Question:

Bark refers to

Updated On: Jul 28, 2022
  • phellem + phellogen + phelloderm
  • periderm + cortex
  • phellem + phelloderm + secondary phloem
  • periderm + cortex + pericycle + secondary phloem
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The Correct Option is D

Solution and Explanation

Bark is a loose term and is used to define all the tissues outside vascular cambium. It is differentiated into outer bark or rhytidome (consisting of dead cells) and inner bark (of living cells, i.e., periderm, cortex, pericycle and secondary phloem). The outer layers of the bark are being constantly peeled off on account of the formation of new secondary vascular tissues in the interior. Bark is insect repellent, decay proof, fire-proof and acts as a heat screen.
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Concepts Used:

Secondary Growth

Secondary growth is an increase in girth of thickness of the axis due to activity of lateral meristem. The tissues involved in secondary growth are two lateral meristems are vascular cambium and cork cambium.

Secondary Growth in Stem:

Formation of Cambium Ring-

  1. The cambium present between the xylem and phloem in vascular bundles is called Fascicular or Interfascicular cambium.
  2. In secondary growth, few medullary ray cells that are located between vascular bundles also become meristematic, and it is known as Interfascicular cambium.

Formation of the secondary tissues-

  1. The cambium ring cuts off cells on both sides. It produces secondary phloem outwardly and secondary xylem inwardly.
  2. The amount of secondary xylem cut off is more than the secondary phloem.

Secondary Growth in Root:

Vascular Cambium-

  1. The cambium is absent initially but develops later at the time of secondary growth.
  2. The pericycle cells lying outside the protoxylem and the conjunctive parenchyma cells on the lateral side of the phloem bundle become meristematic to form many cambial strips.

Cork Cambium-

  1. The cork cambium develops because of the tangential division of cells of the pericycle.
  2. The activity of cork cambium is like that found in the dicot stem, so it produces cork cells or phellem towards the outside and phelloderm or secondary cortex towards the inside.

Abnormal secondary growth:

  1. Due to absence of the cambium in monocots, monocots don’t show secondary growth and the vascular system is totally composed of primary tissues.
  2. The vascular bundles are irregularly scattered in the ground tissues, such that the cortex and other ground tissues can’t be differentiated.
  3. Dracaena, Yucca, Agave are examples of monocots that exhibit a peculiar type of secondary growth in thickness.