SAC FUNGI IN PARTICULAR - 1 33 



result of the parasitism of species of Exoascus. The "hexenbesen" 

 are brush-Uke, or tufted masses of branches, which suggest the presence 

 of other plants (Uke the mistletoe) parasitically or epiphytically 

 growing. They result mainly by the infection of a bud which de- 

 velops a branch with increased growth. On this branch, all dormant 

 buds are stimulated to activity and the whole infected system of 

 branches consists of negatively geotropic branches. These brush- 

 like excrescences are called the thunder-bushes, and are -sometimes 

 nest-like in appearance. An anatomic study shows that the parenchy- 

 matous tissues — pith, hypodermis, etc. — are greatly increased; wood 

 and bark are traversed by abnormally broad medullary rays, the ducts 

 have short members, the wood fibers wide lumina, which are sometimes 

 thin-walled and septate. The bast fibers are few, or entirely wanting. 

 The cork cells are enlarged and retain their protoplasmic contents a 

 longer time. The form of the witches' brooms are various. Many 

 of them are pendent, some are nest-like, owing to the death of some of 

 the branches. In some the branches are elongated, while some have 

 short twigs. The end of the original branch from which the lateral 

 branches developed usually dies and its food substances are absorbed 

 by the hypertrophied branches. The family includes three genera, 

 distinguished, as follows: 



A. Asci found at the end of intercellular mycelial branches. 



I. Magnusiella. 



B. Asci developed on a more or less subcuticular ascogenous mycel- 

 ium. 



(o) Asci eight- (exceptionally four-) spored. 2. Exoascus. 



ib) Asci many-spored by gemmation of the spores. 3. Taphrina. 



The genus Magnusiella comprises five species, four of which are found 

 in Europe and two in America. Magnusiella flava forms small pale 

 yellow specks on the leaves of the gray birch, Betula populifolia in 

 North America. The genus Exoascus includes about thirty species 

 arranged in two subgenera, the first of which includes those species which 

 deform fruits, which form witches' brooms, and the second those which 

 cause a spotting of the leaves of various plants. It would lengthen 

 this book unduly to enumerate all of the species of Exoascus with an 

 account of the deformities of branches and fruits which they produce. 

 Only a few of the more important species will be enumerated here, 



