140 DISEASES OF CULTIVATED PLANTS 



between the cells of the leaf, and in the bark, medullary rays, 

 and pith of the shoots. 



The brooms should be removed both from cultivated and 

 wild trees. 



Witches' brooms of birch, produced by Exoascus turgidus 

 (Sadeb.), are very abundant in this country on Betula verrucosa, 



FIG. 33. Exoascus pruni. i, portion of a branch bearing 

 diseased plums ; 2, a diseased plum cut in two. Reduced. 



and closely resemble crows' nests in appearance, although 

 sometimes much larger in size. The formation of these 

 brooms takes place as follows, according to Smith : 



1 1 find that a broom results from a prolific development of 

 small twigs on one or a few knotty, swollen parts of a branch. 

 Each central 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- 



