Tissues in dicotyledonous stems and rootsSpec B3.2.9, B3.2.10
In short
In a transverse section of a dicotyledonous stem, vascular bundles are arranged in a ring, each with phloem towards the outside and xylem towards the inside, surrounded by cortex and an outer epidermis. In a dicotyledonous root, xylem and phloem are in a central vascular region, with xylem in a star shape, surrounded by a wide cortex and epidermis.
| Tissue | Main function |
|---|---|
| Epidermis | Protective outer layer; in roots it bears root hairs that absorb water and mineral ions. |
| Cortex | Packing and support tissue between epidermis and vascular tissue; can store food such as starch; in roots water crosses it to reach the xylem. |
| Xylem | Transports water and mineral ions from roots to leaves; lignified walls give support. |
| Phloem | Transports sap containing sugars and other carbon compounds by translocation. |
| Vascular bundle | A strand of xylem and phloem together. |
Stem
- Vascular bundles are arranged in a ring near the outside of the stem.
- In each bundle, phloem is on the outside and xylem on the inside.
- Cortex lies between the epidermis and the ring of bundles; the centre of the stem is pith.
(opens full size in a new tab)Root
- Xylem and phloem form a central vascular region, not a ring: xylem in a star or X shape with phloem between its arms.
- The guide asks you to identify vascular bundles in the root too. In a root they are radial: the groups of xylem and phloem alternate on different radii inside one central vascular cylinder (the stele), instead of xylem and phloem sharing a bundle as in the stem.
- A wide cortex surrounds the vascular region, with the epidermis and root hairs on the outside.
- The central position of vascular tissue resists the pulling forces on a root.
(opens full size in a new tab)Draw plan diagrams from micrographs: outline each tissue with a single clear line, keep proportions accurate, no cells and no shading. Label the tissues the guide lists (epidermis, cortex, vascular bundles, xylem and phloem; pith in the stem) and, for the stem, annotate each label with its main function.
Quick check
What are fenestrations in capillaries?
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Pores in the capillary wall that allow particularly rapid exchange, for example in the kidney glomerulus.
How do elastic fibres in artery walls help maintain blood pressure?
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They stretch when blood is pumped in and recoil between beats, pushing blood on and keeping the pressure up.
What causes tension in the xylem?
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Loss of water by transpiration from leaf cell walls draws water out of the xylem by capillary action.
Where is xylem found in a dicotyledonous root?
Show answer
In the centre, in a star or X shape, with phloem between its arms.
Written and checked against the IB Biology SL specification · Updated October 2026