Anatomy of Flowering Plants

Secondary Growth

  • Secondary growth in dicot root:
  • Secondary growth is found only in dicots which enhances secondary lateral meristem to increase the circumference / girth of a plant. Exception: Palm, Yucca, smilax, coconut.
  • Stage I : Conjunctive parenchyma becomes meristematic and give rise to secondary Xylem and Phloem.
  • Stage II : Conjunctive parenchyma cells form a wavy band.
  • Stage III: Primary Vascular rays / medullary rays from pericycle is formed opposite to protoxylem ends.
  • Stage IV: Ring of vascular cambium gives secondary Xylem and Phloem.
  • Secondary growth in dicot stem:
  • Secondary vascular tissues are formed by vascular cambium that is produced by Fascicular and Interfascicular cambium.
  • Vascular cambium undergoes additive and multiplicative division to undergo dilation. [Movement of Vascular cambium gradually outside by adding new cells].
  • 2 cells of vascular cambium (i) Ray initials and (ii) Fusiform initials which develop secondary Xylem inside and secondary phloem out side.
  • Secondary Xylem forms the bulp of the wood, where it has no distinction into proto and meta xylem.
  • Width of 2nd Xylem grows with the age.
  • Yearly growth is distinct in the areas with 2 seasons.
Soft wood(gymnosperm wood) Hard wood (dicot wood)
→ devoid of vessels → Porous wood two types
→ Non porous → (i) Ring porous. → Eg: Cassia fistula,
→ Easy porous → (ii) Diffuse porous. Eg: Sihyzinium cumini
→ Eg: Cedrus, Pinus  
  •  Sap wood: → Outer light coloured
    Heart wood: → Inner dark coloured
  • Annual / yearly growth appear in the form of annual ring. Shedy of age of trees with regard to the annual rings is called dendrochronology.
  • Periderm formation = Phellogen → (aerating pore) + Phellem + Phelloderm
  • All dead cells lying outside phellogen is called bark [which acts as insect repellent / fire proof] which is of 2 types (i) hard / soft.

View this video for the topic from 3:50 to 54:48

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