Here, using light microscopy, we studied the cellular basis of growth of Arabidopsis cotyledons during postembryonic development. • Analysis of serial paradermal sections of developing cotyledons revealed that early growth involved cell enlargement and some cell division. The daughter cells remained tightly packed in growing clusters, which enabled us to monitor both divisions and cell enlargement. • By contrast to other epigeal cotyledons, cells of all mesophyll layers could enter division. The cells in the cotyledon margins displayed a higher proliferative activity; however, the orientation of the division plane did not permit cells at the margins to give rise to tissue internally, nor to generate new cell layers. Exogenous cytokinin stimulated both cell expansion and cell division in attached dark-grown cotyledons. Light stimulated cell growth. Higher-levels of endogenous cytokinins in the amp1 mutant were confirmed to enhance the light-stimulated process of cell enlargement in cotyledons of dark-grown seedlings. • This work demonstrates an approach to use Arabidopsis cotyledons as an experimental system and provides a framework for molecular–genetic analysis of cellular dynamics during the growth of this organ.