96 BRITISH GALLS 



explained by the theory of immunity. Mites lack wings, and , 

 being feeble creatures, probably depend entirely on chance 

 aid in dispersal. It is known that the Currant-bud mite 

 assumes an erect position and waits an opportunity for 

 "getting a lift." Does Eriophyes vihurni adopt the same 

 tactics ? Its present distribution in the wood alluded to 

 should be investigated by local cecidologists. The mite 

 Pkyllocoptes oblongus Nalepa is a commensal in this gall. 



The bundles of small twiggy outgrowths not infrequently 

 seen on the branches of various trees are known as 

 " witches' brooms." They are caused in most cases by 

 mites, but some are induced by parasitic fungi. They are 

 particularly numerous on the Common Birch, and are 

 most apparent in the leafless season, when they resemble 

 birds'-nests. Plate XX. shows a Birch bearing numerous 

 brooms caused by Eriophyes rudis. The majority of, if 

 not all, the brooms on Birches in the south of England 

 are caused by E. rudis, but it would appear that the 

 fungus Exoascus turgidus is responsible in Scotland for these 

 curious outgrowths. These galls exhibit the same dis- 

 continuous distribution as those of E. vibumi ; perhaps but 

 one tree in a coppice bears them, whilst hundreds near by are 

 totally exempt. The beginning of a " witch's broom " is a 

 swollen bud. The axis of the shoot is seen to be pubescent 

 in March, and yields mites in abundance late in the month 

 if the weather is mild. The buds on afflicted shoots are 

 shorter, more globose, and open earlier than normal ones. 

 The attraction of sap to the spot causes the development of 

 an enormous number of twigs, which grow from a core that 

 increases slowly year by year. Some of the very large 

 brooms occasionally seen on Birches must be of many years' 

 duration. Some that I have had under observation for the 

 past fourteen years at Haslemere are depicted in Plate XX., 

 and are not very large. Mites are probably also responsible 

 for the development of the more or less rounded bosses, 

 usually from 4 to 6 inches in diameter, covered with buds, 

 that frequently occur on the stems of these trees. They are 



