Secondary Growth in Dicot Stem & Root Notes | Free Biology Notes

Secondary Growth in Dicot Stem & Root Notes

This article we will discuss about Secondary Growth in Dicot Stem & Root Notes

  • By the activity of lateral meristems, formation of secondary tissue which leads to increase in girth is called secondary growth
  • Two types of lateral meristems involved in secondary growth:- vascular cambium and cork cambium
  • Secondary growth takes place in root and stem of dicotyledons and gymnosperms
  • Secondary growth in stellar region due to activity of the vascular cambium
  • Secondary growth in extra stellar region due to activity of cork cambium

Secondary Growth in Dicot Stem

Secondary Growth in Dicot Stem

Secondary growth in stelar region by vascular cambium

Formation of cambium ring

  • Cambium is present between xylem and phloem. It is known as intrafascicular cambium
  • During secondary growth, some cells of medullary rays become active and form a strip of cambium in between vascular bundles called interfascicular cambium
  • Both the intrafascicular and interfascicular cambium unite together to form a complete ring called the cambium ring
  • Two types of cells are found in cambial ring:- fusiform initials & ray initials

Activity of cambial ring

I) Fusiform initials

  • Numerous cells formed towards the centre and periphery regions
  • Cells in centre mature into secondary xylem & cells in periphery mature into the secondary phloem
  • Secondary xylem is formed 8-10 times more as compared to secondary phloem

II) Ray initials

  • It’s give rise to parenchyma cells is called secondary medullary rays
  • Development of vascular rays are both centripetal and centrifugal
  • They conduct water and food in radial direction

Formation of annual rings

  • Activity of cambium is affected by variations in temperature

In spring season

  • Vascular cambium is very active and produces large number of secondary xylem
  • Secondary xylem elements having vessels with wider cavities
  • Wood formed during this season is called spring wood or early wood
  • Spring wood is lighter in colour and has lower density

In autumn season

  • Vascular cambium is less active and forms fewer secondary xylem
  • Secondary xylem elements having vessels with narrow cavities
  • Wood formed during this season is called autumn wood or late wood
  • Autumn wood is darker in colour and has higher density
  • Ring of autumn wood & spring wood are collectively known as annual ring
  • The number of annual rings formed in a tree give the idea of age of the tree
  • The study of determination of age of a tree is called dendrochronology
  • Clear annual rings are not formed in India except Himalayan regions

Formation of heart wood and sap wood

  • In old trees where sufficient amount of secondary growth has taken place
    the secondary wood of inner side lose the power of conduction
  • Their cells get filled with tannins, resins, gums, essential oils
  • These cells makes the plant part hard and darker called the heart wood or duramen
  • Heart wood ceases the function of conducting tissue and simply provides mechanical support
  • The outer region of secondary wood, which consists of younger living xylem cells, remains yellow in colour called the sap wood or laburnum
  • It functions as the conducting tissue and also as the food storage tissue

Secondary growth in extra stellar region by cork cambium

  • Cork cambium is also known as phellogen or extrastelar cambium
  • Cork cambium is develops from cortex by dedifferentiation
  • It’s form new cells towards outside and towards inside
  • Those cells which are formed towards outside, become suberized.
  • These cells become dead cells are known as cork or phellem
  • Those cells which are formed inside, are differentiated into parenchyma. These are called secondary cortex or phelloderm
  • Phellem, phellogen and phelloderm are collectively known as periderm
  • Cork cambium is more active towards outside, cork is formed in high quantity and secondary cortex is less quantity
  • At certain regions, the cork cambium cuts off and forms parenchymatous cells on outer side instead of cork cells
  • These parenchymatous cells are not suberized. These cells are called complementary cells
  • Complementary cells increase in number, pressure is exerted on the epidermis due to which it ruptures, forming lens shaped openings is called lenticels & Its helps exchange of gases and transpiration

Secondary Growth in Dicot Stem & Root

Secondary growth in dicot root

  • Secondary growth in dicot roots is essential to provide strength
  • It is similar to secondary growth in dicot stem
  • However, there is marked difference is manner of the formation of vascular cambium

Origin and activity of vascular cambium

  • Vascular cambium is completely secondary in origin
  • It originates from conjunctive tissue located just below the phloem and as a portion of pericycle tissue present above the protoxylem
  • Its form a complete and continuous wavy ring
  • This wavy ring produces secondary xylem and secondary phloem similar to the secondary growth in stems

Origin and activity of cork cambium

  • Cork cambium is developed from pericycle in roots
  • Cork is formed towards the outside and secondary cortex is formed towards inner side by the cork cambium
  • Lenticels are also found in roots but less in number
  • Clear annual rings are not seen in root because root are not affected by the changes of environment
  • In dicot roots all cambia and pith rays are secondary in origin

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