Cambridge A‑Level Biology – Structure of Transport Tissues
Structure of Transport Tissues
Learning Objective
Students will be able to draw accurate plan diagrams of transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants using information obtained from microscope slides and photomicrographs.
Key Anatomical Features to Recognise
Vascular bundles – consist of xylem (inner) and phloem (outer) separated by a cambium in dicots.
Xylem – usually composed of tracheids and vessels; transports water and minerals upward.
Phloem – composed of sieve‑tube elements and companion cells; transports organic solutes downward or laterally.
Cambium – a thin layer of meristematic cells that adds secondary xylem (inward) and secondary phloem (outward).
Epidermis – outer protective layer; may possess stomata (leaf) or root hairs (root).
Ground tissue – parenchyma, collenchyma and sclerenchyma that fill the space between vascular bundles and epidermis.
Steps for Constructing a Plan Diagram
Examine the photomicrograph or slide and note the overall shape of the organ (circular for stem, oval for root, flattened for leaf).
Identify the outermost layer (epidermis) and any specialised structures (stomata, root hairs).
Locate the vascular bundles. In dicot stems they appear as discrete, radially arranged units; in roots they form a central stele; in leaves they are arranged in a single marginal or central vein.
Within each bundle, distinguish xylem (usually darker, towards the centre) from phloem (lighter, towards the periphery). Mark the intervening cambium.
Sketch ground‑tissue regions, indicating any specialised cells (e.g., collenchyma beneath the epidermis of stems).
Label all parts clearly; use a scale bar if dimensions are known. For quantitative work you may need the area of a tissue: \$A = \pi r^{2}\$ for a circular region.
Check the diagram against the original image for proportion and placement accuracy.
Suggested Diagrams
Suggested diagram: Transverse section of a herbaceous dicot stem showing epidermis, cortex, vascular bundles (xylem, cambium, phloem), and pith.
Suggested diagram: Transverse section of a herbaceous dicot root displaying epidermis with root hairs, cortex, endodermis, pericycle, and central stele (xylem and phloem).
Suggested diagram: Transverse section of a herbaceous dicot leaf illustrating epidermis (upper and lower), palisade mesophyll, spongy mesophyll, and a central vascular bundle (xylem, cambium, phloem).
Comparative Table of Transport Tissues in Stem, Root and Leaf
Feature
Stem (Transverse)
Root (Transverse)
Leaf (Transverse)
Overall shape
Circular, with multiple discrete vascular bundles arranged radially
Circular, with a single central stele (xylem‑phloem cylinder)
Flattened, with a single mid‑rib vascular bundle (or marginal veins)
Epidermis
Simple, may have a thin cuticle; no stomata
Simple, often bearing root hairs
Two layers (upper and lower); lower epidermis bears stomata
Ground tissue
Cortex (parenchyma, collenchyma) surrounding bundles; pith in centre
Cortex with endodermis; pericycle just inside endodermis
Palisade mesophyll (upper) and spongy mesophyll (lower)
Xylem position
Innermost part of each bundle, towards centre of organ
Inner portion of stele, often star‑shaped in dicots
Upper side of the vascular bundle (closer to upper epidermis)
Phloem position
Outer part of each bundle, towards periphery of organ
Outer portion of stele, surrounding xylem
Lower side of the vascular bundle (closer to lower epidermis)
Cambium
Thin layer between xylem and phloem in each bundle
Present as a continuous cylinder between xylem and phloem
Often absent in herbaceous leaves; if present, a thin layer between xylem and phloem
Common Errors to Avoid
Reversing the positions of xylem and phloem in the vascular bundle.
Omitting the cambial layer, especially in stems where it is thin.
Confusing the pith with central xylem in stems; pith is parenchymatous ground tissue.
Neglecting the endodermis and pericycle in root sections.
Placing stomata on the upper epidermis of a leaf diagram.
Assessment Checklist
All major tissue types are represented and correctly labelled.
Relative proportions of tissues reflect the photomicrograph.
Diagram includes a scale bar or dimension annotation.
Positions of xylem, phloem and cambium follow the radial pattern for dicots.
Specialised structures (root hairs, stomata, palisade cells) are shown where appropriate.