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In page Xylem:

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There are four primary patterns to the arrangement of protoxylem and metaxylem in stems and roots.

  • Centrarch refers to the case in which the primary xylem forms a single cylinder in the center of the stem and develops from the center outwards. The protoxylem is thus found in the central core, and the metaxylem is in a cylinder around it.[1] This pattern was common in early land plants, such as "rhyniophytes", but is not present in any living plants.[citation needed]