The Scottish Naturalist. 205 



(^g) The Witch-knots, so familiar to every one on the birch, re- 

 sembling rook's nests from a little distance, have been 

 shown by Miss E. A. Ormerod to be the work of gall-mites 

 ('Entomologist,' vol. x. pp. 83-86, April 1877). Dr Thomas 

 (1. c, pp. 257-59, figs. 1-8) has described very fully and figured 

 the earlier stages of the same deformity. Single buds are ' 

 atacked, and show the effect of the attack by becoming 

 swollen to the size of a pea; the bud-scales become en- 

 larged and covered with a thick coating of silky adpressed 

 hairs; they also stand some distance apart, so that the bud 

 loses the compact form and glossy surface natural to it. On 

 examination with the aid of the microscope, the mites are 

 found in multitudes between the inner scales, chiefly towards 

 the base where the hairs are replaced by spherical papillae. 

 The affected buds may occur singly, but they usually, after 

 a time, form compact masses an inch or more in diameter, 

 composed of abortive twigs beset with buds each attacked 

 by the mites. After a considerable time (some years) the 

 twigs grow to a greater or less length, but swollen and dis- 

 torted in form, and form the Witch-knot, sometimes reaching 

 a diameter of two or three feet. The attacks of the mites 

 seem to stimulate the production of woody tissue at the 

 point affected, and to favour the development (in distorted 

 state certainly) of the young buds in the axils of the bud- 

 scales, while hindering the development of the leaves them- 

 selves. The Phytoptiis is of the usual size and form (.0064 

 X .0012 inch). These galls are common in most localities, 

 and the various stages of development may be well traced in 

 winter while the branches are bare. 



Alnus glutinosa L. — {c) Erinemn alneiun, Persoon, is one of 

 the mite-galls formerly looked on as fungi, but now known 

 to be the work of Phytoptiis. It occurs scattered over the 

 lower surface of the leaf in slight depressions (which corre- 

 spond to very low elevations of the upper surface) in scat- 

 tered patches of irregular form, y^- Yz inch across, between 

 the veins. Sometimes the whole lower surface of the leaf 

 may be covered, sometimes the patches are very few and 

 small. At first yellowish-white, a rusty or red tinge begins 

 to appear round the margins, and soon spreads all over the 

 patch. Under the microscope it is found to consist of vesi- 

 cles, each supported on a slender stalk, from the top of 

 which it branches irregularly, each lobe ending in a rounded 



