446 



BASIDIOMYCETES. 



bark of birch. Spathulate cystidia 

 Spores colourless, and acutely ovate 



and forming crusts on the 

 occur between the basidia. 

 in shape. 



This is parasitic on birch. The mycelium kills and permeates 

 the wood-parenchyma which forms the greater mass of the 

 later-formed parts of each year-ring, with the result that the 

 various year-rings of the wood separate from each other as 

 concentric hollow cylinders. The mycelium varies according as 

 its pabulum consists of cells just killed, or of wood, or of 

 elements in the last stages of decomposition ; in this latter case 

 it sufifers from want of food. In woody elements in contact 



with air, or those destroyed 

 by Polyponis betulinus, the 

 mycelium is brown and 

 forms vesicular tyloses 

 similar to Agaricvs melleus. 

 Polypoms betulinus Fr. 

 (Britain and U.S. America). 

 The sporophores are annual, 

 and emerge as spherical 

 structures from the unin- 

 jured bark, or from bore- 

 holes of Beetles, or other 

 wounds. When mature 

 they are hoof-like or semi- 

 circular and short-stalked ; 

 when dead they become 

 The upper side is light-brown in colour, the 

 A section through the sporophore shows it 



Lanceolate cystidia 

 The pore- 



FlG. 278. — Polypm-us betulinus on Betula verrv.CQ$a. 

 The sporophore was developed horizontally on a 

 fallen stem ; it is here, however, set up vertically 

 and photographed from the lower side. (v. Tubeuf 

 phot.) 



soft and break off. 



pore-layer is white. 



to be white and homogeneous without zones. 



occur between the basidia. The spores are rod-like. 



layer and the upper brown layer are easily detached, and strips of 



the remaining tissue are sometimes utilized as razor-strops. 



This parasite frequents living birches, ultimately causing death. 

 It is known to occur on both Betula verrucosa and B. puhescens 

 in Britain, America, and Europe. Its parasitism and injurious 

 results were first demonstrated by Eostrup.^ Mayr^ investigated 



'Rostrup, " Snyltesvamper Angreb paa Skovtraeerne," 

 hurg, 1883. 

 -Mayr, Botan. Centralblatt, xix., 1884. 



pa Skov- 



