BUD-GALLS. 545 



{Asperula galioides, tinctoria, &c.) as on Fir-trees. The infected parts of the shoot 

 remain stunted, and white spongy cushion-shaped growths, which are somewhat 

 grooved, arise at the bases of the leaves. Since the growing tissues of neighbouring 

 leaves touch one another the grooves or channels form small cavities in which live 

 the larvaa of the gall-producing gnats (Cecidomyia Galii and Asperulce). In the 

 common Bedstraw (Galium Mollugo) these spongy growths arise, not from the 

 bases of the leaves, but from the green cortex of the stem round the insertion of the 

 leaves and lateral branches. They rise up as cushions and lobes, and several join 

 together to form a sort of dome, under which the larvae of the gall-gnat dwell. The 

 foliage-leaves are scarcely altered in form, and when lateral twigs arise from the 

 place they also are unchanged. It not infrequently happens that short lateral 

 axes terminated by flowers spring up quite unmodified above the spongy white 

 cuckoo-gall. Cuckoo-galls also occur on Cruciferae, viz. on Barbarcea vulgaris, 

 Nasturtium palustre, sylvestre, and Sisymhriuvi SopJda. They are produced by 

 Cecidomyia Sisymbrii, and originate principally at the bases of the flower-stalks 

 half-way up the inflorescences. They look like spongy white bodies which surround 

 the pedicels like the brim of a hat. As the growths from neighbouring pedicels 

 meet together they roof over chambers which serve as habitations for the larvae of 

 the gall-gnats. Viewed from outside the galls appear like irregular white bodies 

 inserted in the inflorescence, which remind one of the fruit of the white Mulberry- 

 tree. 



The term cluster-gall is reserved for that type of bud-gall in which the axis is 

 much restricted or stunted and covered with densely crowded leaf -structures; it 

 is in the chinks and recesses between the crowded leaves of these galls that the 

 insects concerned pass the whole or a portion of their lives. The animals which 

 cause the galls belong to very different classes. Gnats, leaf-fleas, leaf-lice, and mites 

 are the commonest varieties. The gnats only live in the galls during the egg and 

 larval stages, but the others pass their whole life there. They invariably settle 

 on the end of a shoot while it is still undeveloped in the bud. The axis of the 

 shoot remains more or less stunted in consequence of the influence the animals 

 exercise on it and its leaves undergo fundamental alterations. The blade or sheath 

 of the leaf is deepened and hollowed to afford sufficient space to the animals which 

 have established themselves between them, and as these parts of the leaves touch 

 one another recesses are formed not unlike those which are developed in fir-cones 

 for the growing seeds. The sheathing part of the leaf is often rather tiiickened, 

 and its succulent cells serve as food for the animals living in the gall; in other 

 instances the hollowed leaf-blades are thickly covered with hairs, and this coat 

 then has the same significance with regard to the insects as the felt of haii-s on 

 isolated leaves already described. Very different forms of galls are produced 

 according as to whether the free ends of the leaves turn back or remain in contact, 

 and whether the axis from which the leaves spring is more or less contracted. 

 Sometimes they remind one of open rosettes, sometimes of closed balls, bunches and 

 tufts, sometimes of pig-tails and witches' brooms. 



