EXOASCUS 



141 



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.' 



At one time these brooms were very well developed on a 



FIG. 34. Witches' brooms of birch, produced 

 by Exoascus tiirgidus. Reduced. 



birch-tree in Kew Gardens. One of the brooms measured 

 two yards in diameter, and when removed was found to con- 

 tain seven nests, belonging respectively to blackbirds and 

 thrushes. On this broom some of the more elongated 

 branches bore clusters of buds forming miniature brooms, 

 caused by the mite Eriophyes rudis. 



Unless considered ornamental, the brooms should be 



