22 



RESEARCHES ON FUNGI 



can take place in these groups. The mechanism for liberating 

 spores is of such a nature as to limit the possible forms of the fruit- 

 bodies in question. 



Comparison of the Basidium with the ASCIIS. The vertical or 

 downwardly-looking position of the hymenial surfaces of Hymenomy- 



cetes may be contrasted with 

 the upwardly-looking hymenial 

 surfaces of Discomycetes. From 

 the physiological point of view, 

 the ascus in this great group 

 of fungi is significant in that 

 it is an apparatus by which 

 spores may be liberated suc- 

 cessfully, when it looks upwards. 

 It is an explosive mechanism 

 of considerable efficiency. In 

 many instances it shoots out 

 its spores en masse to a distance 

 of one or several centimetres, 

 and thus causes them to be- 

 come effectively separated from 

 the ascocarp. 1 It seems to be 

 the development of the explosive 

 ascus which has permitted of the 

 fruit - bodies of Discomycetes 

 taking on their saucer- or 

 cup-like shapes. Here again, 

 as in the Hymenomycetes, 

 spore- liberating mechanism and 

 fruit -body structure go hand 



FlG. 2. Group of young fruit-bodies of 

 Pleurotus ostreatnx (the Oyster Fungus) 

 growing from a wound on the trunk of 

 a Beech. The gills are developing in 

 vertical planes in response to a geo- 

 tropic stimulus. Photographed at 

 Sutton Park, Warwickshire, by J. E. 

 Titley. About natural size. 



in hand. 



There appears to be just as strict a correlation between the 

 general structure of an Agaricus or Polyporus and its basidia as 

 between the general structure of a Peziza and its asci. If the 

 basidia and asci in these types were interchanged, each fruit-body 

 would lose its efficiency. The spores could not be liberated, but 



1 Vide infra, Part II. 



