Secondary growth in Roots

  • Secondary growth in thickness of root is due to the addition of new tissues by the activity of vascular cambium and cork cambium.
  • Monocot roots do not show secondary growth.
  • Dicot roots have radial and exarch vascular bundles.
  • Conjunctive tissue is present between xylem and phloem.

Activity of Cambium

  • The conjunctive tissue becomes meristematic and forms a small strip of cambium just below the primary phloem.
  • At the same time, cells of pericycle present outside protoxylem also become meristematic and forms small cambial strips.
  • These two cambial strips join together and form a wavy band of cambium.

  • The cambial cells below the phloem become active first. It cuts off cells to either side, with more cells on the inner side.
  • This causes the phloem and cambium is pushed outwards.
  • Hence the wavy cambium gradually becomes a circular cambium.
  • Then it forms secondary xylem and phloem tissues.
  • But, it does not form any annual rings.

Periderm formation

  • The cells of pericycle become meristematic and forms secondary meristems called cork cambium or phellogen.
  • These cells cut off phellem towards the outer region and phelloderm towards the inner region.
  • These three tissues together form the outer protective layer of a root known as the periderm.

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