404 COMPARATIVE MORPHOLOGY OF FUNGI 



dicaryons participating in mitoses and by an increased septal formation, 

 the cells in the lamellae become binucleate (Hirmer, 1920). 



Except for clamp formation, true branching, vegetative anastomoses 

 and the virtual binucleate character of the coenocytic cells, which 

 characteristics are not similar in all forms and may not always be recog- 

 nized with sufficient certainty, the appearance of the secondary diploid 

 hyphae agrees so closely with the primary haploid hyphae that it has been 

 possible only by the use of cytological methods to demonstrate the cor- 

 rectness of the differentiation into primary and secondary mycelia. In 

 rare cases, e.g., Corticium roseo-pallens (Lyman, 1907), the secondary 

 hyphae may form conidia which appear entirely similar to those formed 

 on the primary mycelium ; or they break up into binucleate oidia which 

 are not separable from the uninucleate oidia (Fig. 355, 1); or they may 

 form gemmae which rarely show a further morphological differentiation 

 from the haploid gemmae; or they may develop into sprout cells, only 

 distinguishable from the uninucleate cell by cytological study; or, in 

 certain cultural conditions, apparently by a process of degeneration, they 

 may regain a uninucleate condition. In the Uredinales, the secondary 

 mycelium differs markedly in biological relations and in secondary spore 



forms. 



There is no fundamental difference between haploid and diploid myce- 

 lium in the Basidiomycetes such as there is in the Ascomycetes. In 

 many species, it is almost impossible to determine microscopically the 

 point of origin of the diploid mycelium, which grows gradually from the 

 haploid mycelium and develops similarly. In other forms, as in Merulius 

 lacrymans and in some species of Coprinus, the types of mycelium show 

 small physiological differences, e.g., different moisture relations, so that 

 the diploid mycelium is elevated as an aerial mycelium above the haploid 

 mycelium which is repent or submersed. Both are culturable in the same 

 substrate, both are adapted to independent existence; the diplont, in 

 contrast to the ascogenous hypha of the Ascomycetes, no longer needs 

 to be nourished by the haplont. It is rather the vegetative mycelium 

 par excellence which winters over in the earth in many species, e.g., mush- 

 rooms, and forms lichens and mycorrhizas. Where we find free Basidio- 

 mycete mycelium in nature, we are generally dealing with secondary 

 mycelium. This has led many authors to call the clamp-bearing, diploid 

 mycelium the true characteristic mycelium of the Basidiomycetes. 



Since it is vegetative mycelium (in contrast to the ascogenous hyphae 

 of the Ascomycetes), the secondary mycelium of the Basidiomycetes is 

 the final mycelial form only in the parasitic forms (as in the simpler 

 Auriculariales and Polyporales as well as the Uredinales, Ustilaginales 

 and Exobasidiaceae) ; in these the hyphal ends are transformed directly 

 to basidia. In all higher forms, the secondary mycelium does not proceed 

 as such to the formation of basidia but its hyphae intertwine with exten- 



