DEFBCTS ASD TKS JCSISES?. 41 



of all 50 far as the number of affected trees is concerned, s'nce 

 from a single attacked tree as focus it spreads rapidly through the 

 soil from tree to tree in an ever-widening circle, and biirro\nng 

 animals of all kinds carrv a^ar the infection (spores) to long dis- 

 tances in their for. Somerimes. especiaJj when the rot has spread 

 from a dead or broken bono^h above, it extends outwards at the base 

 of the tree as far as the bark itself, where it br^iks out in the form 

 of a sore (gcaiffrene), from which a dark foul liquid, containing the 

 acids above alluded to, ojzes ouL Internal rot due to saprophytes 

 alone is aiwavs local. Its origin is always an occluded wound that 

 has remained open long enough for the fungus to attack the dry, 

 and therefore dead, wood at the surface. After occlusion, owing to 

 the air being shut otit. the rot spreads only slowly. Bot of this 

 kind nearly always results in the formation of a concealed hollow 

 or pocket (&ee Ftp. 7 above). 



External rot usually takes tlie form of canker. Canker is the 



Fig. 10. 



Canker. (After SartifJ. 



consequence of imperfect healing of small wounds, the exposed 

 cortex and cambium being attacked by some parasitic or semi- 

 parasitic fungus, as they try to form over the wound. The local 

 disturbances in growth kept up by the mycelium nearly always 

 five rise to malformations and excrescences, from which resinous 

 and other fluids often flow. Hence, by analogy, the name by 

 which these sores are known. There is of course no limit to 



