8 
BRITISH GALLS 
on the upper surface of Beech leaves, caused by the dipteron 
Oligotrophus annulipes. Leaves may often be picked up in 
late autumn which are quite brown except for a green zone 
around the gall. The presence of the living parasite pre¬ 
vents the over-oxidation of the adjacent cells and keeps 
them green. 
Galls exhibit great diversity in form and general 
structure. The following arrangement of the chief types 
occurring on Phanerogams and Vascular Cryptogams is 
based on that given in Kerner and Oliver’s “ Natural 
History of Plants.” 
INSECT GALLS 
I. SIMPLE GALLS 
Abode of an animal or colony of animals on a single 
plant organ. 
i. Felt Galls 
Hypertrophied epidermal cells growing out into variously- 
shaped hairy coverings. The felt galls are chiefly caused 
by gall-mites. Very familiar examples are the variously- 
coloured little patches of felt on the under surface of Alder 
leaves. 
2. Mantle Galls 
Hypertrophy forming a chamber around the insect (in 
many cases) and its brood, serving as a protective mantle. 
The chamber is always lined with part of the original 
surface of the affected part. These galls may be arranged 
under three headings: 
(a) Scroll Galls 
Rolling and thickening of the leaf (rarely the petiole), 
usually on one side only. The rolling is always towards 
the surface occupied by the animals, thus forming a 
chamber for their protection. See Plate XVIII., which 
shows the margins of the pinnules of the Bracken rolled 
