272 DEFINITE ACTION OF THE Chap. NXIII. 



Galls. — Another class of facts, not relating to cultivated 

 plants, deserves attention. I allude to the production of galls. 

 Every one knows the curious, bright-red, hairy productions 

 on the wild rose-tree, and the various different galls produced 

 by the oak. Some of the latter resemble fruit, with one face 

 as rosy as the rosiest apple. These bright colours can be of 

 no service either to the gall-forming insect or to the tree, and 

 probably are the direct result of the action of the light, in the 

 same manner as the apples of Nova Scotia or Canada are 

 brighter coloured than English apples. According to Osten 

 Sacken's latest revision, no less than fifty-eight kinds of galls 

 are produced on the several species of oak, by Cynips with its 

 sub-genera ; and Mr. B. D. Walsh 48 states that he can add 

 many others to the list. One American species of willow, 

 the Salix humilis, bears ten distinct kinds of galls. The leaves 

 which spring from the galls of various English willows differ 

 completely in shape from the natural leaves. The young 

 shoots of junipers and firs, when punctured by certain insects, 

 yield monstrous growths resembling flowers and fir-cones ; and 

 the flowers of some plants become from the same cause wholly 

 changed in appearance. Galls are produced in every quarter 

 of the world ; of several sent to me by Mr. Thwaites from 

 Ceylon, some were as symmetrical as a composite flower 

 when in bud, others smooth and spherical like a berry ; some 

 protected by long spines, others clothed with yellow wool 

 formed of long cellular hairs, others with regularly tufted 

 hairs. In some galls the internal structure is simple, but in 

 others it is highly complex ; thus M. Lacaze-Duthiers 49 has 

 figured in the common ink-gall no less than seven concentric 

 layers, composed of distinct tissue, namely, the epidermic, 

 sub-epidermic, spongy, intermediate, and the hard protective 

 layer formed of curiously thickened woody cells, and, lastly, 

 the central mass, abounding with starch-granules on which 

 the larva? feed. 



Galls are produced by insects of various orders, but the 



< s See Mr. B. D. Walsh's excellent 1864, p. 546. 



papers in ' Proc. Entomolog. Soc. 49 See his admirable ' Histoire des 



Philadelphia,' Dec. 1866, p. 284. Galles,' in ' Annal. des Sc. Nat. Bot.,' 



With respect to the willow, sec ibid., 3rd series, torn, xix., 1853, p. 273. 



