Merriam-Webster Collegiate Dictionary - exarch

 I. noun  Etymology: Late Latin ~us, from Late Greek ~os, from Greek, leader, from ~ein to begin, take the lead, from ex- + archein to rule, begin — more at arch-  Date: 1588  1. a Byzantine viceroy  2. an Eastern bishop ranking below a patriarch and above a metropolitan; specifically the head of an independent church  • ~al adjective  • ~ate noun  • ~y noun  II. adjective  Etymology: exo- + -arch  Date: 1891 formed or taking place from the periphery toward the center ~ xylem