Minnesota Plant Diseases. 



255 



The latter are annual and have a very characteristic and beau- 

 tiful appearance. They are hemispherical in shape and the 

 short stalk is always attached to the side. The upper surface is 

 grey to light brown in color, is very smooth and covered by a 

 thin skin. The pores on the under surface are small and rather 

 deep and the layer in which they are found is easily separable 

 from the rest of the fruiting bodies. The flesh of the fruiting 

 body is pure white and somewhat spongy in texture. In old 

 dried fruiting bodies the flesh is very commonly found to be 

 honey-combed by the larvae of insects. 



Fig. 126. — Fruiting body of the birch pore fungus (Polyporus betulinus), on a branch of 

 a white birch. Original. 



The parasitic relationships of this fungus with the birch trees 

 have been established by several investigators and there is little 

 doubt that the fungus causes the death of many birches in this 

 state. The fruiting bodies are usually found on dead birches, 

 often accompanied by other pore fungi. The mycelium in the 

 living tree grows not only through the growing region of the 

 stem and the inner bark but also attacks the wood. When the 

 mycelium which may be growing for years has accumulated 

 sufficient food material a fruiting body is formed. A new crop 

 is produced every year, if conditions are favorable. 



