VI.] DISEASES DUE TO CERTAIN PARASITES. 171 



attacked by these two parasites, though P. igniarius^ 

 at any rate, attacks many other dicotyledonous trees 

 as well. It occasionally happens that an oak is 

 attacked by both of these Polyporei, and their mycelia 



FIG. 20. Vertical section through the wall of one of the pores of P. sulphnreus, 

 showing the ordinary hyphae (e\ tissue of the fructification (a. and b), and the 

 spore-bearing ends (d and above). (After Hartig.) 



become intermingled in the timber : when this is the 

 case the starch-grains remain intact in those cells which 

 are invaded simultaneously by the hyphce of both fungi. 

 I have been shown longitudinal radial sections of 



