SILVER-FIR CANKER. 440 



through the cortex of an infected branch into the stem, but a 

 stem-canker is produced when the stem grows over the 

 infected base of a branch. 



The canker may be distinguished externally by a swelling 

 either on one side of, or all round the stem, on which the bark is 

 deeply cracked and dark brown, showing here and there a little 

 resin ; it crumbles away in parts, exposing the wood. It may 

 be found at any height on young or old trees or their branches, 

 and may attain a large size. The mycelium which grows in 



Fig. 218. Shoot of Silver-fir attacked by M. caryophyllacearum, Schroter. 



a Cankerous swelling, b Needles of the witches-broom. (Natural size.} After 



Hartig, from Proc. of Royal Soc., Vol. 47. 



the cortical parenchyma is the same as that which produces a 

 witches-broom, but the latter is formed only when the 

 mycelium reaches a living bud. If, however, the shoots are 

 old and have no living buds, no abnormal shoot-production 

 takes place, and the canker alone is formed. The infection 

 appears to spring from a wound in the shoots affected. 



The golden or orange-coloured sporocarps (Spermagonia and 

 Stylospores) are formed on the under surface of the diseased 

 leaves. They appear in two rows, open and emit their spores 

 in June, the needles subsequently dying. The witches-broom 

 continues growing for about 16 years, chiefly upwards, and 

 branching freely, resembles a mistletoe plant on the usually 

 horizontal branches of a silver-fir. It at length dies, and only 



F.P. G G 



