542 ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



When several organs of a plant immediately adjacent to one another are con- 

 cerned in the production of a gall it is said to be compound. Compound galls are 

 for the most part produced from buds, and they are all comprehended under the 

 general name of Bud-galls. 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 

 normallj', whilst in others again the axial portion of the structure is much swollen, 

 and the leaves hardly represented at all. It is difficult to give any satisfactory 

 classification of these bud-galls; still, for the sake of arranging our facts, we may 

 distinguish these types, viz.: — the ordinary hud-gall, the cuckoo-gall, and cluster- 

 gall. Ordinary bud-galls involve several, often all, the members of a shoot. The 

 axis of the shoot is alwaj's deformed and abnormally thickened. The swollen portion 

 contains in its interior one or several larval chambers surrounded by a pith-like 

 layer. Two varieties of ordinary bud-gall may be distinguished. The first is leafless; 

 no leaves are present, or, more correctly, they are transfoi-med into tubercles, pegs, 

 and knobs which merge insensibly into the swollen axis which contains the larval 

 chamber. The second possesses leaves, the gall being covered with scale-like bracts 

 or more or less fully developed green foliage-leaves. Amongst the leafless bud-galls 

 the most interesting are those which are armed with special means of protection 

 against the attacks of animals on the watch for the larvfe of the gall-wasps. The 

 gall shown in figs. 36-4 ^ and 36-1 ^, produced by Cynips p)olycera on the leaf-buds of 

 Quercus pubescens and sessiliflora, which to a certain extent aSects a whole lateral 

 shoot, has the form of a young Medlar fruit, and on it may be seen 3-5 metamor- 

 phosed leaf-structures projecting as stift'-pointed pegs which gradually pass into the 

 tissue of the shoot axis. This gall is one-chambered, and the tissue of the wall has 

 separated into an outer layer and an inner spherical pithy gall. The gall shown in fig. 

 364 - is produced bj* the gall-wasp Cynips Hartigii which lays an egg in the middle 

 of the leaf -bud of the Oak {Quercus sessiliflora). The bud does not develop into a 

 leafy shoot, but into a small one-chambered gall with large tooth-like or club-like 

 processes which represent metamorphosed leaves. The thickened angular ends of 

 these projections fit closely to one another so as to form a sort of second outer coat 

 to the gall-chamber through which hostile ichneumon-flies cannot penetrate. The 

 gall much resembles the cone-fruit of a Cypress in the arrangement and form of its 

 superficial processes. The galls produced from the buds of various Oaks {Quercus 

 pendxdina, sessiliflora, fuhescens) by the gall-wasp Cynips lucida are still more 

 peculiar (see figs. 364^ and 364^). They contain several larval chambers with 

 abundant pithy tissue, whilst innumerable slender processes resembling limed twigs 

 in being very sticky on the capitate thickened end project from their exterior. 

 Ichneumon-flies and other animals hostile to the gall-producers take good care not 

 to come into contact with these spikes which are to be regarded probably as trans- 

 formed leaves springing from the swollen axis. Among the galls produced from 

 leaf-buds belonging to this group there are some in which the leaves are merely 

 indicated as tubercles. This is the case, for example, in the many-chambered, 



