🌱 Secondary Growth in Plants

Definition:
Secondary growth is the increase in the girth (thickness/diameter) of stems and roots in plants, caused by the activity of lateral meristems (vascular cambium and cork cambium). It is most prominent in dicotyledonous plants and gymnosperms but absent in most monocots.

1. Significance of Secondary Growth

Provides mechanical support to tall plants.
Increases the capacity of the plant to conduct water, minerals, and food.
Helps plants survive for many years (perennial growth).
Produces wood (xylem) and bark (protective tissues).

2. Meristems Involved

Vascular Cambium

A lateral meristem located between primary xylem and primary phloem.
Produces secondary xylem (wood) towards the inside and secondary phloem towards the outside.

Cork Cambium (Phellogen)

A lateral meristem found in the outer cortex.
Produces cork (phellem) on the outer side and secondary cortex (phelloderm) on the inner side.

3. Process of Secondary Growth in Stems

(a) Formation of Vascular Cambium Ring

In dicot stems, fascicular cambium (present within vascular bundles) and interfascicular cambium (between vascular bundles) join to form a complete ring of vascular cambium.

(b) Activity of Vascular Cambium

Secondary Xylem: Formed towards the inside. Thick and lignified, contributes to wood.
Secondary Phloem: Formed towards the outside. Conducts food.
More xylem is produced than phloem → stem thickens unevenly.
Old phloem gets crushed, while xylem accumulates and forms bulk of wood.

(c) Formation of Annual Rings

In temperate regions, cambium is more active in spring (producing large vessels: spring wood) and less active in autumn/winter (producing small vessels: autumn wood).
Together, spring and autumn wood form an annual ring, used to determine the age of a tree (Dendrochronology).

4. Secondary Growth in Roots

In dicot roots, secondary growth is similar to stems.
The vascular cambium arises from the pericycle (outside xylem) and becomes a wavy ring.
Produces secondary xylem inside and phloem outside → increases root thickness.

5. Activity of Cork Cambium

Cork cambium (phellogen) cuts off:
Cork (phellem) outside → dead, suberized cells, impermeable to water and gases.
Secondary cortex (phelloderm) inside → living parenchyma cells.
Cork + Cork Cambium + Secondary Cortex = Periderm.
Provides protection against mechanical injury, water loss, and pathogens.

6. Special Features of Secondary Growth

Heartwood vs Sapwood:

Heartwood: Central, dark-colored, non-functional xylem (for strength).
Sapwood: Outer, lighter-colored, functional xylem (for conduction).
Lenticels: Small pores in cork that allow gaseous exchange.
Bark: All tissues outside the vascular cambium (secondary phloem + periderm).

7. Secondary Growth in Monocots

Most monocots lack secondary growth due to absence of vascular cambium.
However, some (e.g., Dracaena, Yucca, Palm) show anomalous secondary growth through activity of a special cambium.