ORIGIN OF AGARICALES C53 



by hymenium, the Polyporaceae. By the radial elongation of these pores 

 so that they extended from the point of attachment (more often a central 

 stipe) to the edge of the spore fruit a system of gills or lamellae was 

 developed, the Agaricaceae. The lateral branches producing the Exo- 

 basidiaceae and Clavariaceae were not in the main line of upward evolu- 

 tion and need not be given further consideration in this hasty review. Up 

 to the Agaricaceae and the closely related Boletaceae practically all the 

 foregoing fungi were gymnocarpous in their ontogeny. The hymenium 

 w^as developed externally over the whole surface of the spore fruit or over 

 part of the surface beginning as a plane which in the more advanced forms 

 became thrown into teeth, pores, or gills. Naturally, therefore, those 

 Agaricaceae were considered the most primitive in which this gymno- 

 carpous ontogeny continued. Upon study of the development of the 

 fruiting body from very early stages of growth it has been shown that in 

 many Agaricaceae and in the Boletaceae the primordium of the hymenium 

 is laid down gymnocarpously, but that by the outward spreading and 

 downward and inward curving of the edge of the pileus this hymenial 

 surface is finally enclosed in a circular tunnel surrounding the top of the 

 stipe and sealed from the exterior by the inturned edge of the pileus which 

 has grown fast to the stipe. Into this circular space the gills grow from the 

 pileus and the basidia begin to form on them but before they have ma- 

 tured sufficiently to produce their spores the edge of the pileus breaks 

 loose from the stipe and pulls away from it so that the gills at their 

 maturity are exposed to the exterior. This is called the pseudoangio- 

 carpous mode of development. Beyond this type of development we find 

 many Agaricaceae in which the hymenial primordium originates in a 

 circular layer entirely closed from its first inception within the tissues of 

 the spore fruit. Not until the gills have reached almost their full develop- 

 ment does the growth of the fruiting body bring about a circular rupture 

 when the expanding pileus pulls away from the stipe, so that finally the 

 gills are exposed to the air and the basidia shed their spores. This angio- 

 carpous mode of development is found especially in those genera with a 

 volva or with a well-developed annulus or cortina or a universal veil, even 

 though this may be rather thin. Since the Gasteromyceteae have a perid- 

 ium that in some forms, at least, seems homologous to the universal veil 

 and since, as mentioned above, other Agaric-like structures are present in 

 some members of this group it has been suggested that these represent 

 further stages of evolution from the Agaricaceae. Thus Heim (1937, 1948) 

 suggested that the Asterosporales are a closely related group, containing 

 the genera Russula and Lactarius of the Agaricales and Arcangeliella, 

 Elasmomyces, Maccagnia, and some others usually included in the Gas- 

 teromyceteae. He indicates his belief that these last mentioned genera 

 have been derived from Russula and Lactarius. 



