FORMS OF MANTLE-GALLS. 531 



Trioza Rhamni on the margin of Buckthorn (Rhamnus cathartica) leaves is very- 

 hard and thickened like cartilage. In many plants the epidermal cells lining the 

 gall elongate into hairy structures, as in the felt-galls previously described. Their 

 juicy contents are used as food by the young gall-mites. This is the case, for 

 , example, in the Alpine Rose (Rhododendron ferrugineum, cf. fig. 360 ^). Poclcet- 

 are closely allied to the scroll-like forms. The tissue of the leaf -lamina or 



Fig. 360.— Galls. 



^ Covering-galls on the petiole of the Black Poplar {Populvs nigra) produced by Pemphigus spirotheca. 2 Scroll-galls on the 

 leaves of an Alpine Rose (Ehododendronjerrugineum) produced by gall-mites, s Transverse section of one of these galls. 

 * and 5 Bud -galls on the branchlets of the Wild Thyme (Thymus Serpyllvm) produced by gall-mltes. « Blister-like 

 galls on the leaf of the Red Currant (Ribes rubrum) produced by Myzus ribis. ' ' Part of the leaf seen from below. 

 8 Vertical section of a portion of this gall. 9 Solid gall on the leaf of the Gray Willow (Salix incana) produced by Nematus 

 pedunculi. 10 The same gall cut open. " Part of the wall of this gall in vertical section. 1. 2, 4, 6, and 9 natural size ; 

 » and 6 X 4; » and ' x 8; » and " x 60. 



petiole and sometimes that of the cortex in young twigs is subjected to a stimulus 

 where the animals (gall-mites, leaf -lice, diptera) settle, with the result that a hollow 

 protuberance arises whose excavated cavity serves as a temporary dwelling for the 

 insects. The protuberances exhibit a great variety of form and shape, and they 

 differ considerably in their internal structure. The following are the most notice- 

 able forms. First, the plaited galls. They form deep, plaited, sometimes twisted 

 channels in the leaf -tissue which open on the upper side by a narrow hole, and 



