Woody plants primarily grow by increasing their girth and length through the activity of specialized dividing tissues called meristems, particularly two lateral meristems: the vascular cambium and the cork cambium.
Woody plant growth is a fascinating process that combines two distinct types of development: primary and secondary growth. While primary growth extends the plant vertically, secondary growth focuses on increasing its diameter, building the robust woody structure we associate with trees and shrubs.
The Dual Nature of Woody Plant Growth
Understanding how woody plants grow involves recognizing these two fundamental processes:
- Primary Growth: This refers to the increase in the length of the plant's stems and roots. It occurs at the tips (apical meristems) and is responsible for the plant's initial vertical extension and branching. While not the focus of secondary thickening, woody plants initiate new shoots and roots annually through primary growth.
- Secondary Growth: This is the characteristic increase in the girth or thickness of stems and roots. It's what makes woody plants sturdy and allows them to grow tall and wide. As stated in the reference, at the start of each growing season, woody plants resume primary growth, adding additional secondary tissues through growth of two lateral meristems: the vascular cambium and the cork cambium. This continuous addition of secondary tissues over years leads to the formation of wood and bark.
The Role of Lateral Meristems
The continuous thickening of woody stems and roots is primarily driven by the activity of two vital lateral meristems, which are layers of actively dividing cells.
1. The Vascular Cambium
The vascular cambium is the main growing tissue of stems and roots in most plants. This cylindrical layer of meristematic cells is located between the plant's existing xylem (wood) and phloem (inner bark). Its division results in:
- Secondary Xylem (Wood): Produced inwards towards the center of the stem or root. This is the main component of wood, providing structural support and transporting water and minerals from the roots to the rest of the plant. Each year's growth forms a distinct layer, visible as annual growth rings.
- Secondary Phloem (Inner Bark): Produced outwards, towards the exterior of the stem or root. This tissue is responsible for transporting sugars produced during photosynthesis from the leaves to other parts of the plant.
The persistent activity of the vascular cambium year after year is what accounts for the increasing diameter of a tree trunk and the formation of distinct tree rings, which can be used to determine a tree's age and past environmental conditions.
2. The Cork Cambium
As the stem or root increases in girth due to the vascular cambium's activity, the outermost tissues (epidermis) would eventually rupture. To prevent this and provide new protection, the cork cambium develops. This meristem forms within the outer cortex of the stem or root and produces:
- Cork (Phellem): Produced outwards, forming the protective outer layer of the bark. Cork cells are dead, suberized (waxy), and provide insulation, protection against pests, diseases, and water loss.
- Phelloderm: Produced inwards, a layer of living cells that is part of the inner bark.
Together, the cork cambium, cork, and phelloderm constitute the periderm, which is the technical term for the outer bark that replaces the epidermis in woody plants. This continuous formation of new protective layers is crucial for the long-term survival of the plant.
Seasonal Growth and Development
The growth of woody plants is typically seasonal, correlating with periods of favorable environmental conditions. As noted, at the start of each growing season, often in spring, the lateral meristems become active, leading to a surge in both primary and secondary growth. This cyclic activity results in the formation of distinct growth increments, such as the visible tree rings, which reflect the annual rhythm of cambial activity.
Summary of Lateral Meristem Functions
| Meristem Type | Location | Primary Function | Resulting Tissues | Impact on Plant |
|---|---|---|---|---|
| Vascular Cambium | Between xylem and phloem in stems and roots | Produces new transport tissues (wood and inner bark) | Secondary Xylem (wood), Secondary Phloem (inner bark) | Increases girth, forms tree rings, provides structural support and water/nutrient transport. |
| Cork Cambium | Within the outer cortex/bark region | Produces protective outer layers of bark | Cork (outer bark), Phelloderm (inner bark) | Forms protective outer bark (periderm), shields against damage, disease, and water loss. |
In essence, woody plants owe their robust structure and longevity to the precise and coordinated activity of these two lateral meristems, which continuously add new tissues, allowing them to expand in diameter year after year.
