Secondary growth in dicot stems and roots

1. Introduction to Secondary Growth

1. Secondary growth refers to the increase in the girth (thickness) of a plant due to the activity of lateral meristems.
2. It occurs in dicotyledonous stems and roots, but is absent in monocots.
3. Involves two types of meristematic tissues: vascular cambium and cork cambium.
4. Leads to the formation of secondary xylem, secondary phloem, and periderm (bark).

2. Secondary Growth in Dicot Stems

Formation of Vascular Cambium

1. The vascular cambium develops from:
   • Fascicular cambium (inside vascular bundles).
   • Interfascicular cambium (between vascular bundles).
2. These join to form a continuous ring of cambium.

Activity of Vascular Cambium

1. Divides to produce:
   • Secondary xylem on the inner side (forms wood).
   • Secondary phloem on the outer side.
2. More secondary xylem is produced than secondary phloem.
3. Old secondary phloem gets crushed, but secondary xylem accumulates to form annual rings.

Formation of Cork Cambium

1. Cork cambium (phellogen) arises from the cortex.
2. Produces:
   • Phellem (cork) on the outer side – protective layer.
   • Phelloderm (secondary cortex) on the inner side – living parenchymatous cells.
3. Cork cells contain suberin, making them waterproof.
4. Small openings called lenticels allow gas exchange.

Formation of Bark

1. Bark consists of all tissues outside the vascular cambium.
2. It includes secondary phloem, cork cambium, cork, and phelloderm.

3. Secondary Growth in Dicot Roots

Formation of Vascular Cambium

1. In young dicot roots, the vascular bundles are **radial** (xylem and phloem alternate).
2. Parenchyma cells between xylem and phloem become meristematic, forming a vascular cambium ring.

Activity of Vascular Cambium

1. The cambium divides to produce:
   • Secondary xylem on the inside.
   • Secondary phloem on the outside.
2. The original **primary xylem remains intact** in the center.

Formation of Cork Cambium

1. Cork cambium arises from the **pericycle** (not cortex, as in stems).
2. Produces:
   • Cork (phellem) on the outer side.
   • Phelloderm on the inner side.
3. Forms protective outer layers as the root ages.

4. Comparison: Secondary Growth in Dicot Stems vs. Roots

5. Importance of Secondary Growth

1. Provides **mechanical strength** to stems and roots.
2. Increases **conduction capacity** for water and nutrients.
3. Forms **wood** used in construction, furniture, and paper.
4. Protects plant by forming **bark and cork**.
5. Contributes to **long life** of trees.

6. Conclusion

1. Secondary growth occurs in **dicot stems and roots**, leading to increased thickness.
2. It is controlled by **vascular cambium and cork cambium**.
3. Plays a key role in **structural support, water transport, and survival**.