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 3 ). Pocket- 

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



Fig. 360. Galls. 



i Covering-galls on the petiole of the Black Poplar (Populus nigra) produced by Pemphigus spirotheca. 2 Scroll-galls on the 

 leaves of an Alpine Rose (Rhododendron ferrugineum) produced by gall-mites. 8 Transverse section of one of these galls. 

 * and 6 Bud -galls on the branchlets of the Wild Thyme (Thymus Serpyllum) produced by gall-mites. 'Blister-like 

 galls on the leaf of the Red Currant (Ribes rubrum) produced by Myzus ribis. 7 Part of the leaf seen from below. 

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

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

 5 and e x 4; and 7 x 8; s and " x 50. 



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 



