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). [source: ISBN:0471245208, POC:Laurel_Cooper]