22 



HKSEAK'CHES OX FUNGI 



can take placu in ihesc gr()U)).s. '["he iiicchanisni lor liliri'aling 

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

 bodies in question. 



Comparison of the Basidium with the Ascus. — The vertical or 

 downwardly-looking position of the hynienial surfaces of Hynienoniy- 



cetcs may be contrasted with 

 the upwardly-looking hynienial 

 surfaces of Discomycetes. From 

 the physiological point of vieAv, 

 the ascus in this great group 

 of fuuLri is siynificant in that 

 it is an apparatus by which 

 spores may bo 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.^ It seems to be 

 the development of the explosive 

 ascus which has permitted of the 

 fruit - bodies of Discom3'cetes 

 taking on their saucer- or 

 cup-like shapes. Here again, 

 as in the Hyuienomycetes, 

 spore- liberating mechanism and 

 fruit - body structure go hand 



Fig. 2. — Gruup of young fruit-bodies of 

 Fleurotns ostreatux (the Oyster Fungus) 

 growintj; from a wound on the trunk of 

 a JJeech. The gills are developing in 

 vertical ])lanes in response to a geo- 

 tropic stimulus. Photographed at 

 Sutton Park. Warwickshire, hy J, E. 

 Titley. About ^ natural size. 



in hand. 



There appears to be just as strict a correlation between the 

 •jfeneral structure of an Acjaricus or Polvi)orus and its basidia as 

 between the genei'al 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 



' J'ide iiifi-ii, I'art 11. 



