The procambium is of two types the xyloic procambium and the phloic procambium. The xyloic procambium give rise to the xylem and the phloic procambium forms the phloem. Usually the phloem elements matures first and it develops in the acropetal manner. But the xylem elements develop both in the acropetal and basipetal pattern. The differentiation of the phloem elements occurs centripetally towards the center of the axis, whereas the xylem shows three different pattern of differentiation. They are:
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Exarch: The exarch pattern of differentiation occurs in the centripetal manner that is towards the axis, so that the initial xylem elements are located far away from the axis. It is the characteristic feature of the root.
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Endarch: The endarch pattern of differentiation occurs in the centrifugal manner that is towards the periphery, so that the initial xylem elements are located nearer to the axis. It is the characteristic feature of the stem.
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Mesarch: The mesarch pattern of differentiation proceeds in two directions that is both centrifugally and centripetally. The protoxylem is found at the center and the metaxylem is seen towards the periphery. This is the characteristic feature of the certain fern rachis.
