Autograph Letter Signed [to W.A.Leighton, English botanist, author of Lichen Flora of Great Britain], answering his letter and discussing lichens and recent articles on the subject. In French. 2 pp. 8 x 5 inches, with blemishes and a tear (without loss of text), mounted at lower edge on part of an old album page. Paris, 19 June 1867. Uncommon. William (Wilhem) Nylander (1822-1899), Finnish botanist and entomologist. Nylander taught at the University of Helsinki for a number of years before later moving to Paris. Nylander pioneered the technique of determining the taxonomy of lichens by the use of chemical reagents, such as tinctures of iodine and hypochlorite, still used by lichenologists to this day. Nylander was also the first to realise the effect of atmospheric pollution on the growth of lichens.