The formation of leaves involves a transition from indeterminate growth in shoot apical meristems into a determinate growth programme. The indeterminacy of the shoot apical meristem is maintained by KNOX transcription factors; in different angiosperms, the initiation of leaf determinate growth requires that KNOX genes are negatively regulated by ARP proteins in leaf primordia. A similar KNOX–ARP mechanism operates during microphyll development in lycophytes, despite the independent origin of microphylls and megaphylls