In temperate latitudes, tree-ring series and, in some instances, patterns of width variation provide a direct means of establishing timespans of direct relevance to geomorphologists. During growth, trees regularly increase their size by the addition of secondary woody tissue. In a given growth period this material is initially produced as large, thin-walled, cells, but is then followed by a gradual transition to the production of small thick-walled cells. Since the growth pattern is cyclic, with active periods being followed by dormancy, discontinuities, delimited as abrupt changes in cell size, occur between successive cycles, and these constitute the familiar tree-ring series. The growth cycles are normally seasonal in character and are determined by the regular alternation of winter and summer climatic conditions, and hence the rings record annual events.