Cambridge A-Level Biology – Structure of Transport Tissues
Structure of Transport Tissues
Objective
Describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants.
Key Concepts
Vascular bundles contain both xylem (water‑conducting) and phloem (food‑conducting) tissues.
In dicots, bundles are arranged in a ring (collateral) with xylem towards the centre and phloem towards the periphery.
Additional tissues such as cambium, cortex and pith modify the appearance of each organ.
Transverse Section of a Herbaceous Dicot Stem
The typical arrangement is shown in the schematic below.
Suggested diagram: Transverse section of a herbaceous dicot stem showing a ring of collateral vascular bundles, each with xylem (inner) and phloem (outer), surrounded by cortex and pith.
Region (from centre outwards)
Primary tissue
Notes on xylem / phloem distribution
Central pith
Parenchyma
Usually no vascular tissue; may contain scattered xylem strands in some species.
Vascular bundle ring
Collateral bundles (xylem inside, phloem outside)
Protoxylem (outermost xylem) – narrow vessels and tracheids.
Metaxylem (inner xylem) – larger vessels.
Phloem consists of sieve‑elements and companion cells.
Interbundle fibres may be present between bundles.
Cambium (if secondary growth present)
Vascular cambium
Produces secondary xylem (inward) and secondary phloem (outward).
Cortex
Parenchyma with collenchyma
Non‑vascular; surrounds the vascular ring.
Epidermis
Single layer of cells
Protective outer layer.
Transverse Section of a Herbaceous Dicot Root
Root anatomy differs markedly from the stem; the vascular tissues are centrally located.
Suggested diagram: Transverse section of a herbaceous dicot root showing a central stele with xylem on the inner side and phloem on the outer side, surrounded by pericycle, endodermis, cortex and epidermis.
Region (from centre outwards)
Primary tissue
Distribution of xylem and phloem
Stele (central cylinder)
Vascular tissue
Xylem is positioned towards the centre of the root (inner side of the stele).
Phloem lies peripheral to the xylem (outer side of the stele).
Both are arranged in a single, radial, collateral configuration.
Pericycle
Parenchyma
Thin layer surrounding the stele; origin of lateral roots.
Endodermis
Single layer of tightly packed cells with Casparian strip
Regulates entry of water and solutes into the stele.
Cortex
Parenchyma (often with aerenchyma in some species)
Stores food; may contain collenchyma for support.
Epidermis
Single layer of cells (often with root hairs)
Increases surface area for absorption.
Transverse Section of a Herbaceous Dicot Leaf
Leaves exhibit a flattened lamina with a distinct vascular pattern.
Suggested diagram: Transverse section of a dicot leaf showing a central midrib with collateral bundles (xylem towards the adaxial surface, phloem towards the abaxial surface) and smaller lateral bundles.
Region (from upper surface downwards)
Primary tissue
Vascular arrangement
Upper (adaxial) epidermis
Single layer of cells
Protective; often covered by a cuticle.
Palisade mesophyll
Columnar parenchyma
Located just below the adaxial epidermis; contains many chloroplasts.
Midrib and lateral veins
Collateral vascular bundles
Xylem positioned towards the adaxial side of each bundle.
Phloem positioned towards the abaxial side.
Each bundle is surrounded by a sheath of sclerenchyma (bundle sheath).
Spongy mesophyll
Irregular loosely packed cells with air spaces
Facilitates gas exchange; contains some chloroplasts.
Lower (abaxial) epidermis
Single layer of cells
Often bears stomata for transpiration.
Summary of Distribution Patterns
Stem: Vascular bundles form a peripheral ring; each bundle is collateral with xylem internal, phloem external.
Root: A central stele with a single radial arrangement; xylem interior, phloem exterior.
Leaf: Vascular bundles (midrib and lateral veins) are collateral; xylem towards the upper (adaxial) side, phloem towards the lower (abaxial) side.