10 BRITISH GALLS 



II. COMPOUND OR BUD GALLS 



Several adjacent plant organs are involved in the 

 production of these galls, which chiefly arise from buds. 

 " They are extraordinarily varied in their characters, some 

 being merely abbreviated axes clothed with scale-like leaves ; 

 in others only the base of the shoot is involved, and above 

 the gall it continues its growth quite normally; whilst in 

 others, again, the axial portion of the structure is much 

 swollen, and the leaves hardly represented at all " (Kerner). 

 It is difficult to arrange them in groups, but three fairly 

 well marked may be distinguished. 



I. Bud-like Galls 



Several or all the parts of a shoot are involved, its axis 

 is deformed and thickened, and elongation is suppressed. 



(a) Modified Foliage Buds 



i. Apparently leafless, the leaves transformed into tuber- 

 cles and knobs. This section includes the various bud galls 

 of the Oak — e.g., the Oak-apple caused hy Biorrhiza pallida 

 (Plate IV.) and swellings on Poplar branches caused by the 

 beetle Saperda populma (Plate VI.). 



ii. Galls covered with scale-like bracts, or more or less 

 fully developed green foliage leaves. 



A familiar representative of this section is the Articholje 

 gall caused by the hymenopteron Andricus fecundator in Oak 

 buds, figured in Chapter II. 



(b) Metamorphosed Flower Buds 



In these the corolla does not open ; the calyx becomes 

 enlarged and often fleshy, the gall resembling a bud or 

 bulbil. The gall caused by the dipteron Contarinia loti in the 

 flower buds of the Bird's-foot Trefoil is a typical example. 

 See Fig. i, p. 15. 



