178 ANNALS OF SCOTTISH NATURAL HISTORY 



face of the member forming or bearing the gall. The cavity 

 serves to protect the maker of the gall. The Mantle-galls 

 show three chief divisions corresponding to the modes in 

 which they may be formed, as indicated above. They are : 



a. Roll~ga.\\s, formed by the margins of the part rolled 

 upwards or downwards. 



b. Pocket- or Pouc/t-galls, in which stimulus of a limited 

 part of the galled member has caused increased growth of 

 that part. Owing to the surrounding tissues not having 

 increased in corresponding measure, the galled portion bulges 

 outwards on one surface, and forms a pouch on the other 

 side. The form and depth of this pouch depend on the 

 extent of the part galled, and on the activity of growth 

 within it. The Pouch-galls differ much in form. Often the 

 pouches are wide and shallow, as on the leaves of many 

 plants (e.g. the galls of Cecidomyia piistulans on Meadow- 

 sweet, and those of Aphides, so common on Currant bushes 

 in gardens). Sometimes they occupy most of the leaf, which 

 is then usually curved round, as one sees so often in the 

 leaves of Elms. On other leaves the pouch is a narrow 

 ridge lying between two chief veins (e.g. in Hornbeam), or 

 near the margin (in Honeysuckle). In this form the opening 

 of the pouch is a long narrow slit, kept nearly closed by its 

 edges. More striking forms are like a nail or bullet attached 

 to the surface, usually of a leaf, by a narrow neck, through 

 which is the exit. In these, the part affected has been very 

 small, but growth in it has been very active, so as to disguise 

 at first sight their true origin. Galls of these latter forms, 

 the work of mites, are often abundant on leaves of Sloe, 

 Birdcherry, Lime and Maple. 



c. Covering-galls, or >//>-galls, as the third class of 

 Mantle-galls may be termed, owe their form to the growth 

 ceasing, or nearly so, at the point where the gall-maker or 

 the egg is fixed, while it becomes very active around this 

 point. A wall is thus built up, which forms at first a cup, 

 but may elongate into a tube, or may arch over the cavity so 

 as almost (seldom altogether) to shut it in. Sometimes the 

 growth is much more rapid on one side, the position of the 

 opening (as in the galls of Tetraneura Ulmi on Elm leaves) 

 showing the true apex of the gall. 



