446 



BASIDIOMYGETES. 



and forming crusts on the bark of birch. Spathulate cystidia 

 occur between the basidia. Spores colourless, and acutely ovate 

 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 suffers from want of food. In woody elements in contact 



with air, or those destroyed 

 by Polyporus betulinus, the 

 mycelium is brown and 

 forms vesicular tyloses 

 similar to Agaricus me Ileus. 

 Polyporus 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 

 soft and break off. The upper side is light-brown in colour, the 

 pore-layer is white. A section through the sporophore shows it 

 to be white and homogeneous without zones. Lanceolate cystidia 

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

 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 Bdula verrucosa and B. pubescens 

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

 results were first demonstrated by llostrup. 1 Mayr 2 investigated 



1 Rostrup, "Snyltesvamper Angrel) pan Skovtraeerne," Tldsskrift pa Skor- 



/////;/, ISSS. 



-Alnyr, /loftui. ('< iilnilliliill, MX., 1SS4. 



FIG. 278. Polyporus betulinus on Bi:tulti verrucosa. 

 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.) 



