Primary xylem

  • During secondary growth, the primary xylem persists in centre of Axis?

    Why doesn't it get crushed like primary phloem? Please explain.

  • @anag
    Secondary growth in stem:

    The vascular cambium, located between the primary xylem and primary phloem of a vascular bundle, is also known as the intra-fascicular cambium. When the stem is young, the vascular cambium is present as a patchy, single layer. However, in the later stages, it develops into a complete ring also known as the cambial ring.

    Formation of cambial ring

    *During the secondary growth of a stem, there occurs differentiation in the cells of the medullary rays. *These cells, located next to the intra-fascicular cambium, become meristematic and form the inter-fascicular cambium.

    *The fusion of intra-fascicular cambium and the inter-fascicular cambium results in the formation of a continuous ring of cambium or the cambial ring.

    • The cambial ring has primary xylem on its inner surface and primary phloem on its outer surface.

    *Cell division takes place on both the sides of the cambial ring which results in the formation of secondary xylem on the inner surface and secondary phloem on the outer surface.

    **As the cambium is more active on the inner side, the formation of secondary xylem is more than secondary phloem.

    *Due to the continuous formation of secondary xylem, primary phloem and secondary phloem get crushed.

    Primary xylem remains in the centre.

    *The cambium forms a narrow band of parenchyma at some places. This band passes through the secondary xylem and the secondary phloem in radial directions and forms secondary medullary rays. *The secondary xylem tissues formed during cambial activity give rise to commercial wood.

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