532 ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



project like weals on the lower surface of the leaf. The growing tissue which 

 forms the floor of the channel is yellow and often lined with short hairs. The 

 channel usually follows the course of the larger veins of the lamina, and some- 

 times actually traverses one. Plaited galls are produced by gall-mites. The best 

 known are those on the foliage of Garpinus Betulus, Clematis Flammula and C 

 recta, and Ribes alpinum. Wrinkled galls come next to the plaited form. The 

 protuberance is here limited to the green tissue shut in by strong rib-like 

 projecting veins, and is only shallow; the upper side of the leaf has bulgings 

 and protuberances and the lower pits and cavities. The protuberances are 

 always developed in numbers close together, so that the leaf looks very 

 much wrinkled in that region. Examples of this form are furnished by the 

 wrinkled galls on the Elm (Ulmus campestris; see fig. 361 *) produced by the leaf- 

 louse Schizoneura Ulmi, and on the Red Currant (Ribes rubrvmi; see figs. SQO^''''^) 

 by another leaf-louse, Myzus ribis. In the latter several wrinkles are usually 

 united into large blister-like protuberances, red on the outside, and covered with 

 jointed cellular structures bearing glands which look to the naked eye like short 

 hairs. This form, though resembling certain felted galls, is distinguished from 

 them by the different form of the hairs arising in consequence of the stimulation. 

 In the Mouse-ear Hawkweed (Hieracium Pilosella) leaf-fleas (Psyllodes) produce 

 minute protuberances with narrow mouths, which stand out from the lower side 

 turned towards the ground like small warts, and when they occur close together 

 give a corrugated appearance to the leaf. Hollow protuberances of this sort 

 arising upon restricted areas of the leaf-surface, and growing very actively, give 

 rise to bag or sack-like structures attached by a very narrow neck. From their 

 resemblance to a head such galls are sometimes termed capitate galls (Cephalonion). 

 In others, where the outgrowth is fairly thick- walled and in form horn-like, the 

 designation horn gall (Geratonion) has been given. Between these forms numerous 

 intermediate forms exist which may be compared to pockets, bags, nails, &c. 

 Many of these galls project from both the upper and lower side of the leaf, as 

 though a nail had been driven through it — hence the last-mentioned name. The 

 capitate-gall of the Sloe {Prunus spinosa), caused by a gall-mite, projects almost 

 as much from the under as from the upper side, whilst the similar gall on the 

 foliage of the Bird Cherry (Prunus Padus) rises on the upper side as a long 

 pocket, but below only projects like a small wart. Many capitate and horn-like 

 galls are developed only on one side, and here again there is a very great variety. 

 When the protuberances are due to mites the cavity always opens on the lower 

 side of the leaf. Both the inner wall and mouth of the cavity are covered with 

 hairs, and sometimes the aperture seems to be actually plugged up by them. In 

 the bag-shaped protuberances produced by the leaf-louse Tetraneura Ulmi on 

 Elm leaves, a relatively large slit is formed just at the narrow part of the bag at 

 the moment when the insects leave the cavity (see fig. 361 ^). The external surface 

 of the protuberances caused by mites on the foliage of Alders (Alnus), Maples 

 (Acer), and Limes (Tilia) is smooth, in those of the Bird Cherry (Prunus Padus), 



