92 DISEASES OF TREES 



embraced under the term"" Canker" being due to N. ditissima. 

 This canker- fungus appears most frequently upon the beech, oak, 

 hazel, ash, hornbeam, alder, maple, lime, apple, dogwood, and bird- 

 cherry. Although as a rule this parasite only gains an entrance 

 to the cortical tissues of trees through wounds, I have also been 

 able to infect young leaves by means of gonidia and ascospores. 

 Abrasions caused by hailstones are probably the commonest kind 

 of wounds, Fig. 39. Should no infection supervene on such a 

 wound, a callus forms, and occludes the injured part in a short time, 

 Fig. 39 a. If, however, it is infected by the gonidia or ascopores 

 of Nectria, death and brownness spread in all directions, but most 

 rapidly in the direction of the long axis of the stem. Although 

 in rare instances the mycelium may advance 3 cm. in a year, it 

 is comparatively seldom that the annual rate of progress in any 

 direction exceeds one third of that amount. The apparent 

 deepening of the diseased spot in the course of time is to be ex- 

 plained from the fact that not only does the contiguous healthy 

 tissue continue to increase in thickness, but it even displays an 

 augmented rate of growth. This is satisfactorily enough ex- 

 plained when we remember that during their movements in the 

 bast the plastic materials assimilated by the leaves are neces- 

 sarily confined to the sound side of the stem. As the canker- 

 spot dries up, their passage is chiefly confined to its margin, which 

 is consequently very richly nourished, and projects as a well- 

 marked prominence. Thus in the course of years very striking 

 malformations are formed. 



It also frequently happens that the base of a lateral branch, 

 whose cortex has been injured in the upper angle, proves the in- 

 fection-spot from which death of the tissues annually proceeds, 

 Fig. 40. In the case of the hazel especially it often happens that 

 in pulling down the branches to get at the nuts a split is formed 

 at the point of bifurcation. This then develops into a canker- 

 spot, which constantly increases in size, as is represented in 



Fig. 41- 



I believe that I am justified in assuming that under certain 

 circumstances, with which I am not yet familiar, the mycelium 

 spreads from the cortex to the wood, in which it progresses up- 

 wards, and at certain places attacks the tissues of the cortex and 

 cambium from within. In this way canker-spots may be pro- 



