycelium is well developed in most 

 cases, and is typically septate. It is usually less conspicuous, 

 however, than the reproductive bodies, the condition in this 

 respect being the reverse of that in the Phycomycetes. Motile 

 cells are never formed. The group is distinguished by the ascus, 

 which is a sac forming endogenous spores, usually in small and 

 definite number (typically eight, but less frequently some other 

 number which is usually a multiple of two). Though a sac 

 forming endogenous spores, the ascus is not homologous with 

 the sporangium, and differs from it in several important respects. 

 It is sexual in character, in it two nuclei, usually regarded as 

 sexually different, fuse, and in it reduction division occurs. 

 Following the reduction process the ascospores are cut out by 

 means of the astral rays in a peculiar process known as free cell 

 formation. The spores thus delimited are surrounded by unused 

 cytoplasm termed epiplasm. In the sporangium, epiplasm is 

 absent, and the process of progressive cytoplasmic cleavage, 

 which accomplishes spore formation there, is a wholly different 

 phenomenon. In the lower Ascomycetes the asci are formed 

 without order throughout a mould-like mycelium, or exist as 

 isolated cells as in the yeasts. In the great majority of higher 

 forms they are collected into a more or less spherical, flask- 

 shaped, cup-shaped or disc-shaped fruit body, usually conspic- 

 uous and termed the ascocarp (apothecium, perithecium). 

 Asexual reproduction takes place by means of spores termed 

 conidia. Conidia are exogenous and are adjointed at the ends 

 of more or less specialized hyphae called conidiophores. The 

 conidium of the Ascomycetes and Basidiomycetes is regarded 

 as homologous with that of the Phycomycetes and hence with 

 the sporangium. The conidiophores in some forms are scattered 

 over the mycelium. In others, they are aggregated into definite 

 sori or fruit bodies (acervuli, sporodochia, coremia, pycnidia). 

 In some forms the hyphae break apart at the septa to form 

 unicellular asexual spores termed oidia. The group as a whole 

 may be roughly split into the Discomycetes and Pyrenomycetes. 

 It contains the yeasts, morels, cup fungi, truffles, and many forms 



