<P> The other three terms are used where there is more than one strand of primary xylem . </P> <Ul> <Li> Exarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the outside inwards towards the center, i.e. centripetally . The metaxylem is thus closest to the center of the stem or root and the protoxylem closest to the periphery . The roots of vascular plants are normally considered to have exarch development . </Li> <Li> Endarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the inside outwards towards the periphery, i.e. centrifugally . The protoxylem is thus closest to the center of the stem or root and the metaxylem closest to the periphery . The stems of seed plants typically have endarch development . </Li> <Li> Mesarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the middle of a strand in both directions . The metaxylem is thus on both the peripheral and central sides of the strand with the protoxylem between the metaxylem (possibly surrounded by it). The leaves and stems of many ferns have mesarch development . </Li> </Ul> <Li> Exarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the outside inwards towards the center, i.e. centripetally . The metaxylem is thus closest to the center of the stem or root and the protoxylem closest to the periphery . The roots of vascular plants are normally considered to have exarch development . </Li> <Li> Endarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the inside outwards towards the periphery, i.e. centrifugally . The protoxylem is thus closest to the center of the stem or root and the metaxylem closest to the periphery . The stems of seed plants typically have endarch development . </Li>

Where is active xylem found in a mature tree