THE PARASITIC EXOASCEAE. 



159 



duciiig the brooms by artificial infection of alder. The disease 

 is common and epidemic, and a single tree may carry as many 

 as a hundred brooms. 



The witches' brooms are 

 composed of many thickened 

 twigs, beset with an abnor- 

 mal number of lenticels, 

 and the point of infection 

 shows a distinct swelling, 

 from which the broom tends 

 to turn directly upwards. 

 The leaves are somewhat 

 modified, they are larger and 

 thicker than the normal, 

 they unfold later and wither 

 earlier, while their stipules 

 remain attached for some 

 time. The brooms of alder 

 only survive a few years, 

 and by their decay cause 

 the death of large branches, 

 and frequently of the whole 

 tree. 



The asci, which are sunk 

 in a depression of tlieir 

 stalk-cell, form a white 

 coating on both surfaces of 

 the leaves. The mycelium 

 hil)ernates in the l:)uds. 



Exoascus turgidus Sad. 

 causes- the formation of 

 witches' brooms on Bdvla 

 verrucosa. The leaves form- 

 ed on the brooms are some- 

 what crumpled, and the asci 

 are produced on their lower 

 surface. 



Exoascus betulinus 

 Eostr. produces witches' 

 brooms on Bdula puhescais and B. odorata. 



epiphytlits. 



broom 



in first year, shovving swelling at the point of infec- 

 tion. The leaves are already shed in autumn, while 

 the normal still remain 4 natural .size. After 

 V. Tubeiif.) 



