98 BRITISH GALLS 



a section through three galls and one of the larger veins ; 

 Fig. 10 the orifices on the lower surface of the leaf 

 (Figs. 8 to 10 are magnified). There is another common gall 

 on Maple leaves caused by Eriophyes macrochelus. It is 

 abundantly distinct from the preceding in its larger size and 

 isolated habit (Figs, ii, 12, 13), and may also at once be dis- 

 tinguished by the pluricellular character of. the hairs which 

 line its interior (Fig. 14). The galls of both species may 

 occur on one leaf, and leaves quite devoid of galls may often 

 be found adjacent to infected ones. Several species of 

 supposed Phyllerium have been observed on fossil Maple 

 leaves. 



At least three species of Eriophyes frequent Alder leaves in 

 this country, causing familiar galls. In those resulting from 

 the presence of E. Nalepai the pustules are hemispherical, 

 about 3 mm. high, seated on the upper, surface, always at 

 the junction of the lateral veins and midrib, and arranged 

 in pairs. They are glabrous, yellowish-green at first, be- 

 coming red or brown. Each pustule contains a cavity with 

 a wide opening on to the lower surface of the leaf. The 

 hairs are white or yellowish-brown ; when viewed under the 

 microscope they are seen to be of two kinds — one plural- 

 celled, blunt-pointed, and thick-walled, usually much dis- 

 torted and entangled ; the other unicellular, very short and 

 thick. The slight swellings on these leaves are caused by 

 Eriophyes brevitarsus. In the early stages of growth the 

 hairs lining, the depressions on the lower surface are whitish; 

 they become brown at maturity, and resemble, when viewed 

 with a pocket-lens, minute crystals of Demerara sugar. 

 Under the microscope the crystal-like bodies are seen to be 

 the enormously swollen he^ds of the hairs. In the inter- 

 stices between these growths the mites live in hundredsj 

 browsing upon the "sugar" so generously provided for 

 them by the plant. The busy colony is a most fascinating 

 object of contemplation. It is difficult to realize that the 

 remarkable alteration of epidermal cells, from their normally 

 flattened shape, to these peculiar club-shaped bodies, is 



