312 A TEXTBOOK OF THEORETICAL BOTANY 



Aphyllophorales 



The Aphyllophorales are Basidiomycetes in which the basidia are un- 

 divided, but in which the hymenium is exposed from the first and spreads 

 over the surface of either teeth, anastomosing gills or a smooth surface, or 

 alternatively it may line, in the higher members, the inner surface of tubes. 



The group is a large one containing many of the common Bracket Fungi 

 which are so very characteristic of woodlands. It must be realized that 

 these brackets are only the fruiting bodies borne on an invisible mycelium. 

 This mycelium begins life either in the soil, from whence it gains entrance 

 into the host through the roots, or it may enter the host tissue through a 

 wound. The mycelium ramifies mainly in the phloem and the layers of thin- 

 walled cells just below the bark, but some have the power of attacking lignin, 

 so that they live not only at the expense of the elaborated food material 

 produced by the host, but also upon the host tissue as well. In many 

 instances, moreover, it is only when the life of the host tree is threatened, 

 or in some cases when it is dead, that any fruiting body is formed at all. In 

 fact prior to that time the presence of the Fungus may be quite unsuspected. 



The fruiting bodies or sporophores are usually hard and woody in 

 consistency, though in some species they may be more or less fleshy. They 

 may develop rapidly and mature in a few months or they may be perennial, 

 in which case a fresh hymenial layer is developed each year. 



In addition to the normal fruiting bodies special fructifications of very 

 variable form may be produced, particularly in the dark. Such fruiting bodies 

 are often found developing on pit props. Though these bodies appear to 

 be often sterile, it is said that sometimes they develop conidiospores. Coni- 

 diophores may also develop from the mycelium ; they are spherical bodies 

 and each bears a cluster of oval or spherical conidiospores which germinate 

 to produce a fresh mycelium. These conidiospores are stated to be diploid. 



The mycelium, too, may form spores directly by the aggregation of the 

 protoplasm into small areas, which then become surrounded by a wall. 

 The spores so formed are usually spherical and thick walled. They are 

 termed oidia, and are stated to be monoploid and to give rise only to mono- 

 ploid mycelia. 



Many of the species have been grown successfully on agar in pure culture. 

 Under such circumstances it is often difiicult to induce them to produce 

 normal fruiting bodies, though some, after years of growth, do produce 

 typical fructifications. It is not very surprising, therefore, that our knowledge 

 of the developmental history of the various species is extremely scanty 

 and no species has been adequately investigated. The following account of 

 Polyporus betulinus serves to illustrate the more important features. 



Polyporus betulinus (The Birch Polypore) 



This species may be found growing very commonly in Birch woods, 

 forming large fructifications on the trunks and larger branches of diseased 



