Secondary growth is the increase in thickness or girth of a plant stem and root, contrasting with primary growth, which is the increase in length.
Here's a breakdown of the differences:
Primary Growth
- Definition: The increase in the length of a plant, occurring at the apical meristems (shoot and root tips).
- Location: Occurs at the tips of roots and shoots (apical meristems).
- Cell Division: Results from cell division in the apical meristems.
- Result: Leads to the formation of primary tissues: epidermis, ground tissue, and vascular bundles (xylem and phloem).
- Plants Affected: Occurs in all vascular plants, including herbaceous (non-woody) plants.
Secondary Growth
- Definition: The increase in thickness or girth of a plant stem and root.
- Location: Occurs in the lateral meristems, specifically the vascular cambium and cork cambium.
- Cell Division: Results from cell division in the vascular cambium and cork cambium.
- Result: Leads to the formation of secondary vascular tissues (secondary xylem or wood and secondary phloem) and the periderm (bark).
- Plants Affected: Primarily occurs in woody dicots and gymnosperms, allowing them to grow in size over many years. Monocots typically lack secondary growth.
Key Differences Summarized in a Table:
| Feature | Primary Growth | Secondary Growth |
|---|---|---|
| Definition | Increase in length | Increase in thickness/girth |
| Location | Apical meristems (shoot & root tips) | Lateral meristems (vascular & cork cambium) |
| Cell Division | Apical meristem cells | Vascular & cork cambium cells |
| Tissues Formed | Primary xylem, primary phloem, epidermis, ground tissue | Secondary xylem (wood), secondary phloem, periderm (bark) |
| Plants Affected | All vascular plants | Woody dicots and gymnosperms (mostly) |
In essence, primary growth helps a plant reach for sunlight and explore the soil for water and nutrients, while secondary growth provides the structural support necessary for a plant to grow larger and live longer. The development of wood through secondary xylem contributes significantly to a plant's ability to withstand environmental stresses and support its increasing size.
