Plant Science The Arabidopsis plant, when grown under intense light, develops thicker leaves than when grown under dim light. Hoshino et al. parse the various processes involved. Cell elongation and organized divisions underpin the thickness of leaves growing in intense light. Later in development, cell expansion and increases in intercellular spaces finalize the respective dimensions of leaves growing in sunlight or in shade. Blue-light signaling drives initial unequal cell elongation in sun-exposed leaves, whereas sucrose, a photosynthetic product, promotes the late phase of leaf thickening for both sun-exposed and shaded leaves. With leaf development linked to environmental signals, the plant's leaf structure can be fine-tuned to best suit its microenvironment. Plant J. 10.1111/tpj.14467 (2019).