describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants
Transport Tissues – Distribution in Herbaceous Dicotyledonous Plants
Learning Outcomes (Cambridge AS & A‑Level Biology 7.1)
- 7.1 a – Draw labelled plan (transverse) diagrams of a stem, root and leaf.
- 7.1 b – Describe the distribution of xylem and phloem in each organ.
- 7.1 c – Draw and label the main cell types: xylem vessel element, tracheid, phloem sieve‑tube element and companion cell.
- 7.1 d – Relate the structure of each tissue to its function.
Colour Key (use the same colours in every diagram)
- Blue – Xylem (water‑conducting)
- Red – Phloem (food‑conducting)
- Green – Cambium (meristematic)
- Orange – Sclerenchymatous bundle sheath (leaf)
Master Schematic (Stem | Root | Leaf)
1. Transverse Section of a Herbaceous Dicot Stem
| Region (centre → outward) | Primary tissue | Distribution of xylem / phloem |
|---|---|---|
| Central pith | Parenchyma (often stores starch) | No vascular tissue (occasionally isolated xylem strands in some species). |
| Vascular bundle ring (collateral bundles) | Proto‑xylem (outer), meta‑xylem (inner), phloem, inter‑bundle fibres |
|
| Vascular cambium (if secondary growth) | Meristematic cylinder | Produces secondary xylem inward and secondary phloem outward. |
| Cortex | Parenchyma (often with collenchyma for support) | Non‑vascular, surrounds the vascular ring. |
| Epidermis | Single layer with cuticle | Protective outer covering. |
Structure → Function (stem)
- Lignified secondary xylem provides mechanical strength and a low‑resistance conduit for water.
- Collateral arrangement keeps xylem and phloem close, allowing efficient radial exchange of solutes.
- Companion cells actively load sugars into sieve‑tube elements, enabling long‑distance translocation.
2. Transverse Section of a Herbaceous Dicot Root
| Region (centre → outward) | Primary tissue | Distribution of xylem / phloem |
|---|---|---|
| Stele (central cylinder) | Collateral vascular tissue |
|
| Pericycle | Thin layer of parenchyma | Origin of lateral roots; may give rise to secondary growth. |
| Endodermis | Single layer with Casparian strip | Regulates entry of water and solutes into the stele. |
| Cortex | Parenchyma (often with aerenchyma in wet soils) | Carbohydrate storage; may contain collenchyma for support. |
| Epidermis (root hairs) | Single layer of cells | Increases surface area for water and mineral absorption. |
Structure → Function (root)
- Central xylem allows efficient upward transport of water under tension.
- Casparian strip forces solutes to cross a membrane, giving selective uptake.
- Phloem positioned just outside the xylem facilitates loading of photosynthates from the surrounding cortex.
3. Transverse Section of a Herbaceous Dicot Leaf
| Region (upper → lower) | Primary tissue | Vascular arrangement |
|---|---|---|
| Upper (adaxial) epidermis | Single layer with cuticle | Protective; may contain a few stomata in some species. |
| Palisade mesophyll | Columnar parenchyma, densely packed chloroplasts | Principal site of photosynthesis. |
| Midrib & lateral veins | Collateral bundles surrounded by a sclerenchymatous bundle sheath (orange) |
|
| Spongy mesophyll | Irregular loosely packed cells with intercellular air spaces | Facilitates gas exchange between stomata and photosynthetic cells. |
| Lower (abaxial) epidermis | Single layer, many stomata | Regulates transpiration and gas exchange. |
Structure → Function (leaf)
- Upper‑side xylem places water close to the palisade mesophyll where evaporation occurs, maintaining the transpiration stream.
- Lower‑side phloem lies adjacent to the stomata‑rich abaxial epidermis, allowing rapid export of photosynthates.
- The sclerenchymatous bundle sheath protects delicate conducting elements and reinforces the thin lamina.
4. Concise Distribution Table (Answer‑style)
| Organ | Xylem position | Phloem position |
|---|---|---|
| Stem (dicot) | Inner side of each peripheral collateral bundle (proto‑ and meta‑xylem) | Outer side of each peripheral collateral bundle |
| Root (dicot) | Innermost part of the central stele | Just outside the xylem, still within the stele |
| Leaf (dicot) | Adaxial (upper) side of each vein | Abaxial (lower) side of each vein |
All three organs have collateral bundles – xylem is always internal to phloem.
5. Main Cell Types (AO1)
When answering exam questions, label each part clearly (e.g., “perforation plate”, “bordered pit”, “sieve plate”, “dense cytoplasm”).
6. Practical / Microscopy Tips (AO3)
- Sample preparation: Cut a thin hand‑section of fresh herbaceous stem, root or leaf. Stain with 1 % safranin (xylem) and 0.5 % fast green (phloem) for 1–2 min, then rinse.
- Low power (×40): Locate the epidermis, cortex and the vascular bundle(s).
- High power (×400):
- Identify vessel elements – large lumen, perforation plates, thick lignified walls.
- Identify tracheids – narrower lumen, bordered pits.
- Identify sieve‑tube elements – thin walls, sieve plates, no lignin.
- Identify companion cells – dense cytoplasm, prominent nucleus, close association with a sieve tube.
- Sketching technique: Use consistent symbols – solid blue lines for lignified xylem walls, dashed red lines for phloem, thin green line for cambium, orange shading for bundle sheath.
- AO3 marks: Mention the purpose of the stain, describe what you see at each magnification, and relate the observed structures to their functions.
7. Summary (Key Points for Revision)
- Stem: Peripheral ring of collateral bundles; xylem on the inner side, phloem on the outer side. Secondary xylem (wood) provides strength; cambium adds secondary growth.
- Root: Central stele with a single radial collateral arrangement; xylem innermost, phloem outermost. Endodermis with Casparian strip controls entry of water and minerals.
- Leaf: Flattened lamina with collateral veins; xylem towards the adaxial side, phloem towards the abaxial side. Bundle sheath (sclerenchyma) protects and supports the veins.
- All three organs share the collateral pattern (xylem internal, phloem external) – a high‑yield point for exam answers.
- Remember the four cell types and their distinctive features; they are frequently asked in AO1/ AO2 questions.