Leaf tissues at Higher Level: The same statement as SL. HL papers pair a leaf image with questions on what each tissue does, including xylem and phloem.
Practise this as you read
- Suggest the functions of the upper epidermis and the palisade mesophyll.
- State the functions of xylem and phloem in a leaf.
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The big idea: A dicot leaf is built in layers, always in the same order from the upper surface to the lower one, with veins running through the middle.
| Tissue | Where | What it does |
|---|---|---|
| Upper epidermis | Top surface, under the waxy cuticle | Protects; transparent, lets light in |
| Palisade mesophyll | Just under the upper epidermis | Most of the photosynthesis |
| Spongy mesophyll | Below the palisade layer | Air spaces for gas exchange |
| Xylem | In each vein, towards the upper side | Brings water into the leaf |
| Phloem | In each vein, towards the lower side | Carries sugars out of the leaf |
| Lower epidermis | Bottom surface, with the guard cells | Holds the stomata |
The points to remember
- The order never changes: upper epidermis, palisade, spongy, lower epidermis.
- Veins (vascular bundles) sit in the mesophyll; the largest is the midrib.
- In a vein, xylem faces the upper surface and phloem the lower one.
Remember it as: Up the leaf: lower epidermis, spongy, palisade, upper epidermis.
Real example: a maize leaf. Maize is not a dicot. Its mesophyll is not split into palisade and spongy layers, and its many veins run side by side, so the dicot plan does not fit it.
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What a plan diagram is: A plan diagram shows where each tissue is, as outlines. It never shows single cells.
The points to remember
- Draw the outline of each tissue with clear, continuous lines.
- Keep the proportions: a thick palisade layer is drawn thick.
- Show upper and lower epidermis, palisade, spongy mesophyll and a vascular bundle (xylem above, phloem below).
- Label with ruled lines that touch the tissue and never cross each other.
- No shading and no cells.
Three traps: Drawing cells turns it into a cell drawing, not a plan.
A thin palisade band drawn as thick as the spongy layer loses the proportion mark.
A label line that stops short of the tissue does not count.
How it is asked: Expect a micrograph of a leaf: draw a plan diagram, identify labelled tissues, or give the function of one.
IB-style questionSuggest[2 marks]
Suggest the functions of the cells in the upper epidermis and the palisade mesophyll.
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