8 BRITISH GALLS 



on the upper surface of Beech leaves, caused by the dipteron 

 Oligotvophus 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 oa 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, 



1. 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 XVIIL, which 

 shows the margins of the pinnules of the Bracken rolled 



