426 THE FUNGI WHICH CAUSE PLANT DISEASE 



hjTnenium: context fleshy-tough, elastic, homogeneous, 3 cm. thick, 

 milk-white; tubes 0.5 cm. long, 2-3 to a mm., sodden-white, sepa- 

 rated from the context by a thin pink layer; mouths very irregular, 

 dissepiments thicker than the pores, obtuse, entire, crumbling 

 away in age, leaving the smooth, white context; spores white, 

 cylindrical, curved, 4-5 ix in length. The mycelium penetrates 

 lignified cell walls entering the living cells and causing death. 



On birch it causes a decay of the sap wood similar to that caused 

 by Fomes fomentarius. 



P. adustus (Wild.) Fr. is a common saprophyte of deciduous trees. 



Polystictus Fries (p. 417) 



Sporophore leathery, usually thin; pores developing from the 

 center to the circumference of the hymenophore. The thicker 

 forms are quite close to some species of Polyporus. 



About four hundred fifty species. 



P. versicolor (L.) Fr.«'' ^* 



Pileus densely imbricate, very thin, sessile, dimidiate, conchate, 

 2-4 X 3-7 X 0.1-0.2 cm.; sinrface smooth, velvety, shining, marked 

 with conspicuous, glabrous zones of various colors, mostly laterice- 

 ous, bay or black; margin thin, sterile, entire; context thin, mem- 

 branous, fibrous, white; tubes punctiform, less than 1 mm. long, 

 white to isabelline within, mouths circular to angular, regular, 

 even, 4-5 to a mm., edges thick and entire, becoming thin and 

 dentate, white, glistening, at length opaque-isabelline or slightly 

 umbrinous: spores allantoid, smooth, hyaline, 4-6 x 1-2 n; 

 hyphse 2-6 /*; C5'stidia none. 



Von Schrenk regards this as strictly a saprophyte except when 

 on catalpa, where it causes a heart-rot. It is common on almost 

 any kind of wood. 



Catalpa wood under its action becomes straw-colored and finally 

 soft and pithy. Both cellulose and lignin are dissolved. 



P. sanguineus (L.) Fr. & P. cinnabarinus (Jacq.) Fr. are sapro- 

 phytes on dead parts of live trees. 



P. velutinus (Pers.) Fr. is a common saprophjrte which is perhaps 

 sometimes parasitic. 



P. occidentalis Klachb. is recorded as a parasite on Pterocarpus 

 indicus in the Malay peninsula.*^ 



