plas·to·chron \ˈplastəˌkrän\noun (-s) Etymology:plasto- + Greek chronos time : a unit of time corresponding to the interval between two successive similar, periodically repeated events (as the emergence of leaf primordia at any two successive nodes in a stem apex) • plas·to·chron·ic\| ̷ ̷ ̷ ̷|kränik\adjective