REPORT OF THE SOCIETY 37 



Immunity. — At the Arboretum we have about 25 different varieties of alder, 

 the majority of which are subjected to various degrees of attack. There are a 

 few, however, which I have found during the three seasons work to be immune. 

 They are: — 



Alnus serrulata Alniis oblongata 



Alnus incana Alnus macrophylla 



Alnus carpinua hetula 



This immunity, in all probability, may be due to the thick and hard texture 

 of the leaves of these varieties, as the flies are never found on them, although 

 these trees are closely situated to those that are very severely attacked. 



