SOLID GALLS. 535 



are caused by leaf-lice on various species of Pistacia, also belong to the covering 

 class. The rudiment of a foliage-leaf, which in the normal course of events would 

 have developed into a pinnate leaf with dark-green elliptical leaflets, grows out 

 into a pod-hke structure not unlike a locust-bean (fruit of Ceratonia Siliqua). 

 These galls are longitudinally grooved, and it can be seen more or less distinctly 

 that the furrows correspond to the edges of the leaflets, only here the leaflets 

 have become wrapped in, very much thickened and elongated, and fused with one 

 another. In the cavity inclosed by the fused leaflets lives a colony of leaf-lice 

 {PeTnphigus cornicularius) which have developed under the protection of the gall. 

 When it is time for them to leave the cavity the top of the pod opens by the 

 separation and bending back of the tips of the fused leaflets which form the wall 

 of the cavity (see fig. 362*). The Chinese galls of commerce, produced also by 

 Aphides (on Rhus semialata), develop much in the same way. They are hollow, 

 irregularly pear-shaped structures with thin walls covered externally with a gray 

 down. Two other covering galls which deserve special mention on account of their 

 form arise on the petioles of the Poplar, particularly on the species Populus nigra, 

 pyramidalis, and dilatata. The one, caused by a leaf-louse, PeTnphigus bursarius 

 (see fig. 362 ^), consists of a smooth expansion, red in colour externally, on the upper 

 side of the grooved petiole. If the local swelling be cut through it is seen to be 

 hollow, the cavity in which the leaf-lice live being shut in by thick fleshy walls. 

 The fleshy tissue of the walls is formed by a growth of the cells round the place 

 where the gall-producing insect has settled. A hole is formed at a point remote 

 from the petiole (where the growing tissue met and formed a dome) as soon as 

 the time comes for the inhabitants to make their exit. This is bordered by thick 

 lips as shown in fig. 362 ^. The other gall which appears on Poplar petioles, pro- 

 duced by Pemphigus spirotheca, is formed by the thickening of the edges of the 

 grooved petiole, which rise up as fleshy cushions and meet above the depression. 

 At the same time the petiole undergoes a spiral twisting, and a gall is thus pro- 

 duced whose cavity is spirally twisted like the interior of a snail's shell. The 

 thickened edges of the petiole do not fuse; at first they fit close to one another, 

 but later on they separate, and a spiral hole out of which the white, downy leaf- 

 louse can creep is the result (see fig. 360^, p. 531). 



We will now leave the mantle-galls and pass on to a consideration of the solid 

 or tubercular galls. These are of the nature of swellings of limited size on single 

 plant-organs, and are produced by insects which pierce the plant-tissue and lay 

 their eggs in the wound. In this way either the epidermis of the chosen spot alone 

 is injured, or the egg is inserted into the deeper-lying tissues. In both cases an 

 active cell-division is incited in the neighbourhood of the injury. If, however, the 

 egg has only been deposited in the epidermis, the larva which arises from it must 

 penetrate into the interior of the now swollen tissue; when the egg is laid at once 

 deep down this farther penetration on the part of the larva is of course unnecessary. 

 The cavity in which the larvee dwell may be called the larval chamber, and this sort 

 of gall can be classified according to the number of chambers which it contains, 



