NECROTIC DISEASES. 241 



cambium and cortex around them then begin to 

 push in callus towards the centre of the necrotic 

 area ; but since this callus is formed under the 

 pressure of the cortical tissues it does not form a 

 thick lip or margin to the healing wound, as it 

 does in a Canker, but insinuates itself with thinned- 

 off edges between the wood and the dead tissue, or 

 at most traps a little of the latter in the final closing 

 up of the wound. It is easy to see how such an 

 area of Necrosis may become a Canker if the dead 

 tissues split or slough off, atid fungi or insects ob- 

 tain access to the callus at the margins of the area, 

 setting up the disturbances described on p. 222. 

 As matter of fact many Cankers e.g. those of 

 the Larch disease, and those due to Nectria, or 

 Aphides, etc. often begin as flattened or de- 

 pressed areas of Necrosis started by frost, and 

 many small necrotic patches would eventually 

 become Cankers if not healed up by the callus. 



Necrosis may also be due to the bruising of the 

 tissues by large hailstones, to gun-shot wounds, or 

 to any form of contusion which kills the living 

 cells of cortex and cambium. 



Necrosis is a natural and common result of fire, 

 and it frequently happens after forest-fires which 

 have run rapidly through the dry underwood, 

 fanned by steady winds, that the lower parts of the 

 boles are scorched on one side only. The killed 

 cambium and cortex then dry up in black necrotic 

 patches, which may eventually heal up by intrusion 

 of callus from the uninjured parts. 



Sun-burn or Scorching. If thin-barked trees, 



