AECIDIUM-FOKMS. 



405 



prey to wood-destroying fungi.' The presence of such rotting 

 spots renders the tree liable to break over in tlieir neighbour- 

 hood, while they, as well as the swellings on the trunks, cause 

 a considerable depreciation in the value of the timber. 



The malformations of the branch-system known as witclies' 

 brooms are frequently induced by this fungus. They occur as 

 a rule on tlie liorizontal branches and form a richly branched 

 busli easily distinguished, even at a distance, l)y a marked 



Fui. 2-il.— Witches' Broom oj Silvei- Fir (winter condition). The needles, with 

 spores of Atcidium, datinv.m, have fallen off, but the normal foliage remains. 

 (V. Tubeuf phot.) 



negative geotropy of its twigs. The brooms not unfrequently 

 start from a marked basal swelling. They may be found of all 

 sizes, on young as well as old trees, on any part of tlie branch- 

 system, and in all localities where the fir occurs (Figs. 241 

 and 242). 



The aecidia of Aaidium datinum are developed onl}' on the 

 deformed needles of the witches' brooms. These needles are 

 produced anew each spring, live only one season and are cast 



' Polyporwf Harthjii and A<jaricm adiposuf in paiticular acconipany tliis 

 canker and bring about decay of the wood. 



