draw plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides and photomicrographs

Structure of Transport Tissues – Cambridge International AS & A Level Biology (9700)

Learning Objective

Students will be able to draw accurate plan (cross‑section) diagrams of stems, roots and leaves of herbaceous dicotyledonous plants from microscope slides or photomicrographs and label every structure required by the syllabus.

Key Anatomical Features to Recognise

• Epidermis – outer protective layer; may bear a cuticle, stomata (leaf) or root hairs (root).
• Cuticle – waxy layer on the epidermis that reduces water loss.
• Cortex – parenchymatous ground tissue between epidermis and vascular tissue (present in stems and roots).
• Endodermis – single layer surrounding the stele in roots; contains a Casparian strip.
• Pericycle – thin layer of parenchyma just inside the endodermis; origin of lateral roots.
• Stele – the central vascular cylinder of a root (xylem + phloem, often star‑shaped in dicot roots).
• Pith – parenchymatous ground tissue occupying the centre of many herbaceous stems.
• Vascular bundles (stem) / Stele (root) / Mid‑rib (leaf)

  • Xylem – inner side of the bundle/stele; mainly vessels and tracheids; conducts water and minerals upward.
  • Cambium – thin meristematic layer between xylem and phloem (present in stems and roots; usually absent in herbaceous leaves).
  • Phloem – outer side of the bundle/stele; sieve‑tube elements and companion cells; conducts organic solutes.

• Mesophyll (leaf)

  • Palisade mesophyll – tightly packed columnar cells beneath the upper epidermis.
  • Spongy mesophyll – loosely arranged cells with intercellular air spaces beneath the palisade layer.

• Stomata – pores, usually on the lower leaf epidermis, surrounded by guard cells.
• Root hairs – extensions of epidermal cells that increase surface area for water absorption.

Step‑by‑Step Procedure for Constructing a Plan Diagram

1. Determine the organ’s overall shape – circular for stems and roots; flattened for leaves.
2. Identify the outermost layer – locate the epidermis and note any specialised structures (cuticle, stomata, root hairs).
3. Spot organ‑specific features (quick checkpoint):

   • Stem – cortex, pith, discrete radial vascular bundles.
   • Root – cortex, endodermis, pericycle, central stele (often star‑shaped).
   • Leaf – upper & lower epidermis, palisade mesophyll, spongy mesophyll, mid‑rib vascular bundle.

4. Locate the vascular tissue – xylem is always the innermost part, phloem the outermost, with cambium in between (when present).
5. Sketch the ground‑tissue regions – draw cortex, pith or mesophyll layers with realistic relative thicknesses (see optional Teacher Tip).
6. Label every structure – use the exact terminology required by the syllabus (e.g., “endodermis”, “pericycle”, “cuticle”).
7. Include a scale bar or dimension – if the slide provides a magnification, indicate it on the diagram.
8. Cross‑check the drawing against the original slide or photomicrograph for proportion, placement and completeness.

Optional Teacher Tip: Quick Proportion Guide (Herbaceous Dicots)

These percentages are useful for teachers when modelling a “typical” section, but they are not part of the syllabus requirement.

Suggested Diagrams (Plan View)

Comparative Table of Transport Tissues (Stem, Root & Leaf)

Functional Overview (Brief)

• Xylem – conducts water and dissolved minerals from roots to aerial parts; vessels provide low‑resistance pathways.
• Phloem – transports photosynthates (mainly sucrose) from source (leaf) to sink (roots, growing buds); movement can be bidirectional.
• Cambium – adds secondary xylem inward and secondary phloem outward, allowing stems and roots to increase in girth.
• Epidermis & cuticle – protect against desiccation and pathogen entry.
• Endodermis – with its Casparian strip forces water and solutes to cross the plasma membrane, regulating entry into the stele.
• Mesophyll – site of photosynthesis (palisade) and gas exchange (spongy).

Common Errors to Avoid

• Reversing the positions of xylem (inner) and phloem (outer) in any organ.
• Omitting the cambial layer in stems or roots.
• Confusing pith with central xylem in stems; pith is parenchymatous ground tissue.
• Failing to label the endodermis and pericycle in root sections.
• Calling the star‑shaped central region of a dicot root “pith” – it is the stele (xylem + phloem).
• Placing stomata on the upper leaf epidermis.
• Neglecting the cuticle on epidermal surfaces.
• Drawing xylem and phloem of equal thickness; in dicot bundles xylem occupies roughly twice the area of phloem.

Assessment Checklist

1. All mandatory structures are present and correctly labelled (epidermis, cuticle, root hairs, cortex, endodermis, pericycle, pith, stele, cambium where required, xylem, phloem, palisade & spongy mesophyll, stomata).
2. Relative proportions of tissues reflect the original photomicrograph (use the optional proportion guide for guidance).
3. A scale bar or dimension annotation is included.
4. Positions of xylem, phloem and cambium follow the correct radial pattern for each organ.
5. Organ‑specific features (e.g., stomata on lower leaf epidermis, Casparian strip in root endodermis) are accurately depicted.