7.1 Structure of Transport Tissues
- Syllabus
- 9700–2028–2029
- Topic
- 7.1
- Level
- AS
Learning objectives
7.1.1Plan diagrams of transverse sections• Plan diagrams of transverse sections: stems, roots, leaves (herbaceous dicots)7.1.2Xylem and phloem distribution• Distribution of xylem and phloem in transverse sections7.1.3Xylem and phloem cell drawings• Draw and label: xylem vessel elements, phloem sieve tube elements, companion cells7.1.4Structure and function• Relate structure to function
A plan diagram maps tissue boundaries from visible evidence
A transverse-section plan diagram is a large, simple map of tissue boundaries and proportions. It records what a stem, root or leaf section shows without drawing individual cells or inventing unresolved structures.
- Inspect the whole section and identify the organ only from supported overall patterns.
- Locate vascular bundles or the central vascular region and distinguish xylem/phloem from position, boundaries, size and contrast.
- Draw a large outer outline and main tissue-region boundaries with single continuous lines and correct relative proportions.
- Do not shade, sketch individual cells or reproduce every colour/texture.
- Use ruled label lines ending on the correct region; add magnification or scale only when supplied or required.
- Check that every labelled structure is actually visible or justified by the section.
The evidence chain is visible tissue pattern to vascular position to supported tissue identity to simplified plan. Functional knowledge may help interpretation, but the drawing must stay within the observed section.
A plan diagram is not a photograph, shaded cell drawing or memorised organ template. Separate direct observation from inference and preserve boundaries and proportions rather than decorative detail.
Dicot roots, stems and leaves position xylem and phloem differently
In herbaceous dicotyledonous plants, identify the organ pattern first and then the relative xylem/phloem positions; one universal vascular-bundle rule does not fit root, stem and leaf.
| Organ transverse section | Xylem distribution | Phloem distribution |
|---|---|---|
| root | central star- or cross-shaped xylem | separate groups between the arms of the xylem |
| stem | vascular bundles in a ring; xylem on the inner side of each bundle | phloem on the outer side of each bundle, nearer the epidermis |
| leaf | vascular bundles in midrib and veins; xylem towards the upper epidermis | phloem towards the lower epidermis |
Use the whole-organ layout, locate the vascular region, then apply central/between-arms, inner/outer or upper/lower comparisons. Label only where the section supports the identification.
Do not draw all three organs with the same bundle layout. In a dicot root, phloem lies between xylem arms rather than forming one vague ring around the central core; in a leaf, use upper/lower rather than inside/outside.
Draw transport cells from visible diagnostic evidence
A biological drawing of transport cells must reproduce the structures visible in the supplied slide, photomicrograph or electron micrograph. Identify diagnostic evidence first, then draw and label only what the source resolves.
- Check image type, orientation and scale; distinguish longitudinal from transverse views.
- Xylem vessel element clues include a large lumen, thick lignified wall, pits when visible, no protoplasm and open/end-to-end continuity in longitudinal view.
- Sieve-tube element clues include an elongated conducting cell, cellulose wall, reduced contents and a sieve plate with pores when resolved.
- Companion-cell clues include a smaller adjacent living cell with nucleus, dense cytoplasm and organelles; only label mitochondria or plasmodesmata if visible at the supplied resolution.
- Draw large single clear outlines with correct relative proportions, no shading and no sketching.
- Use ruled non-crossing label lines ending exactly on observed structures; do not label a remembered feature that is absent or unresolved.
| Cell | Highest-value labels when visible |
|---|---|
| xylem vessel element | lignified wall, lumen, pit, absent contents/open end |
| sieve-tube element | sieve plate/pore, thin peripheral cytoplasm, cellulose wall |
| companion cell | nucleus, dense cytoplasm, mitochondria, connection to sieve tube |
Do not turn an observation drawing into a memorised ideal diagram or a function essay. Resolution controls what may be labelled, and companion cells are adjacent support cells rather than the conducting sieve tube itself.
Transport-cell structures match conduction and support roles
Xylem vessels provide a strong low-obstruction route for water and mineral ions; sieve-tube elements provide an assimilate pathway whose living function is supported by companion cells.
- Xylem vessel elements join end to end; mature elements have no protoplasm or end plates, reducing resistance to water flow.
- Lignified walls strengthen vessels and resist collapse, while pits permit lateral water movement.
- Sieve-tube elements join through porous sieve plates; reduced contents leave space for assimilate flow while the cells remain living.
- Companion cells retain a nucleus and many mitochondria and connect through plasmodesmata; their ATP-supported transport and metabolism maintain sieve tubes and support loading/unloading.
- Xylem transport is mainly root-to-shoot; phloem follows source-to-sink demand and can occur in either direction in different tubes.
Hollow lignified pitted vessels link structure to water/mineral conduction and support. Sieve tubes plus metabolically active companion cells link a low-obstruction living pathway to controlled assimilate translocation.
Do not call sieve-tube elements dead and hollow like xylem or call companion cells the conducting tube. This card stops at structure-function relationships; transpiration, cohesion-tension and mass-flow mechanisms belong to 7.2.