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Secondary Growth in Dicot Stem - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:34 AM | #NEET

Quick Facts

  • Secondary Growth in Intrastelar Region of Dicot Stem, Secondary Growth in Intrastelar Region of Dicot Stem - Heartwood and Sapwood is considered one of the most asked concept.

  • 53 Questions around this concept.

Solve by difficulty

Age of a tree can be estimated by:

Given below are two statements : One is labelled as Assertion A and the other is labelled as Reason R:

Assertion A : Late wood has fewer xylary elements with narrow vessels.

Reason R : Cambium is less active in winters. In the light of the above statements, choose the correct answer from the options given below :

Identify the wrong statement in context of heartwood:

Concepts Covered - 4

Secondary Growth in Intrastelar Region of Dicot Stem

Secondary Growth in Intrastelar Region of Dicot Stem

  • The vascular bundles present in the Dicot stem are conjoint, collateral and open.
  • That means, they have a fascicular or intrafascicular cambium present the primary xylem and primary phloem.
  • The intrafascicular cambium is primary meristem having lateral position in the plant body.
  • Events During the Intrastelar Secondary Growth in Dicot Stem:

Formation of Cambium Ring

  • The intrafascicular cambium gets activated to perform meristematic activity.
  • The parenchymatous cells of the pith rays that are present adjacent to the intrafascicular cambium dedifferentiate to become meristematic.
  • The meristem so formed through the dedifferentiation of the cells of the pith rays is called secondary meristem or interfascicular meristem.
  • The interfascicular meristem joins with the strip of intrafascicular meristem to form a complete cambium ring or vascular cambium.

 

2. Formation of Secondary Vascular Tissues

  • The vascular cambium produces secondary xylem towards inside, that is, towards the pith.
  • It produces secondary phloem towards the outside.
  • The continuous formation of secondary xylem crushes the primary phloem. The primary xylem remains intact at some places.
  • The vascular cambium also produces radial parenchymatous cells that passes through the secondary xylem and secondary phloem. These are called secondary medullary rays.
  • The secondary xylem has axial and radial (horizontal) system while radial system is absent in primary xylem.
  • Primary xylem has long vessels and tracheids as compared to secondary xylem.
     

Secondary Growth in Intrastelar Region of Dicot Stem - Annual Rings

Secondary Growth in Intrastelar Region of Dicot Stem - Annual Rings

  • The activity of the cambium is markedly affected by environmental fluctuations.     
  • In temperate regions, these fluctuations have a great role in defining the activity of the cambium. 
  • During the spring season, the activity of thee cambium increases. 
  • It produces a large number of xylary elements with wider lumen.
  • The wood formed during this season is called spring wood or early wood.
  • During the autumn season, the activity of cambium reduces.
  • It produces narrow vessels in secondary xylem. A large amount of xylem fibres are produced.
  • The wood formed during the autumn season is called autumn wood or late wood.
  • The early wood and late wood occur in the form of two concentric rings, forming a single annual ring.
  • An annual ring is called so because it is produced in a span of a year.
  • The annual rings reflect the age of trees.
  • Determination of age of tree by counting the annual rings in called dendrochronology.
  • The activity of cambium is also affected by disease, drought, defoliation, etc. In such conditions false rings can be observed.
  • In tropical trees, there is no annual rings formation because of uniformity in climate.
  • In roots, the annual rings are not visible because inside the soil the temperature remains constant throughout the year.
  • The roots of Salix are an exception because the annual rings are visible in them.
     

 

 

 

Secondary Growth in Intrastelar Region of Dicot Stem - Heartwood and Sapwood

Secondary Growth in Intrastelar Region of Dicot Stem - Heartwood and Sapwood

 

  • When a transverse section of a woody trunk is taken, two regions are distinctly visible.
  • In the centre, there is dark coloured wood called heartwood. 
  • Heartwood is made up of dead and non-functional components of wood. 
  • Heartwood is also called duramen.
  • The lumen of vessels in the heartwood has been blocked by tyloses.
  • Tyloses are balloon-like structures formed due to protrusion of xylem parenchyma cells into the lumen of vessels.
  • These parenchyma cells become lignified and dead, thereby making the vessels non-functional.
  • In the tracheids of gymnosperms, the tylosoids are present.
  • The heartwood is durable and resistant to parasitic and microbial attack. 
  • It gives mechanical support to the tree trunk.
  • It appears dark due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils.
  • In the periphery of the tree trunk, surrounding the heartwood, there is lighter colored wood made up of functional xylem elements.
  • This is called sapwood. It is involved in the conduction of sap. 
  • Sapwood is also called alburnum.
     

Secondary Growth in Extra Stelar Region

Secondary Growth in Extra Stelar Region

  • The continuous secondary growth in the intrastelar region adds more and more secondary xylem and secondary phloem.
  • This exerts pressure on the tissues lying towards the outer side, that is, cortex and epidermis.
  • The epidermis ruptures due to the exertion of the pressure and the underlying tissues are exposed to the environment.
  • At this stage, a secondary tissue called periderm develops.
  • Periderm develops through the activity of cork cambium or phellogen.
  • Cork cambium or phellogen is the secondary meristem that develops generally from the collenchymatous cells of hypodermis, sometimes from the cells of the epidermis (apple), pericycle (Cimatis), phloem (vitis).
  • Phellogen gives rise to cork or phellem on the outer side and secondary cortex of phelloderm on the inner side.
  • Phellem or cork is the dead tissue with suberised cell walls. It is impermeable to water and is used commercially. 
  • Quercus suber is commercial cork.
  • Secondary cortex or phelloderm is living tissue. It stores food and replaces the damaged primary cortex. 
  • Phellem, phellogen and phelloderm are called periderm.
  • Periderm and fragments of epidermis form the protective tissues in an old dicot stem.

Bark:

  • This term is used to refer to all the tissues outside the vascular cambium.
  • It includes secondary phloem, periderm and epidermis.
  • Removal of bark , called the ringing experiment, causes serious injury to the plant because it removes the secondary phloem which restricts the conduction of food.
  • Roots die first in the ringing experiment.

Rhytidome:

  • All the tissues outside the cork cambium is called rhytidome
  • It consists of cork and epidermis.

 

Study it with Videos

Secondary Growth in Intrastelar Region of Dicot Stem
Secondary Growth in Intrastelar Region of Dicot Stem - Annual Rings
Secondary Growth in Intrastelar Region of Dicot Stem - Heartwood and Sapwood

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