INJURIES INDUCED BY PLANTS 



133 



at its base, which, under the influence of the parasite, enlarges or 

 increases abnormally. On the other hand, the mycelium arrives 

 too late at the apex of the shoot to be able to exert a similar 

 influence there. 



Exoascus Insititice produces witches' brooms on Prunus in- 

 sititia. 



Exoascus bullatus induces bladder-like swellings, which after- 

 wards become mealy underneath, on the leaves of pear-trees. 

 In the case of the hawthorn it produces formations like witches' 

 brooms, which bear leaves of a reddish colour. 



Exoascus alnitorquus (Ascomyces Tosquinetit) often appears in 

 great abundance both on the 

 leaves of Alnus glutinosa and 

 on the scales of the female 

 catkins of that tree and of 

 Alnus incana. Not only does 

 it cause the leaves to become 

 crumpled and corrugated, but 

 also to increase in size in 

 every way. On the cones of 

 the alders it produces pocket- 

 like outgrowths, which, when 

 fresh, are of a brilliant red 

 colour, and remind one some- 

 what of the pockets of plums 

 (Fig. 72). 



Exoascus flavus (Sadebeckii) 



causes the formation of yellow blotches, in this case also on the 

 leaves of A. glutinosa and A, incana. 



Exoascus epiphyllus, which infests the leaves of Alnus incana 

 and A. glutinosa, is only with difficulty to be distinguished from 

 the former species by its broader stalk-cells. It induces sinuous 

 crumplings of the leaves, the outgrowths usually appearing on 

 the upper side. 



Exoascus borealis produces witches' brooms on Alnus incana, 

 These are very numerous near Munich and at other places in 

 Bavaria. It is probably identical with E. epiphyllus. 



Exoascus turgidus (Taphrina betulina} very often produces 

 witches' brooms on the birch. 



FIG. 72. Malformation of the fruit of 

 Almis incana induced by Exoasciis. 



