FELT- GALLS. 509 



numerous 



has to bore its way out. Needless to say, of both these types there are 



modifications, but they fall into the two classes (of mantle and solid galls) according 

 to their mode of development. 



The majority of felt-galls are produced by gall-mites. They form cottony or 

 felted growths on limited and sharply defined areas of green leaves and stems, 

 the surface of which is otherwise smooth, or possesses but few hairs. Some- 

 times they have the form of small tufts, bands, or stripes, sometimes of large spots 

 with irregular contour. In most instances the felt is situated on the under side 

 of the foliage-leaf, and the gall-mite usually prefers the projecting veins to the 

 green surface. In the Lime, Alder, Hornbeam, and Horse -Chestnut, the mites 

 usually establish themselves in the angles formed by the lateral strands where 

 they arise from the midrib, the projecting veins forming the framework for the 

 felted hairs. In the Bramble (Rubus) and the Burnet (Poterium) it sometimes 

 happens that the felt is continued down from the lamina to the leaf-stalk, and 

 occasionally the green cortex of the succulent twig is covered with felted bands 

 and spots. In some Brambles and Cinquefoils the sepals become furred by the 

 action of gall-mites, the usual consequence being that the outline also becomes 

 distorted. A swelling or slight hollowing of the green leaf-tissue very frequently 

 accompanies the formation of felted galls, in which case the hairy covering is 

 only visible on the concave side whilst the other remains smooth. This is most 

 remarkable in the foliage of the Avens (Geum), Vine (Vitis), and Walnut-tree 

 (Juglans), where a dozen white or brown-felted pit-like depressions are sometimes 

 to be seen on the under side of a single leaf. The colour of the felted hairs is 

 white in the leaves of Beeches, Limes, Bird Cherry, Brambles, Cinquefoils 

 and Burnets, green in the common Maple, yellow in the Spindle-tree (Euonymus 

 vewucosus), sulphur-yellow in Alnus orientalis and Black Poplar (Populus nigra), 

 carmine red at first and then violet in Alnus viridis and in the Birches (Betula 

 alba, carpatica, &c.), and brown in the Avens (Geum macrophyllum), Horse- 

 Chestnut (JEsculus Hippocastanum), and in the Aspen (Populus tremula). The 

 felted galls which are light in their young stages usually take on a brown tint 

 afterwards. Microscopic investigation has shown that in the formation of felted 

 galls, the epidermal cells, originally tabular in shape and closely fitting, swell 

 out and become transformed into bent and twisted tubes generally shaped like 

 a club or retort, the stimulus being afforded by a minute gall-mite (Phytoptus). 

 These cells look like short hairs to the naked eye, and as they stand side by 

 side in large numbers the covering has a velvety or felted appearance. The mites 

 which produce the felt, deposit their eggs in the juicy hair-shaped cells, and their 

 young live on the materials contained in them. It should be mentioned that 

 formerly these velvety and felted coverings were regarded as Fungi, and were 

 described as distinct genera under the names Erineum and Phyllerium (e.g. the gall 

 known as Erineum quercinum on the leaves of Quercus Cerris). To this group 

 belongs also the gall occurring on the Wood Meadow Grass (Poa nemoralis) con- 

 sisting of cells which resemble root-hairs, which is produced by the gnat Hormo- 



VOL. II. 



