The paper explores the adaptation of plants to different irradiance levels, focusing on traits that are significant when considered at the whole-plant level rather than just at the leaf level. It reviews traditional approaches to demonstrating or suggesting adaptation to irradiance levels and outlines three energetic trade-offs likely to shape such adaptations: the economics of gas exchange, support, and biotic interactions. The author re-evaluates the classic study of acclimation of the photosynthetic light response in *Atriplex* and shows that it does not fully support the central dogma of adaptation to sun v. shade unless analyzed in terms of whole-plant energy capture. Calculations for *Liriodendron* demonstrate that the traditional light compensation point has little meaning for net carbon gain and is influenced by the costs of night leaf respiration, leaf construction, and associated support and root tissue. The effective compensation point is significantly affected by these costs, with a 140 μmol m⁻² s⁻¹ increase in trees 1 m tall and nearly 1350 μmol m⁻² s⁻¹ in trees 30 m tall. The paper also discusses new models for the evolution of canopy width/height ratio and mottled leaves as defensive measures in understory herbs.The paper explores the adaptation of plants to different irradiance levels, focusing on traits that are significant when considered at the whole-plant level rather than just at the leaf level. It reviews traditional approaches to demonstrating or suggesting adaptation to irradiance levels and outlines three energetic trade-offs likely to shape such adaptations: the economics of gas exchange, support, and biotic interactions. The author re-evaluates the classic study of acclimation of the photosynthetic light response in *Atriplex* and shows that it does not fully support the central dogma of adaptation to sun v. shade unless analyzed in terms of whole-plant energy capture. Calculations for *Liriodendron* demonstrate that the traditional light compensation point has little meaning for net carbon gain and is influenced by the costs of night leaf respiration, leaf construction, and associated support and root tissue. The effective compensation point is significantly affected by these costs, with a 140 μmol m⁻² s⁻¹ increase in trees 1 m tall and nearly 1350 μmol m⁻² s⁻¹ in trees 30 m tall. The paper also discusses new models for the evolution of canopy width/height ratio and mottled leaves as defensive measures in understory herbs.