American Arctic Lichens:  2.The Microlichens is the second volume of John W. Thomson's fundamental reference work on the lichens of the American Arctic, the product of almost four decades of field research and analysis. Thomson, the world's foremost lichenologist of the Arctic region, has undertaken fourteen expeditions to the U.S. and Canadian Arctic to find and identify 623 species in 106 genera of microlichens. Complementing his taxonomy and classifications are 201 black and white illustrations and 512 maps.     By bringing together in a single volume keys, descriptions, maps, illustrations, statements of color reactions, chemistry, and ecology, Thomson makes available a compendium of knowledge hitherto available only in scattered literature. American Arctic Lichens 2 will serve as a manual for identification of the crustose lichens of the Canadian and Alaskan Arctic--as well as of other circumpolar regions of the world--providing an invaluable aid to ecologists, conservationists, naturalists, plant taxonomists, and students of Arctic resources and wildlife.     Complementing this volume is American Arctic Lichens: 1. The Macrolichens (Columbia University Press, 1984). Together, the two-volume work covers 965 species of lichens in 166 genera, flora Thomson considers an "untapped resource of nature." Thomson notes that lichens, under systematic biochemical explorations, provide ecological indicators, and could provide possible new treatments for cancer and other human diseases. The availability of this manual will greatly accelerate future botanical studies of the vast and fascinating American Arctic.