First, stress is laid on the point of view of the ecologist and geographer, as distinct from the physiologist. The desirability of maps showing abundance as well as range is emphasized and it is suggested that such terms as rare and common be restricted to indication of abundance only. The direct or indirect effect of temperature is considered and the vegetation of hot springs is quoted in support of the contention that resistance to extreme conditions depends on the constitution of the colloidal protein complex. The differential effects of temperature relations on the vegetative and reproductive functions of species influence their distribution. A species towards its northern boundary presents 3 zones with respect to temperature: the central area of maximum density where the temperature suffices for growth and reproduction; a zone in which the temperature suffices for vegetative increase but for reproduction only in exceptional years; and an outermost zone reaching a limit where the species is maintained only by artificial aids to regeneration. The role of such factors as length of frostless season, cold air drainage, soil texture, topography and especially wind action are considered, and the modifying effect of temperature, wind and soil texture on the efficiency of a particular rainfall is emphasized. The varying plasticity of species in respect to their root systems, osmotic suction force, and transpiration checks, tends toward an ill-defined relation between a species range and climatic limits. Specialization, however, in any of these respects entails a loss of plasticity which reacts on the capacity to withstand competition. It is concluded that the phenomenon of acclimatization rarely occurs and that the climatic boundaries of species are of a comparatively secular stability.