94 



HEART-ROT 



always small and are more or less circular in cross-section, 

 though they extend to greater length in the longitudinal 

 direction. The cross-section has a diameter of 0'1-0*4 mm., 

 and covers one to a dozen or more tracheides. The length 

 is fairly regular, being 0*5-1 mm. 



These black specks may occur in the spring or summer 

 wood, and do not bear any definite relation to each other, 



except that they some- 

 times appear in longi- 

 tudinal series, with a dis- 

 tance of 1 mm. or so 

 between them. Under 

 the high power of the 

 microscope they are seen 

 to be composed of dense 

 masses of hyphae, all of 

 the broader kind, whose 

 walls have become more 

 than usually thick, and 

 dyed with a deep brown 

 or black colour similar to 

 Brunswick black. The 

 hyphae at these points 

 branch very freely, and 

 on the outskirts of the 

 patch they are seen with 

 rounded ends and have 



FIG. 33. Black specks in the wood of 

 the larch caused by Fomes annosus. 

 Transverse section ( x 85). 



a tendency to grow in 

 fascicles. Sometimes, however, the brown hyphae may be 

 seen continuing as colourless hyphae, and in such cases 

 the coloration ceases somewhat abruptly at the limit of 

 the black patch. These hyphae are presumably the original 

 hyphae which were present before the black patch was 

 formed, and which gave rise by branching to the rest of 

 the hyphae forming that patch ; but, in addition, hyaline 

 branches are also sent to the neighbouring tracheides, and 

 these may give rise to new black patches after the parent 

 ones have disappeared. 



