FORMS OF MANTLE-GALLS. 531 



Trioza Mhariini 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 ^). Pocket- 

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



Fig. 360.— Galls. 



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

 leaves of an Alpine Kose (Rhododendron Jerrugineum) produced by gallmites. s 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. ' Part of the leaf seen from below. 

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

 pedunculi. lo The same gall cut open, n Part of the wall of this gall in vertical section, i. 2, 4, 6, and s natural size ; 



* and 6 X 4 ; s and ^ x 8 ; 8 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 



