Botany · Ch 9 — Tissue and Tissue System
Vascular Tissue System
Vascular Tissue System
The vascular tissue system is the plant's transport network: xylem and phloem, always bundled together rather than scattered as separate tissues, forming discrete vascular bundles. In both gymnosperm and angiosperm stems these bundles are arranged as a eustele - typically one ring of bundles separated from each other by interfascicular regions (medullary rays) - while in roots the vascular tissue instead forms a solid protostele at the centre. A vascular bundle's internal geometry is described by three independent choices: how xylem and phloem sit relative to each other on the same or different radii (radial, with xylem and phloem alternating on separate radii, typical of roots; or conjoint, with both on the same radius, typical of stems); whether phloem occurs on one side of the xylem only (collateral) or on both the outer and inner sides (bicollateral, e.g. Cucurbitaceae); and whether a cambium sits between xylem and phloem (open, allowing secondary growth, typical of dicot/gymnosperm stems) or not (closed, typical of monocot stems). A separate, less common geometry, concentric bundles, has one tissue completely encircling the other - amphivasal/leptocentric, with xylem surrounding a central phloem (e.g. Dracaena, Yucca), or amphicribral/hadrocentric, with phloem surrounding a central xylem (e.g. ferns s …
What this figure shows. A comparative panel of vascular-bundle cross sections illustrating: Radial bundles, where xylem and phloem sit on separate, alternating radii separated by parenchyma (roots); Conjoint bundles, where xylem and phloem sit together on the same radius (all stems) - either Collateral (phloem to one side of xylem only) or Bicollateral (phloem on both the outer and inner sides of xylem, e.g. Cucurbitaceae) - and either Open (a cambium present between xylem and phloem, as in dicot/gymnosperm stems) or Closed (no cambium, as in monocot stems); and Concentric bundles, where one tissue completely surrounds the other - Amphivasal/Leptocentric (xylem surrounding a central phloem, e.g. Dracaena, Yucca) or Amphicribral/Hadrocentric (phloem surrounding a central xylem, e.g. ferns such as Polypodium). NOTE ON RECOVERY: the source page for this figure (page 29, between the vascular-bundle introduction and Table 9.3) is a near-image-only page, so this cover is reconst …
| Comparison | First term | Second term |
|---|---|---|
| Protoxylem vs Metaxylem | Protoxylem: first-formed primary xylem, found in developing organs, elements relatively smaller | Metaxylem: later-formed primary xylem, found in developed primary organs, elements relatively larger |
| Protophloem vs Metaphloem | Protophloem: first-formed primary phloem, found in developing organs, elements relatively smaller | Metaphloem: later-formed primary phloem, found in developed primary organs, elements relatively larger |
| Primary vs Secondary xylem | Primary xylem: derived from the procambium of the apical meristem | Secondary xylem: derived from the vascular cambium, a lateral meristem |