Anatomy of Flowering Plants - Secondary growth

Secondary growth refers to the increase in the girth (diameter) of the plant axis, primarily occurring in dicotyledonous stems and roots due to the activity of lateral meristems: Vascular Cambium and Cork Cambium.

The Vascular Cambium develops from the intrafascicular cambium (present between primary xylem and phloem) and interfascicular cambium (formed from medullary ray cells). Together they form a continuous $Cambial\ Ring$.

Activity of the Cambial Ring: The cambium cuts off cells on both sides. Cells produced towards the pith differentiate into $Secondary\ Xylem$, while cells produced towards the periphery differentiate into $Secondary\ Phloem$.

Growth Rings: In temperate regions, cambial activity varies with seasons. Spring wood (early wood) has wider vessels produced during favorable conditions, while Autumn wood (late wood) has narrower vessels. One spring wood ring and one autumn wood ring together constitute an $Annual\ Ring$, used in $Dendrochronology$.

Heartwood ($Duramen$): The central, non-functional, dark-colored part of the secondary xylem, filled with tannins, resins, and oils, making it resistant to pests and decay.

Sapwood ($Alburnum$): The peripheral, functional, light-colored part of the secondary xylem involved in the conduction of water and minerals.

Cork Cambium ($Phellogen$): Arises in the cortical region to provide protection. It produces $Phellem$ (cork) towards the outside and $Phelloderm$ (secondary cortex) towards the inside.

Periderm: A collective term for the three layers formed by the cork cambium, expressed as $Periderm = Phellem + Phellogen + Phelloderm$.

Bark: A non-technical term referring to all tissues exterior to the vascular cambium, which includes $Secondary\ Phloem$ and $Periderm$.

Lenticels: Lens-shaped openings in the bark that permit gas exchange between the internal tissues of the stem and the atmosphere.