1 3 2 DISEASES OF TREES 



fruit. Numerous mycelial branches penetrate between the 

 epidermis and cuticle, where, by the formation of transverse 

 branches, they form short chambers. In this way an almost 

 uninterrupted layer of fungal mycelium is formed under the 

 cuticle. Each fungus-cell next grows outwards to produce a 

 short cylindrical ascus, and the cuticle after being detached 

 from the epidermis is ruptured, and the ascogenous layer 

 becomes completely exposed. 



Each ascus becomes separated from the basal part, or " stalk," 

 by a transverse septum ; and, by free cell-formation, six to eight 

 roundish spores are formed in its interior, to be afterwards 

 ejected through the ruptured apex. The spores either germi- 

 nate forthwith, or multiply by budding, and form a kind of yeast. 



The pocket-plums decay owing to the concurrence of numerous 

 saprophytic fungus-forms. 



Exoascus deformans is closely related to the foregoing species, 

 but lives partly in the leaves and shoots of Persica vulgaris and 

 Amygdalus communis, and partly in the leaves and shoots of 

 Prunus avium, P. Cerasus, P. Chamcecerasus , and P. domestica. 

 On these trees, according to the investigations of Rathay, 1 it 

 causes the so-called witches' brooms. Whether the Exoascus 

 that occurs on cherries is really a new species {Exoascus Wies- 

 nert), as Rathay assumes, or whether the distinctions that have 

 been noted are not perhaps due to differences in the host-plants, 

 must remain doubtful until infection-experiments have been 

 carried out. Peculiar crumpling is induced in the leaves, similar 

 to that which is sometimes caused by Aphides. The branches 

 that have been taken possession of by the fungus anastomose 

 freely, and usually exhibit decided negative geotropism, while 

 the basal portion is often hypertrophied. These constitute 

 the thunder-brooms and witches' brooms. Towards the base, 

 the branches of these witches' brooms are often double the 

 thickness of the branches from which they spring. Towards 

 the apex, on the other hand, they become normal. A possible 

 explanation of these phenomena is that, as the mycelium grows 

 more slowly than the young shoot, it finds immature tissue only 



1 Rathay, Ueber die Hexenbesen der Kirschbaume und iiber Exoascus 

 Wiesneri, Rath., im Sitzber. d. Wien. Akad. d. Wissensch., vol. Ixxxiii., March 

 1881. 



