🌳 Secondary Growth in Plants

The big idea:

  • Primary growth → increase in length (height/depth) → due to apical meristems at tips.
  • Secondary growth → increase in girth/thickness → due to lateral meristems (on the sides).

👉 Lateral meristems involved:

  • Vascular cambium → makes secondary xylem & secondary phloem.
  • Cork cambium (phellogen) → makes cork (phellem) and secondary cortex (phelloderm).

📌 Where does secondary growth occur?

  • Typical in dicot stems and dicot roots.
  • Also present in gymnosperms.
  • Generally absent in monocots.

🌿 1️⃣ Vascular Cambium and Secondary Growth

The vascular cambium is the meristematic layer responsible for formation of secondary xylem (wood) and secondary phloem.

🔹 Origin of Cambium

In a young dicot stem (like sunflower):

A. Intrafascicular cambium (inside bundles)

  • Present within each vascular bundle, between primary xylem and primary phloem.
  • This cambium is primary in origin and is already there in young stem.

B. Interfascicular cambium (between bundles)

  • Initially, the region between two vascular bundles contains medullary rays (parenchyma).
  • These ray cells become meristematic (start dividing) and form interfascicular cambium.

C. Cambial Ring

  • Intrafascicular (inside bundles) + Interfascicular (between bundles) cambium join → form a continuous ring called the cambial ring.

👉 Memory line: “Intra + Inter = Cambium Ring”

🔸 Cambial Activity

Once the cambial ring is formed, it becomes very active and divides in two directions:

  • Towards inside (pith side) → forms secondary xylem.
  • Towards outside (cortex side) → forms secondary phloem.

Important points:

  • Cambium is more active towards the inside → more secondary xylem than phloem.
  • Primary phloem is gradually crushed and pushed outwards.
  • Primary xylem remains more or less intact near the centre.
  • At some places, cambium cuts off parenchyma cells in radial rows → these form secondary medullary rays, which pass through both secondary xylem and phloem.

👉 Function of secondary medullary rays:

  • Help in radial (sideways) transport of water and food.

🌲 Early Wood and Late Wood (Spring Wood & Autumn Wood)

Cambial activity is not uniform throughout the year (especially in temperate regions):

A. Spring / Early in the growing season:

  • Cambium is highly active → produces large, thin-walled xylem vessels.
  • Wood is lighter, less dense.
  • Called Early wood / Spring wood.

B. Autumn / Later in the growing season:

  • Cambium is less active → produces small, thick-walled xylem vessels.
  • Wood is darker, denser.
  • Called Late wood / Autumn wood.
Growth Season Wood Type Characteristics
Spring Early wood (spring wood) Large vessels, thin walls, less dense, light coloured
Autumn Late wood (autumn wood) Small vessels, thick walls, more dense, dark coloured

🌲 2️⃣ Annual Rings and Growth Measurement

  • In one year, cambium produces one layer of early wood + one layer of late wood.
  • Together they form one annual ring.
  • In temperate regions (clear seasons), these rings are clearly visible in a cross section of a tree trunk.

👉 Counting the number of annual rings in the stem can give an estimate of the age of the tree.

  • The scientific study of tree rings and age is called Dendrochronology.

🌳 3️⃣ Secondary Growth in Root

In a dicot root, the vascular cambium is entirely secondary in origin.

A. Origin of Cambium in Root

  • The root has radial vascular bundles (xylem and phloem in separate patches).
  • Cambium arises from:
  • Conjunctive tissue (parenchyma between xylem and phloem) just below each phloem patch.
  • Pericycle cells located above the protoxylem.
  • These cambial cells join to form a wavy cambial ring, which later becomes circular.

B. Activity of Cambium in Root

  • Exactly like in stems:
  • Secondary xylem is formed towards the inside.
  • Secondary phloem is formed towards the outside.
  • This leads to increase in thickness of the root.
  • Secondary growth is present in dicot and gymnosperm roots, but absent in monocot roots.

👉 Key idea: In both stem and root, vascular cambium does the same job: makes secondary xylem inward and secondary phloem outward.

🌿 4️⃣ Cork Cambium and Bark Formation

As the stem keeps getting thicker due to secondary growth, the outer tissues (epidermis, cortex) cannot stretch indefinitely. They rupture and are replaced.

A. Cork Cambium (Phellogen)

  • A new lateral meristem called cork cambium or phellogen arises.
  • It usually develops in the outer cortex.
  • Phellogen is a thin, meristematic layer, only a few cells thick.

B. Activity of Cork Cambium

Phellogen divides both ways:

A. Outwards (towards periphery) → forms cork (phellem)

  • Cells are dead, compactly arranged.
  • Walls heavily deposited with suberin (waxy substance).
  • Impermeable to water and gases → gives protection.

B. Inwards (towards centre) → forms secondary cortex (phelloderm)

  • Made of living parenchyma.
  • Can store food and help in minor functions.

C. Periderm

  • Combination of:
  • Phellogen (cork cambium)
  • Phellem (cork)
  • Phelloderm (secondary cortex)
  • Together called the Periderm → protective outer covering formed during secondary growth.

D. Bark

  • Bark is a non-technical term.
  • It includes all tissues outside the vascular cambium, usually:
  • Secondary phloem
  • Periderm (phellogen, phellem, phelloderm)
  • Bark formed early in the growing season → soft bark.
  • Bark formed later in the season → hard bark.

👉 Memory: "Cambium = inner tissues (xylem & phloem), outside cambium = bark".

E. Lenticels

  • At some places, instead of producing compact cork cells, the phellogen produces loosely arranged parenchyma cells with intercellular spaces.
  • These cells rupture the epidermis and form small, raised, lens-shaped openings called lenticels.

🔹 Function of lenticels:

  • Allow gaseous exchange (O₂ and CO₂) between the internal living tissues and the outside air, especially when stomata are closed.
Tissue Direction of Formation (by phellogen) Nature
Phellem (cork) Outward Dead, suberised, protective
Phellogen — Meristematic cambium layer
Phelloderm (secondary cortex) Inward Living parenchyma

🌲 5️⃣ Heartwood and Sapwood

As the tree becomes older, the inner parts of secondary xylem undergo changes.

A. Sapwood

  • Outer, lighter-coloured part of secondary xylem.
  • Functionally active:
  • Conducts water and minerals from roots to leaves.
  • Contains living parenchyma cells.

B. Heartwood

  • Inner, darker-coloured part of secondary xylem.
  • Non-functional in conduction.
  • Cells of heartwood are filled with tannins, resins, oils, gums, etc.
  • Provides mechanical strength and rigidity.
  • More resistant to decay and attack by microbes/insects.
Feature Heartwood Sapwood
Position Central, inner region Peripheral, outer region
Function No conduction; gives mechanical support Actively conducts water & minerals
Colour Dark Light
Cells Dead, filled with deposits Relatively living (parenchyma present)
Durability Hard, durable, resistant to decay Softer, less durable

👉 Memory trick:

  • Sapwood = “Service wood” (still working in conduction).
  • Heartwood = “Hardwood” (support and strength).

💡 Questions and Answers

Q1. Why does secondary growth not occur in monocots?

A1.

  • Secondary growth requires lateral meristems such as vascular cambium and cork cambium.
  • In most monocots, there is no vascular cambium between xylem and phloem in stems and roots (vascular bundles are closed).
  • Therefore, they cannot produce secondary xylem and secondary phloem, and secondary growth is generally absent.

Q2. What is the function of cork cambium?

A2.

  • Cork cambium (phellogen) is a lateral meristem that arises in the cortex.
  • It produces:
  • Cork (phellem) towards the outside, which is dead, suberised and protective.
  • Secondary cortex (phelloderm) towards the inside, which is living parenchyma.
  • Together with cork and phelloderm, it forms the periderm, which replaces the ruptured epidermis and cortex and protects the stem.

Q3. How are annual rings useful?

A3.

  • Each annual ring = one layer of early (spring) wood + one layer of late (autumn) wood.
  • By counting the number of annual rings in a stem cross-section, we can estimate the age of the tree.
  • The width of rings can also indicate favourable or unfavourable climatic conditions (wide rings = good growth years, narrow rings = poor growth years).

Q4. What are the two lateral meristems responsible for secondary growth?

A4. The two lateral meristems are:

  1. Vascular Cambium
  • Lies between xylem and phloem.
  • Produces secondary xylem (inside) and secondary phloem (outside).
  • Responsible for increase in thickness of the vascular cylinder.
  1. Cork Cambium (Phellogen)
  • Arises in the cortex.
  • Produces cork (phellem) outward and secondary cortex (phelloderm) inward.
  • Forms periderm, which replaces the outer ruptured tissues and provides protection.

Q5. Differentiate between Heartwood and Sapwood.

A5.

Feature Heartwood Sapwood
Location Central, inner region of secondary xylem Outer, peripheral region of secondary xylem
Function Non-functional in conduction; gives mechanical strength Functionally active; conducts water and minerals
Colour Dark coloured Light coloured
Cell condition Cells filled with resins, tannins, oils; mostly dead Contains more living parenchyma
Durability Hard, resistant to decay and insects Less durable, more prone to decay

Q6. What are annual rings? What is their significance?

A6.

  • An annual ring is the ring of wood formed in one growing season, consisting of a zone of early (spring) wood and a zone of late (autumn) wood.
  • In regions with distinct seasons, these rings appear as concentric circles in a cross-section of a tree trunk.
  • Significance:
  • Help in estimating the age of the tree by counting the rings.
  • Give information about past climatic conditions (thicker rings = favourable conditions; thinner rings = unfavourable conditions).