LIFE CYCLE OF A GILLED MUSHROOM 11 



where apparently more than two "sexes" occur, has been designated 

 multipolar sexuality, and the different strains capable of conjugation 

 have been called mating types rather than sexes. The term mating 

 type has come, to a considerable extent, to replace the term sex in 

 discussions of the Basidiomycetes. 



Classes of Fungi. Instead of recognizing only one class of true 

 fungi, the Eumycetes, most mycologists recognize four classes, which 

 together with the Schizomycetes and the Myxomycetes make up the 

 subdivision Fungi. They are Basidiomycetes, Ascomycetes, Phyco- 

 mycetes, and Fungi Imperfecti. This last is an artificial class created 

 to include the fungi whose perfect stage, that is, the stage involving 

 sexual reproduction, has not been observed. The first three classes 

 are divided upon the basis of their sexual reproduction. In the 

 Basidiomycetes the sexual spores, basidiospores, are exogenous and 

 typically four in number. In the Ascomycetes the sexual ascospores 

 are endogenous and typically eight in number. The Phycomycetes 

 form sexual spores which are in some cases single and in others multi- 

 ple. There are also other differences between these three classes. 

 Many modern mycologists divide the Phycomycetes into two or three 

 classes, each of equal rank with the other three classes. 



Basidiomycetes. The class Basidiomycetes comprises in part the 

 large, fleshy fungi, as the mushrooms, the puff balls, and the bracket 

 fungi which grow upon trees. These are the Horaobasidiomycetes. 

 Some species are known to form oidia in cultures, and some form 

 conidia on either the mononucleate or binucleate mycelium or both, 

 but mostly they reproduce by only one type of spore, the basidio- 

 spore. What we call a mushroom consists of only the spore-forming 

 or reproductive part of the plant. There is an extensive vegetative 

 mycelium penetrating the substrate. 



Life Cycle of a Gilled Mushroom. The stalk of a mushroom is 

 composed of numerous filaments of mycelium arranged in a compact 

 bundle. These terminate in the gills in the form of swollen or pear- 

 shaped tips, the basidia. In most species each basidium gives rise 

 to four basidiospores attached each to a minute stalk, the sterigma 

 (Fig. 3). When these basidiospores germinate they give rise to a 

 haploid mycelium which sooner or later becomes uninucleate, i.e., 

 divided by septa into cells with but one nucleus each. In hetero- 

 thallic species, if mycelia from two mating types come in contact 

 the hyphae may anastamose and the nuclei pair. From this point 

 on, the paired nuclei divide conjugately and an extensive binucleate 

 mycelium, in many species with clamp connections at the septa, is 



