SUMMARYTemperature, rainfall and day‐length probably all help to determine limits of plant distributions, but the distributions cannot generally be explained by simple values of any of these quantities alone. There is in most latitudes a negative correlation between annual temperature range and log. winter rainfall and this and other correlations make it difficult to distinguish the effects of the several factors in the delimitation of plant areas.Approximately within the 40o‐70oN. latitude band a strongly marked climatic pattern of increasing annual temperature range inland from the coasts is matched by a pattern of reducing winter temperatures in the same directions. The centre of this pattern, in Eurasia, is in eastern Siberia. This pattern is reflected in many plant areas. Following ideas expressed severally by Vahl, Iversen and Jeffree for plotting plant distribution in terms of temperatures of coldest and warmest months (or of January and July) on Cartesian co‐ordinates it is shown that the areas obtained may be closely related to the above main climatic pattern. Very similar areas may be obtained from values of latitude (which governs day‐length) and log. winter rainfall.Several of these climatic areas have been compared with biological distributions and maps are presented. Some of these can be arranged in progressive series which correspond rather closely with some series of equiformal progressive areas of plant distributions given