In seed plants, leaves are born on radial shoots, but unlike shoots, they are determinate dorsiventral organs made of flat lamina. YABBY genes are found only in seed plants and in all cases studied are expressed primarily in lateral organs and in a polar manner. Despite their simple expression, Arabidopsis thaliana plants lacking all YABBY gene activities have a wide range of morphological defects in all lateral organs as well as the shoot apical meristem (SAM). Here, we show that leaves lacking all YABBY activities are initiated as dorsiventral appendages but fail to properly activate lamina programs. In particular, the activation of most CINCINNATA-class TCP genes does not commence, SAM-specific programs are reactivated, and a marginal leaf domain is not established. Altered distribution of auxin signaling and the auxin efflux carrier PIN1, highly reduced venation, initiation of multiple cotyledons, and gradual loss of the SAM accompany these defects. We suggest that YABBY functions were recruited to mold modified shoot systems into flat plant appendages by translating organ polarity into lamina-specific programs that include marginal auxin flow and activation of a maturation schedule directing determinate growth.