6.1.10Plant Nutrition
Identifying Leaf Structures in a Dicotyledonous Plant — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 11 of 11
Part 1 of 4Reading a leaf cross-section
Dicotyledonous plants
- Dicotyledonous plants A dicotyledonous plant, or dicot, has seeds with two seed leaves and broad leaves with a branching network of veins.
- Examples include sunflowers, beans and oak trees.
Top to bottom order
- Top to bottom order In a cross-section the layers appear in a fixed order: cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis, with stomata and guard cells on the lower surface.
How to approach a diagram
- How to approach a diagram Start at the top surface and work down layer by layer, using the cell shape, the number of chloroplasts and the amount of air space to recognise each tissue.
Part 2 of 4Structures you must be able to identify
| Structure | How to recognise it | Function (Extended tier only) |
|---|---|---|
| Cuticle | Thin waxy layer on top of the upper epidermis | Waterproof barrier that cuts water loss; transparent so light gets through |
| Upper epidermis | Single layer of flat cells with no chloroplasts | Protects the leaf and lets light pass to the palisade layer |
| Palisade mesophyll | Tall column-shaped cells packed with chloroplasts just under the upper epidermis | The main site of photosynthesis, positioned where light is strongest |
| Spongy mesophyll | Irregular rounded cells with large air gaps between them | Air spaces allow carbon dioxide and oxygen to diffuse through the leaf |
| Air spaces | Gaps between spongy mesophyll cells, linked to the stomata | Provide a large surface for gas exchange and speed diffusion |
| Lower epidermis | Single layer of cells containing the stomata | Protective layer containing the pores for gas exchange |
| Guard cells and stomata | Pairs of bean-shaped cells surrounding a pore | Control the entry of carbon dioxide and the loss of water vapour |
| Vascular bundle | Group of tubes in the midrib and veins, containing xylem and phloem | Xylem delivers water and minerals; phloem carries glucose away |
| Chloroplasts | Small green discs inside mesophyll and guard cells | Contain chlorophyll that absorbs light for photosynthesis |
Part 3 of 4Labelling a section: a method
1
Find the outer surfaces first. The surface with a thick cuticle and no pores is the upper surface; stomata are mostly on the lower surface.
2
Locate the layer of tall, closely packed cells directly below the upper epidermis. This is the palisade mesophyll.
3
Find the looser layer beneath it with obvious gaps. This is the spongy mesophyll, and the gaps are air spaces.
4
Look for the circular or oval group of tubes in the middle. This is the vascular bundle; the thicker-walled xylem is usually towards the upper side and the phloem towards the lower side.
5
Finish by marking guard cells on either side of each gap in the lower epidermis.
Part 4 of 4Why the arrangement increases photosynthesis (Extended tier only)
1Light reaches the chloroplasts
- Light reaches the chloroplasts A transparent cuticle and epidermis let most light pass through to the cells below.
2Chloroplasts at the top
- Chloroplasts at the top Palisade cells hold the greatest number of chloroplasts and sit near the upper surface, so they absorb the most light.
3Gases move quickly
- Gases move quickly Open stomata and interconnected air spaces keep carbon dioxide arriving and oxygen leaving.
4Supply lines
- Supply lines Veins bring water to every cell and remove glucose so that the reaction does not stall.
Key terms
| Term | Meaning |
|---|---|
| Dicotyledon | A flowering plant whose seeds contain two seed leaves, usually with broad, net-veined leaves. |
| Palisade mesophyll | The layer of tall, chloroplast-rich cells where most photosynthesis occurs. |
| Spongy mesophyll | The layer of loosely arranged cells with air spaces that allow gas exchange. |
| Vascular bundle | A group of xylem and phloem vessels that transports substances through the leaf. |
| Epidermis | The protective outer layer of cells of a leaf. |
| Xylem | Tissue that carries water and mineral ions up the plant. |
Avoid these mistakes
1
Confusing palisade and spongy mesophyll. Palisade cells are tall and tightly packed; spongy cells are rounded with gaps.
2
Saying the epidermis cells contain many chloroplasts. They are mostly transparent and have few or none, apart from the guard cells.
3
Mixing up xylem and phloem. Xylem brings water up from the roots; phloem carries sugars away from the leaf.
4
Giving a vague function such as 'helps the plant'. State exactly what each structure does, for example 'absorbs light' or 'lets gases diffuse'.
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