9 o 



DISEASES OF TREES 



greater during the season of growth than at other times. As 

 may be seen from Fig. 37, the parasite may advance longitu- 

 dinally more than 2\ inches 

 during a growing season. Later- 

 ally the seat of the disease 

 seldom advances more than 

 I ij inch. The tissues that 

 have been killed by the fungus 

 become separated from the 



...d 



FIG. 38. Cross section of the 

 cortex and wood of a spruce 

 infected a short time previous- 

 ly ; a, the wood, b b, the sieve- 

 tubes containing one or more 

 mycelial filaments ; c, my- 

 celium in the intercellular 

 spaces ; magnified 420 times. 



FlG. 37. A spruce attacked by N. 

 Cucurbitula, At a a wound due to 

 a hailstone has healed over without 

 becoming infected; b, the gallery 

 of a larva of Grapholitha pactoland, 

 over which a callus has been formed, 

 but where infection has occurred 

 two years later ; the mycelium has 

 spread from c to c in the cambium, 

 and from d to d in the cortex ; 

 numerous groups of perithecia have 

 appeared on the dead cortex. 



living parts by the formation of a layer of cork which prevents 

 the further progress of the parasite in the following year. 



If the cortex that has been killed be exposed to wind and sun, 



