THE PJUNCIPI-ES OF OAK GALL FORMATION. 5 



tudinal cylindrical cavity, at the lower end of wliicli 

 is a little, partly-attached larval cell. This cell is 

 undoiibtedlv amenalile to the laws controlling other 

 oak galls, but not so the outer structure. That con- 

 tinues its growth, throws out buds from which leaves 

 of normal size expand, and not infrequently a shoot 

 will grow from the apex, upon which, in the next 

 summer, a gall of the same species will develop (see 

 Plate XI, div. C.) 



The gall of Andricus cur cat or, when upon a twig or 

 leaf petiole, exhibits similar peculiarities, but not to so 

 great an extent ; when a leaf is involved in its growth 

 there is no comparison. This gall is also l)ilocular ; 

 the larval cell, however, is loose within the external 

 gall growth, while that of htflator is attached. 



In the compound gall of ApJiilothru' fecnndatrix 

 the inverse is exemplified. The inner gall, when 

 pushed out in the late summer by the dying leaf 

 scales, falls to the ground, and continues growth in a 

 manner similar to that of the Nevroterus o-alls. The 

 leaf scales, although dead, persist for many months, 

 and only disappear from the twig when pushed off by 

 new growth in the following spring. 



The component parts of galls are variable. 



The simplest forms are those of SjinfJtegoster hac- 

 carum and Trigonaspis crusfaJi.s, in which the tissue is 

 very loose, filled with a copious thin fluid, and the 

 exterior merely a thin epidermis with chlorophyll. 



After the emeroeuce of the imao-o it withers and 

 shrivels to a condition sometimes beyond recognition. 



The next form is that of galls such as Andricus 

 noduli, A. nndvs, Aphilotlirix fecnndatrix, A. quadri- 

 lineatuH, A. soUtarins, in which the inner layer is 

 moderately thick, and is composed of cells containing 

 nutritive materials for the larva, and surrounded by a 

 thin layer of hard woody cells filled with colour pig- 

 ments. The same is found in Aph. (ilohvli, with the 

 addition of an outer sappy green rind. Ap)h. Sieholdi 

 partakes of the same nature, but the rind, instead of 



