Secondary growth in plants is essentially the process that leads to an increase in stem and root thickness.
Understanding Secondary Growth
Unlike primary growth, which is responsible for increasing the plant's length, secondary growth focuses on widening the plant's girth. This thickening is crucial for providing structural support to larger plants and for increasing the capacity to transport water and nutrients.
The Role of Vascular Cambium
The main player in secondary growth is the vascular cambium, a lateral meristem (a type of plant tissue consisting of undifferentiated cells capable of cell division). This cambium produces secondary vascular tissues:
- Secondary xylem: This is wood! It transports water and minerals. Over time, older xylem layers become non-functional and contribute to the heartwood, while newer layers form the sapwood.
- Secondary phloem: This transports sugars. It's formed towards the outside of the vascular cambium.
Process of Secondary Growth
- Vascular Cambium Activation: The vascular cambium, located between the primary xylem and primary phloem, becomes active.
- Cell Division: The cambium cells divide, producing new cells on both the inside and outside of the cambium layer.
- Differentiation: The cells formed on the inside differentiate into secondary xylem cells (wood), while those formed on the outside differentiate into secondary phloem cells.
- Increase in Girth: The continuous production of new vascular tissues causes the stem and root to increase in thickness.
Cork Cambium and Bark Formation
Another lateral meristem, the cork cambium (or phellogen), also contributes to secondary growth. The cork cambium produces:
- Cork cells (phellem): These cells are dead at maturity and are impregnated with suberin, a waxy substance, making them impermeable to water and gases. They provide protection.
- Phelloderm: A layer of parenchyma cells formed inward from the cork cambium.
The cork, cork cambium, and phelloderm together constitute the periderm, which replaces the epidermis as the outermost protective layer of the stem and root in woody plants. The bark includes all tissues outside the vascular cambium, including the periderm and secondary phloem.
Plants Exhibiting Secondary Growth
Secondary growth is most prominent in dicotyledonous (dicot) plants and gymnosperms (conifers). Monocotyledonous plants typically lack secondary growth.
Significance of Secondary Growth
- Structural Support: Allows plants to grow taller and larger.
- Increased Transport Capacity: Enables greater transport of water, minerals, and sugars.
- Protection: The formation of bark provides protection against mechanical damage, herbivores, and pathogens.
