160 



ASCOMYCETES. 



' f Witches' brooms on birch are very common in Scotland. 

 They appear as tangled masses of twigs, which at first sight 

 give the impression of some bird's nest. I have frequently 

 examined the leaves borne on these brooms, and have never 

 failed to find the asci of an Exoascus. Sadebeck gives in 

 his monograph the two above-named species as found on birches 

 bearing witches' brooms. Mites (e.g. Phytoptus) have also been 

 given as the cause of these malformations. On close examination 

 of brooms which undoubtedly bore EMXISCUS, I found that a 

 broom results from a prolific development of small twigs on 

 one or a few knotty swollen parts of a branch. Each central 



FIG. 55. Witches' Broom of the Hornbeam. Exoascus car^ini on Carpinus Betulus. 

 The bush measures about 1 metre across, and arises laterally from a branch, the 

 upper normal part of which has been removed, (v. Tubeuf phot.) 



knot we may regard as the position of the bud which was 

 first infected, and from which the broom system took its 

 origin. As one result of the attack of the fungus, the greater 

 number of the buds in the axils of the scales of the infected 

 bud have grown out as twigs, but not into well-developed 

 ones. In consequence, nearly every twig has been killed back 

 by the winter, but not completely, so that from each twig- 

 base has sprung a new crop of stunted immature twigs like 

 the first, and equally liable to be killed in the following winter. 

 Thus has arisen that tangled mass of dead or sickly birch 

 twigs which we call a witches' broom. [Edit.] 



