The big idea: In a dicot stem the vascular bundles form a ring, between the cortex outside and the pith in the centre.
| Tissue | Where | Main function |
|---|---|---|
| Epidermis | One thin outer layer | Protects the stem; cuts water loss |
| Cortex | Wide band under the epidermis | Support and storage |
| Vascular bundle | In a ring, between cortex and pith | Transport and support |
| Phloem | Outer side of each bundle | Carries sugars and amino acids |
| Xylem | Inner side of each bundle | Carries water and minerals; support |
| Pith | The centre | Storage |
The points to remember
- Order from the outside: epidermis, cortex, vascular bundles, pith.
- In each bundle, phloem faces outwards and xylem faces inwards, towards the centre.
- Phloem carries sugars and amino acids, not 'food' or 'nutrients'.
Remember it as: Phloem out, xylem in.
Real example: alfalfa. Alfalfa (lucerne) is a common subject for stem micrographs: its vascular bundles sit in a neat ring between the cortex and the pith.
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What a plan diagram is: A plan diagram shows where each tissue is, as outlines. It never shows cells.
The points to remember
- Draw the segment you are asked for (area X), not the whole stem, with more than one vascular bundle.
- Outline the epidermis, the cortex under it, each vascular bundle and the pith, in proportion.
- Split each bundle: phloem on the outer side, xylem on the inner side.
- Label with ruled lines; add a short function for each tissue when asked to annotate.
Three traps: Drawing cells, or the whole stem when only part is asked for, loses marks.
The cortex label must point to the band just under the epidermis.
A leaf section is not a stem section: leaves have palisade and spongy layers.
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How it is asked: Expect a stem micrograph: draw a plan of part of it, identify the tissue that carries water, or state what the phloem does.
IB-style questionDraw[3 marks]
The micrograph shows a transverse section of an alfalfa stem. Draw a labelled plan diagram of the area marked X.
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