AECIDIUM-FORMS. 



405 



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

 spots renders the tree liable to break over in their 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 witches' 

 brooms are frequently induced by this fungus. They occur as 

 a rule on the horizontal branches and form a richly branched 

 bush easily distinguished, even at a distance, by a marked 



Fjg. 241. — Witches' Broom oj Silvc}' Fir (winter condition). The needles, with 

 spores of Accidium etatinum^ 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 the branch- 

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

 and 242). 



The aecidia of Aecidium elatimcm are developed only on the 

 deformed needles of the witches' brooms. These needles are 

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



^ Polyporus Hartigii and Agaricun adiposus in particular accompany this 

 canker and bring about decay of the wood. 



