root system exarch protoxylem diarch protoxylem Laurel_Cooper protoxilema diarco (Spanish, exact) plant_anatomy 二原型原生木部 (Japanese, exact) PO:0025559 A portion of root exarch protoxylem (PO:0025555) tissue in which the primary xylem (PO:0005849) of the root system (PO:0025025) has two protoxylem strands or poles, and differentiation progresses centripetally, with the oldest elements (protoxylem) farthest from the center of the plant axis (PO:0025004). PO_GIT:534 2013-04-24T15:01:31Z