l'oKMS OK MAXTl.K-O Al.l.s. 



531 



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



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

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

 juicy contents are used as food l>y 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 



1 Covering-galls on the petiole of the Black Poplar (Poptdus nigra) produced by Pemphigus spirotheca. • Scroll-palls <m tint 

 leaves of an Alpine Rose (Rhododendron Jerrugineum) produced by gall-mites. 3 Transverse section of one of theae galls. 

 * and 5 Bud-galls on the branchlets of the Wild Thyme (Thymus Serpyllum) produced by gall-mites. 6 Blister-like 

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

 6 Vertical section of a portion of this gall. ° Solid gall on tin- leal "i the Gray Willow (Salix incana) produced by Xematus 

 pedunculi. 1° The same gall cut open. Il Part of the wall of this gall in vertical section. *. J . «. *, and 9 natural size ; 

 1 and « x 4 ; » and ; x 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 resuU that a hollow 

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

 insects. The protuberances exhibit a great variety of form ami 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 



