83 



PHYTOPTUS OF THE BIRCH -KNOTS. 

 By E. A. Okmerod. 



Amongst the various fonr.s of galls caused by Phijlopti 

 tbe peculiar growth of twigs in the birch tree (commonly 

 known as witch-knots) is of some interest, from the attack of 

 the gall-mites producing an increased development of woody 

 growth from the infested buds, instead of — as is usually the 

 case — leaf-galls, or diseased leaf-buds alone. 



As far as I am aware attention has not yet been drawn to 

 this point. Dr. F. Low, in his notice of the Pliylopius galls of 

 the Popiilus tremula (Verh. der z.-b. Gesellschaft,Wien, xxiv.), 

 mentions the shortened twigs bearing the leaves in unnaturally 

 close proximity, which in some degree corresponds with one 

 stage of the birch growths, but he does not allude to any 

 increased development of the twigs ; and in the ' Pflanzen- 

 feinde' of Kallenbach, beyond an allusion to these growths, 

 there is no instance given of woody growths under the head 

 of PJiyioptus. 



Dr. J. W. H. Trail, in his papers on Scottish galls, 

 mentions finding multitudes of Phytopti on the Belula alba 

 (Scot. Nat. iv. 17) ; but in this case they were observable 

 amongst the patches of vesicles growing in depressions of 

 the leaves. 



The witch-knot, or great bunch of twigs looking like a 

 large bird's nest fallen at random amongst the branches of 

 the birch, is familiar to all ; and having noticed some of these 

 on birches planted by the road-side in the neighbourhood of 

 Isleworth (Middlesex), an examination of the trees showed 

 them to be suffering so much from the attack of tlie gall-mite 

 as to allow of its effects being traced from the commence- 

 ment. The affected buds are distinguishable by their swollen 

 form, — after a time by the knotted clusters into which they are 

 thrown by the diseased development of great numbers ot 

 them close together ; and in November the four-footed 

 Acariis {Plnjioplus) is to be found in an active state amongst 

 the inner scales, — in considerable numbers, certainly, but 

 still to be counted rather by dozens or scores, than by the 

 hundreds or innumerable quantities noticed in some other 

 cases of Phytoplus bud-possession. The infested buds may 

 be distinguished by their spheroidal shape, greater size, and 



