Secondary medullary rays are radiating strips of parenchyma cells that originate from the vascular cambium and extend through the secondary xylem and secondary phloem in woody plants.
These rays play a crucial role in:
- Storage: They serve as storage sites for carbohydrates and other nutrients.
- Radial Transport: They facilitate the radial transport of water and nutrients across the secondary xylem and phloem.
- Wound Healing: They contribute to wound healing processes in the stem.
Origin and Formation
The vascular cambium, a lateral meristem, is responsible for producing secondary xylem (wood) and secondary phloem (inner bark). It contains two types of initials:
- Fusiform initials: These give rise to the conducting cells (tracheids, vessel elements in xylem and sieve tube elements, companion cells in phloem) and vertically oriented parenchyma cells.
- Ray initials: These produce the cells of the medullary rays.
As the vascular cambium divides and differentiates, the ray initials form new ray cells that extend radially outwards, creating the secondary medullary rays within the developing secondary xylem and phloem.
Function
The main functions of secondary medullary rays are:
- Radial Transport: They are essential for moving water and nutrients radially within the stem, supplementing the longitudinal transport provided by the xylem and phloem.
- Storage: They store starch, fats, and other compounds, acting as a reservoir of resources for the plant. These stored reserves can be mobilized during periods of stress or high demand.
- Structural Support: While not their primary function, they contribute to the overall structural integrity of the wood by providing radial connections between different xylem elements.
Difference between Primary and Secondary Medullary Rays
While both primary and secondary medullary rays exist, their origins and extent differ. Primary medullary rays extend from the pith outwards through the primary xylem and phloem. In contrast, secondary medullary rays are formed in the vascular cambium and exist solely within the secondary xylem and phloem. Primary rays are established during primary growth, while secondary rays are a product of secondary growth.
