Boundaries to the distribution of life forms and species often coincide with isometric lines of climatological variables. Prominent examples of this phenomenon are the restriction of megaphanerophytes to areas where the mean temperature of the warmest month does not fall below about 10 °C, and the restriction of species within the British Isles to areas of specific summer or winter warmth. In the interpretation of these patterns, it is important to realise that the climate at plant surfaces may differ appreciably from that of the atmosphere as a whole, to an extent which depends on structural attributes of the vegetation. I n particular, chamaephytes experience a much higher surface temperature than phanerophytes when comparisons are made in bright sunlight and this contributes to their success in mountains. Temperatures of flowers may similarly be elevated. In the British Isles, temperatures in the summer months may be decisive in determining the success of many species, as physiological processes in C3-plants display near-linear relationships with temperatures in the range which prevails for most of the time. The life-cycle of native plants responds to environmental cues in such a way as to synchronize the development of the plant with the succession of seasons. When species are transplanted to different phytogeographical zones, as in forestry and horticulture, they frequently fail because of asynchrony.