9 6 
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 viburni adopt the same 
tactics? Its present distribution in the wood alluded to 
should be investigated by local cecidologists. The mite 
Phyllocoptes 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. viburni; 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 
