i So 



DISEASES OF TREES 



at a part of a stem or branch where there are no buds capable of 

 developing, the stimulated growth of the cambium induces the 

 formation of a knob-like swelling at that place, which is due 

 both to the increased formation of wood and to the more vigorous 

 development of the cortex (Fig. 109). With the spread of the 

 mycelium the swellings or canker-spots increase in size, and if 

 present on the stem of 

 a vigorous tree they may 

 attain to large dimen- 

 sions. At such places 

 the tissues of the cortex 

 and bast soon become 

 fissured (Fig. no), and 

 dry up here and there 

 as far in as the wood, so 

 that in course of time a 



FlG. 109. Swelling on the silver 

 fir, but without the formation 

 of a witches' broom. Natural 



FIG. no. Longitudinal section of a silver fir 

 showing a swelling, one third natural size, which 

 had originated by infection thirty-one years 

 previously, when the stem was four years old. 

 On the right side the cortex, which has been 

 dead for three years, has withered and dropped 

 off. At the infected part the growth of the 

 cortex and wood has been stimulated. 



door is opened for the entrance of wood-parasites. One of the 

 commonest of these is Polyporus Hartigii, which produces a kind 

 of white rot. A species of Agaricus also namely, A. adiposus 

 frequently appears as a wood-destroying parasite. In consequence 

 of the decomposition of the wood, storms and snow often break 

 the trees. One not unfrequently finds swellings which have 



