THE FUNGI : ASCOIVIYCETES AND BASIDIOMYCETES 317 



are abstricted. These basidiospores are small and oval in shape. It appears 

 that the continuous spore discharge, which is characteristic of these Bracket 

 Fungi, is brought about partly by the upgrowth of fresh basidia from among 

 those which have ceased to function and also by the progressive formation 

 of basidia from the top of the tube downwards, as the thickness of the fruiting 

 body increases and the tubes elongate. The period of spore discharge may 

 continue for weeks or even months, and as a consequence the total number 

 of spores discharged by a single fruiting body may run into millions. 



Asexual reproduction by means of conidiospores occurs ; these arise 

 from the mycelium and may also be produced on the curious sterile fructi- 

 fications, if any of these are actually produced by this species, but since the 

 identity of these fructifications with any definite species is extremely obscure, 

 we do not know whether they are ever produced by P. hetuUmts. 



Polvponis hetiilinus has found certain technical uses. The fruiting bodies 

 are employed at the present time in the manufacture of charcoal crayons, 

 while formerly the surface of the upper tissue was found to make a very 

 efficient razor strop. 



Other Types of Bracket Fungi 



In Polyponts betulinus the fruiting body only lasts for a single season, but 

 in certain other types, for example, in the genus Fomes, the fruiting bodies 

 are perennial. In such cases a completely new zone of tubes is formed below 

 those of the previous year. These tubes may be almost continuous with 

 those already existing, arising by the elongation of the longitudinally running 

 hyphae of the tube walls, or in some species a zone of anastomosing hyphae 

 may be formed first, from which the longitudinally running hyphae develop. 

 Thus in a section of an old fruiting body of Fomes applanatus successive layers 

 of tubes can be seen one below the other, resembling superficially the annual 

 rings of growth of a stem. The age of such fruiting bodies therefore can be 

 calculated by counting the number of successive tube layers. 



In some species, e.g.. Poly poms dryadeus, it appears that liquid is normally 

 exuded from the tubes, and in these drops basidiospores occur (Fig. 310). 

 Whether this exudation assists spore distribution or the germination of the 

 spores is not known. 



Many important plant diseases are caused by Fungi belonging to this 

 large order, which contains several thousand species. Among those best 

 known we may mention Stereum purpureiun, which is the cause of the so- 

 called Silver Leaf disease which is common on trees and most important when 

 it attacks Plums. It first causes a silvering of the leaves due to the separation 

 of the epidermis from the rest of the leaf tissue, which results in an air space 

 being formed which reflects light. Later the branches begin to die back, 

 and finally the whole tree dies. 



Meruliiis lachrymans, the Fungus responsible for Dry Rot of timber is 

 another member of this group. The fruiting bodies of the Fungus, instead 

 of being produced in a bracket, lie flat or resupinate on the surface of the 



