422 



THE FUNGI WHICH CAUSE PLANT DISEASE 



zonate, shining, 3-10 mm. thick; tubes slender, concolorous with 

 the context, about 1 cm. long, mouths regular, angular, 2-3 to a 

 mm., glistening, whitish-isabelline to dark-fulvous, edges thin, 



Fig. 302. — Decomposition of spruce-timber by Polyporus borealis. a, a 

 tracheid containing a strong mycelial growth and a brownish yellow fluid 

 which has originated in a medullary ray ; at b and c the mycelium is still 

 brownish. At d and e the walls have become attenuated and perforated, 

 the filaments delicate; at / the pits are almost destroyed; at g and h only 

 fragments of the walls remain." The various stages in the destruction 

 of the bordered pits are to be followed from i to r; at i the bordered pit 

 is still intact; at k the walls of the lenticular space have been largely dis- 

 solved, their inner boundary being marked by a circle; at I one side of 

 the bordered pit has been entirely dissolved; at m and n one sees a series 

 of pits which have retained a much-attenuated wall on one side only — 

 namely, on that which is provided with the closing membrane. In mak- 

 ing the section a crack has been formed in this wall. Between o and r 

 both walls of the pits are found to be wholly or partially dissolved, only 

 at p and q has the thickened portion of the closing membrane been pre- 

 served; at d the spiral structure of both cell-walls is distinctly recogniz- 

 able. These walls when united form the common wall of the tracheid ; at 

 t hyphffi are seen traversing the tracheids horizontally. After Hartig. 



entire to toothed; spores subglobose, smooth, deep-ferruginous, 

 6-7 m; cystidia scanty and short; hyphse deep-ferruginous. 



It causes a disease of oaks. 



P. fruticum B. & C. occurs on living twigs of the orange and 

 oleander in Cuba. 



