This book reviews the biology of bryophytes and lichens in the polar tundra, where these plants may form a dominant component of the vegetation. It considers adaptation to severe environments in terms of growth form, physiology and reproduction. The role of bryophytes and lichens is discussed in vegetation processes such as colonisation and succession, and in energy flow, nutrient cycling and other functional aspects of polar ecosystems, both natural and as modified by man, The range of microclimates experience by polar cryptograms is described using an energy budget approach, and the environmental relationships of CO2 exchange, stress resistance, growth and other physiological responses are discussed against this background. Reproductive biology is also reviewed as an introduction to a consideration of population ecology, distribution patterns, dispersal potential and the origin and adaptation of polar cryptogamic floras. This book integrates the results of work in the Arctic and the Antarctic, and includes a classification of vegetation zones applicable to both polar regions. The study of plant ecology in these areas has advanced dramatically and the results synthesised here contribute to a general understanding both of polar ecosystems and of the environmental relationships of bryophytes and lichens.