Secondary Growth — Revision Notes

⚡ 30-Second Revision

Secondary Growth: — Increase in girth/diameter.
Lateral Meristems: — Vascular cambium, Cork cambium.
Vascular Cambium: — Between primary xylem & phloem. Produces secondary xylem (inwards) & secondary phloem (outwards).
Cork Cambium (Phellogen): — In cortex (stem) or pericycle (root). Produces phellem (cork, outwards) & phelloderm (secondary cortex, inwards).
Periderm: — Phellem + Phellogen + Phelloderm. Replaces epidermis.
Bark: — All tissues external to vascular cambium (secondary phloem + periderm).
Annual Rings: — Spring wood + Autumn wood = 1 year's growth. Formed by seasonal vascular cambium activity.
Heartwood (Duramen): — Central, dark, non-conducting, durable, supportive.
Sapwood (Alburnum): — Peripheral, light, conducting, living.
Lenticels: — Pores in bark for gas exchange.

2-Minute Revision

Secondary growth is the process by which dicotyledonous plants and gymnosperms increase their girth. It's driven by two lateral meristems: the vascular cambium and the cork cambium. The vascular cambium, a continuous ring formed from fascicular and interfascicular cambia in stems (or conjunctive parenchyma and pericycle in roots), actively divides to produce secondary xylem towards the inside (forming wood) and secondary phloem towards the outside.

Secondary xylem accumulates significantly, pushing primary xylem towards the center. The seasonal activity of the vascular cambium leads to distinct annual rings, comprising spring wood (lighter, wider vessels) and autumn wood (denser, narrower vessels), each ring representing one year's growth.

As wood ages, the central, non-conducting part becomes heartwood, while the peripheral, conducting part remains sapwood. Simultaneously, the cork cambium (phellogen) develops, typically in the cortex of stems or pericycle of roots.

It produces phellem (cork) outwards, which is protective and suberized, and phelloderm (secondary cortex) inwards. The phellem, phellogen, and phelloderm together form the periderm, which replaces the epidermis.

All tissues outside the vascular cambium constitute the bark. Lenticels are specialized pores in the periderm for gas exchange.

5-Minute Revision

Secondary growth is the increase in the diameter of stems and roots, characteristic of dicots and gymnosperms, providing structural support and enhanced transport. It's orchestrated by two lateral meristems: the vascular cambium and the cork cambium.

1. Vascular Cambium and its Products:

Formation: — In dicot stems, it forms from the fascicular cambium (within vascular bundles) and interfascicular cambium (from medullary rays). In dicot roots, it originates from conjunctive parenchyma and pericycle cells.
Activity: — It's a meristematic ring that divides periclinally. Cells cut off towards the inside differentiate into secondary xylem (wood), which is much more abundant and pushes primary xylem towards the pith. Cells cut off towards the outside differentiate into secondary phloem, which is less abundant and often crushed.
Annual Rings: — Due to seasonal variations in cambial activity (e.g., spring wood with wide vessels in favorable conditions, autumn wood with narrow vessels in unfavorable conditions), distinct annual rings are formed, each marking one year's growth.
Heartwood vs. Sapwood: — The older, central, non-conducting, dark, and durable secondary xylem is heartwood (duramen), providing support. The younger, peripheral, conducting, light-colored secondary xylem is sapwood (alburnum).

2. Cork Cambium (Phellogen) and Periderm:

Formation: — As the stem/root expands, the epidermis ruptures. The cork cambium (phellogen) develops, typically from the outer cortex in stems and the pericycle in roots.
Activity: — Phellogen divides to produce phellem (cork) towards the outside and phelloderm (secondary cortex) towards the inside.
Periderm: — The phellem, phellogen, and phelloderm collectively form the periderm, which replaces the epidermis as the protective outer layer. Phellem cells are dead and suberized, providing insulation and protection.
Bark: — All tissues external to the vascular cambium (secondary phloem + periderm) are collectively called bark.
Lenticels: — These are lens-shaped openings in the periderm, formed by localized phellogen activity, allowing for gaseous exchange through the otherwise impermeable cork layer.

Key Differences (NEET Focus):

Stem vs. Root: — Remember the distinct origins of vascular and cork cambia in stems vs. roots.
Primary vs. Secondary: — Primary growth increases length (apical meristems), secondary growth increases girth (lateral meristems).

This entire process ensures the plant's structural integrity, efficient transport, and protection as it grows larger and lives longer.

Prelims Revision Notes

Secondary growth is the increase in girth of dicot stems and roots, and gymnosperms. It is mediated by lateral meristems: vascular cambium and cork cambium.

I. Vascular Cambium:

Origin in Dicot Stem: — Formed by fascicular cambium (intrafascicular, within vascular bundles) and interfascicular cambium (from medullary ray cells between bundles), forming a complete ring.
Origin in Dicot Root: — Formed from conjunctive parenchyma (below phloem) and pericycle (outside protoxylem), initially wavy, then circular.
Activity: — Produces secondary xylem inwards (more abundant) and secondary phloem outwards (less abundant).
Secondary Xylem (Wood): — Composed of tracheids, vessels, xylem fibers, xylem parenchyma. Provides water conduction and mechanical support.

* Annual Rings: Formed by seasonal activity of vascular cambium. * Spring Wood (Early Wood): Formed in spring, wider vessels, thinner walls, lighter, less dense. * Autumn Wood (Late Wood): Formed in autumn/winter, narrower vessels, thicker walls, darker, denser.

* Heartwood (Duramen): Central, older, dark, non-conducting (vessels plugged with tannins, resins), durable, provides mechanical support. * Sapwood (Alburnum): Peripheral, younger, light, conducting, living parenchyma, less durable.

Secondary Phloem: — Composed of sieve tubes, companion cells, phloem parenchyma, phloem fibers. Conducts food. Often crushed.

II. Cork Cambium (Phellogen):

Origin in Dicot Stem: — Typically from outer cortical cells.
Origin in Dicot Root: — Typically from the pericycle.
Activity: — Divides to form phellem (cork) outwards and phelloderm (secondary cortex) inwards.
Phellem (Cork): — Outermost layer of periderm. Dead cells, suberized walls, impermeable to water/gases, protective.
Phelloderm (Secondary Cortex): — Innermost layer of periderm. Living parenchymatous cells.
Periderm: — Collective term for phellem + phellogen + phelloderm. Replaces epidermis as protective tissue.
Bark: — All tissues external to the vascular cambium (secondary phloem + periderm).
Lenticels: — Lens-shaped openings in periderm, formed by phellogen cutting off parenchymatous cells. Facilitate gaseous exchange.

III. Monocots: Generally lack secondary growth due to scattered, closed vascular bundles and absence of a continuous vascular cambium. Some show anomalous thickening.

Vyyuha Quick Recall

Very Clever Cambium Produces Secondary Xylem Inside, Secondary Phloem Outside. Cork Cambium Protects Plants With Periderm.