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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

Stem Tissues Place Transport in Separate Paths

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.

For a plan diagram, draw tissue boundaries with clear single lines and correct relative positions; do not draw individual cells. Label epidermis, cortex, vascular bundles, phloem, cambium, xylem and pith, then annotate their main functions.

Annotate xylem as water/mineral transport and support, phloem as translocation of assimilates, cortex as storage/support and epidermis as the protective outer boundary.

A plan diagram shows distribution and proportion, not cellular detail or shading. In each dicot stem bundle, xylem is inner and phloem outer; do not reverse them.

Stem tissue distribution

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Label / Describe.

Command terms

Identify / Label / Describe

What earns marks

Build the answer around this relationship: Dicot stem vascular bundles are arranged in a ring.

Watch for

Confusing xylem and phloem positions within a dicot stem vascular bundle.

Representative question

Question 1

[Maximum number: 2]

Describe the distribution of vascular tissues in the stem of dicotyledonous plants.

Link Transport Structure To Function

Animal and plant transport answers should link structure to function. In animals, capillaries exchange, arteries maintain high-pressure flow, veins return low-pressure blood, pulse measures arterial pressure waves, and coronary occlusion blocks oxygen delivery to heart muscle. In plants, xylem transports water by transpiration tension and cohesion, while stem and root tissue plans show where xylem and phloem are arranged.

  • Blood vessel answers need structure plus pressure or exchange function.
  • Xylem answers need transpiration pull, cohesion, adhesion, lignin, pits, and hollow vessels when relevant.
  • Plant diagrams should identify tissue distribution: stem vascular bundles in a ring, root xylem cross with phloem between arms.

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.