Summary: Bark thickness is ecologically crucial affecting functions from fire protection to photosynthesis. Bark thickness scales predictably with stem diameter but there is little consensus on whether this scaling is a passive consequence of growth or an important adaptive phenomenon requiring explanation.With a comparative study across 913 species we test the expectation that if bark thickness–stem diameter scaling is adaptive it should be possible to find ecological situations in which scaling is predictably altered in this case between species with different types and deployments of phloem.‘Dicots’ with successive cambia and monocots which have phloem-free bark had predictably thinner inner (mostly living) bark than plants with single cambia. Lianas which supply large leaf areas with limited stem area had much thicker inner bark than self-supporting plants. Gymnosperms had thicker outer bark than angiosperms.\r\nInner bark probably scales with plant metabolic demands for example with leaf area. \r\n\r\nOuter bark scales with stem diameter less predictably probably reflecting diverse adaptive factors; for example it tends to be thicker in fire-prone species and very thin when bark photosynthesis is favored. Predictable bark thickness–stem diameter scaling across plants with different photosynthate translocation demands and modes strongly supports the idea that this relationship is functionally important and adaptively significant.