GENERAL PRACTICE. DISEASES. 231 



at the latter part of summer, a wet season, extreme variations of temperature, and 

 sudden checks occasioned by atmospheric vicissitudes. 



Canker is produced more from the action of external than internal causes, hence is 

 much more easy to reach and cure than diseases which originate internally. Indeed, 

 canker is of traumatic (wound) origin. When a-trec receives a bruise or injury in a 

 shoot, branch, or stem, sufficient to rupture the bark, that part is destroyed, becomes 

 brown, shrunk, cracked, and exposes the wood. Nature always attempts to cover it with 

 new bark. This is effected by the tree throwing out a ridge of callus around the 

 circumference of the wound, which, growing outwards, ultimately heals over. The time 

 occupied depends on the tree's condition. If healthy aud vigorous, its efforts at covering 

 the wood exposed with fresh bark are expeditious ; if enfeebled or unhealthy, the wound 

 is healed over slowly, and the wood thus exposed naturally decays, forming a receptacle 

 for water and other accumulations, to the prejudice of the wound ever healing. 



The causes that give rise to wounds in trees are 1, natural : hailstones, breaking of 

 branches and erosion of bark by wind, sun and frost cracks, punctures of insects and 

 burrowing of Iarva3, pecking of birds, bites and scratches of animals ; 2, cultural : 

 all manipulations, bruises, and abrasions. A tree, however, may have many (it always 

 has some) wounds, yet those innocent of canker. These, in healing over, are not remark- 

 able for abnormal enlargement. American blight, however, sometimes becomes located 

 in the cavities ; then the bark around the wound is considerably distorted, and it has 

 some resemblance to the hypertrophy always associated with canker. Nevertheless, 

 wounds in process of healing over by Nature without, and those with canker, are very 

 different, as will be readily comprehended by an examination of the comparative portrayal 

 of the two forms in the illustration. 



"When the bark of an apple or pear tree is injured by any of the causes mentioned, 

 the spores of the canker fungus gain admission into the wound, and set up disease of 



stem ; p, branch split at stem ; q, branch with bark off : r, pruning wounds ; s, severe bruise in stem ; t, wound in 

 small branch ; u, crack in stem ; v, blow from hailstone. T, wounds in part of stem, branches, and shoots showing 

 cankerous affections : w, arm broken ; x, bark erosed by broken arm ; y, shoot killed by frost ; z, shoot damaged 

 by frost ; a, limb eawn off ; b, wound with American blight ; c, nailed boot injuries ; d, limb cut off close to stem ; 

 e, branch split at stem ; /, branch shortened in severe frost ; g, pruning wounds ; h, size of wound occasioned by 

 bruise ; i, wood exposed by canker ; j, tree's efforts at covering over wood with new bark ; k, small branch killed ; 

 I, hailstone wound ; m, crack in stem being enlarged. U, enlarged shoots, showing extent of a season's growth of 

 canker : n, discoloured portion of bark ; o, point of entrance of canker. F, enlarged shoot, showing canker affection 

 in autumn or early spring : p, bark raised, fissured, and granulated by canker " fruits " about to shed spores. \V, 

 enlarged shoot, showing : q, bark thrown off cankered portion of previoiis year ; r, dent in wood caused by original 

 blow. X, spur twig of enfeebled and aged tree, showing : s, ring of bark thrown off the previous season's canker- 

 affected part. 



