B3.2.9—Stem tissue distribution
Dicot stems arrange vascular bundles in a ring, with xylem, phloem, cambium and supporting tissues positioned for transport and support.
- Syllabus
- First assessment 2025
- Objective
- B3.2.9
- Level
- SL
A Dicot Stem Separates Water and Assimilate Routes
A transverse section of a young dicot stem has an outer epidermis, cortex beneath it, vascular bundles arranged in a ring and a central pith.
Within each vascular bundle, phloem lies toward the outside, xylem toward the centre and cambium between them. The ring links transport around the stem while lignified xylem and supporting fibres add strength.
SL Summary: Flow Structures
| Route | Structural solution |
|---|---|
| artery | elastic-muscular wall for high pressure |
| capillary | one-cell wall + many branches for exchange |
| vein | wide lumen + valves for low-pressure return |
| xylem | hollow lignified tube for tension |
Stem tissue distribution
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Describe the distribution of vascular tissues in the stem of dicotyledonous plants.
Concept essentials
- Dicot stem vascular bundles are arranged in a ring.
- Xylem is usually nearer the centre of each bundle, with phloem toward the outside.
- Phloem is the tissue responsible for sucrose translocation.
- Plan diagrams emphasise tissue positions rather than individual cells.