CHAPTER XXIV. 



EXCRESCENCES {contifiued). 



Cankers Burrs Sphaeroblasts, and other excrescences of 

 woody tissues Witches' Brooms. 



Cankers are irregular excrescences due to the 

 perennial struggle between tissues attempting to 

 heal up a wound, and some organism or other 

 agent which keeps the lesion open. A canker 

 always originates in a wound affecting the cam- 

 bium, and usually in a small wound such as an 

 insect puncture or frost nip ; if undisturbed 

 the dead parts would heal over by cork and 

 callus, but if recurring frost-cracks break open 

 the coverings, or if insects or fungi penetrate the 

 callus and invade the cambium, irregularities of 

 growth due to the occluding tissue on the one 

 hand, and continued growth of the still unim- 

 paired cambium on the opposite side of the 

 injured shoot on the other, result in the canker. 

 Frost cankers occur on fruit-trees, Vines, Beeches, 

 etc. 



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