I 82 



DISEASES OF TREES 



organisms, somewhat after the manner of the mistletoe. The 

 mycelium also spreads slowly backwards in the cortical and 

 bast tissues, so that a swelling or canker-spot, such as I have 



already described, is formed on 

 the stem or branch to which the 

 witches' broom is attached (Fig. 

 112). This swelling increases 

 independently, even after the 

 witches' broom, as such, is dead, 

 an event which is sometimes de- 

 layed for twenty years or more. 

 In young woods all trees that 

 show cancerous swellings on their 

 boles should be removed in the 

 thinnings, even in cases where 

 they belong to the larger class 

 of trees. 



FIG. 112. The appearance of a witches' 

 broom, seven years old, in winter, 

 when, being deciduous, of course it is 

 leafless. Above the point from which 

 the witches' broom springs the fir 

 branch has almost ceased to exist. 



FIG. 113. yEcidia FIG. 114. Theouter 

 of A. strobilinum side of the scale 

 on the upper side of a spruce-cone, 

 of a scale of a showing two pale 

 spruce-cone. patches which had 



previously been oc- 

 cupied by the aeci- 

 dia of A. conorum 

 Picea. 



STROBILINUM 1 



This secidium form develops its mycelium in the green living 

 carpellary scales of the spruce. It destroys the organs of the 

 flower, and produces dark hemispherical brown aecidia, which are 

 densely crowded, for the most part on the inner side of the 

 scales, though to some extent also on the outer side. These 

 aecidia usually rupture transversely (Fig. 113). When such 

 cones fall to the ground they are easily recognized by the fact 

 1 Reess, Die Rostpilzformen der deutschen Coniferen. 



