THE PARASITIC EXOASCEAE. 



161 



Exoascus alpinus Johan. and Ex. nanus Johan. Both occur 

 on Betula nana, and induce formation of hypertrophied twigs. 



Fig. 56. — Witches' Broom of the Cherry. Exoascus cerasi oa Prunus Cerasus. The 

 whole left side forms a large broom. A smaller example occupies the summit of 

 the crown, while another hangs downwards to the right. In winter condition. 

 (t. Tubeuf phot.) 



The mycelium of Ux. names hibernates in twigs, and penetrates 



L 



