Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter II. Histology
2.8. Vascular bundles, their histological composition and types

Xylem and phloem together form vascular, or conducting, bundles within plant Organs. Both Tissues incorporate conducting, mechanical, and fundamental elements. Specifically, the xylem comprises vessels and tracheids, xylem parenchyma, and wood fibers (libriform), whereas the phloem consists of sieve tubes, phloem (bast) parenchyma, and bast fibers.

Depending on the relative arrangement of xylem and phloem, vascular bundles are classified into four main types: collateral, bicollateral, radial, and concentric.

Collateral bundles are characterized by the side-by-side placement of xylem and phloem, where the xylem lies adjacent to the phloem (or vice versa), which explains their name meaning "side-by-side." These are the typical bundles found in the stems and leaves of most flowering plants.

Bicollateral bundles feature a central core of xylem flanked by phloem on both sides. These are less common and are most frequently observed in the stems of plants belonging to the Cucurbitaceae (pumpkin) family.

In radial bundles, the xylem is arranged in radiating arms alternating with strands of phloem (Fig. 24). This type is characteristic of roots with Primary Structure.

Concentric vascular bundles feature a ring-like arrangement of xylem and phloem, most commonly with a ring of phloem surrounding a centrally located xylem core. Concentric bundles are typically found in fern rhizomes.

A complete vascular bundle consists of both xylem and phloem, fully or partially enclosed by sclerenchyma. If any of these components are missing, the bundle is considered incomplete. Incomplete bundles are typically found at the tips of leaf blades.

If cambium is present between the xylem and phloem, the vascular bundle is termed open; if cambium is absent, it is closed. Plant vascular bundles develop from procambium. In monocots, the procambium fully differentiates into primary xylem and phloem elements, rendering the bundle closed, whereas in dicots, a layer of procambium persists as cambium, making the bundles open. Fascicular cambium then produces secondary xylem and phloem elements, allowing for continued plant growth.

Vascular bundles originate in the roots, extend through the stem, and terminate in the leaf Veins, thereby forming a continuous transport system throughout the plant.