How Do Xylem And Phloem Differ In Woody Versus Herbaceous Stems

The primary difference lies in their arrangement and the presence or absence of secondary growth (growth in width).

Herbaceous Stems (e.g., flowers, vegetables)

• Arrangement: The xylem and phloem are typically grouped together in discrete vascular bundles.
• Pattern: The pattern of these bundles differs between plant groups.
  • In eudicots (like sunflowers or beans), the vascular bundles are arranged in a single ring near the edge of the stem.
  • In monocots (like corn or grass), the vascular bundles are scattered throughout the entire stem.
• Growth: These stems primarily undergo primary growth (getting taller). They lack a vascular cambium, so they do not produce secondary xylem (wood) or secondary phloem and do not grow thicker.

Woody Stems (e.g., trees, shrubs)

• Arrangement: In the first year of growth, they may start with vascular bundles in a ring (like an herbaceous eudicot). However, a vascular cambium quickly forms, connecting these bundles into a continuous ring.
• Growth: These stems undergo secondary growth. The vascular cambium continuously divides.
• Secondary Xylem (Wood): The cambium produces new xylem cells to the inside. This secondary xylem accumulates year after year, forming the wood of the tree. The old, non-functional xylem in the center is called heartwood, while the functional, outer xylem is sapwood.
• Secondary Phloem (Bark): The cambium produces new phloem cells to the outside. This secondary phloem does not accumulate; it becomes the inner bark. As the tree expands, the older phloem is crushed and sloughed off as part of the outer bark.