Internal structure of a dicot leaf
A dicot leaf is dorsiventral: upper and lower epidermis, mesophyll of palisade and spongy parenchyma, and veins with xylem facing up and phloem facing down.
Tissues of a dicot leaf, from top to bottom
| Tissue | Structure | Job |
|---|---|---|
| Upper epidermis | Single layer of tightly packed cells with a thick cuticle; few or no stomata | Protects; reduces water loss |
| Palisade parenchyma | Long, column-like cells packed closely below the upper epidermis; many chloroplasts | Main site of photosynthesis |
| Spongy parenchyma | Loose, irregular cells with large air spaces; fewer chloroplasts | Exchange of gases; some photosynthesis |
| Vascular bundles (veins) | Conjoint, collateral, closed; xylem towards the upper epidermis, phloem towards the lower; ringed by a bundle sheath | Xylem brings water; phloem carries food away |
| Lower epidermis | Single layer with a thin cuticle and many stomata, each with two bean-shaped guard cells | Gas exchange and transpiration |
Palisade vs spongy parenchyma
Palisade and spongy parenchyma together make up the mesophyll. Because the two sides of the leaf differ, the leaf is called dorsiventral.
How the leaf is built for photosynthesis
- A thin, flat blade gives a large surface and a short path for light and CO₂.
- Palisade cells, full of chloroplasts, sit near the top where light is strongest.
- Air spaces in the spongy tissue let CO₂ reach every cell.
- Stomata let CO₂ in and O₂ out.
- Veins bring water in and carry sugar away.
How to draw the T.S. of a dicot leaf
- 1Draw two long parallel lines a little apart for the leaf blade. Mark a thick line on top: cuticle.
- 2Under the cuticle, draw one row of box-like cells: upper epidermis.
- 3Below it, draw one or two rows of tall, closely packed cells with dots (chloroplasts): palisade parenchyma.
- 4Below that, draw loose, irregular cells with gaps: spongy parenchyma and air space.
- 5In the middle, draw a vein: a circle with xylem on the upper side and phloem on the lower side, ringed by bundle sheath cells.
- 6Draw the bottom row of cells: lower epidermis. Break it in two places with a pair of guard cells, and label stoma, guard cell and the sub-stomatal cavity above it.
Hold a mango or hibiscus leaf: the upper side is darker and shinier (thick cuticle, palisade cells), and the lower side is paler (spongy cells, many stomata).
1 marks: “In a dicot leaf, where are most stomata found?” In the lower epidermis.
2 marks: “Why is a dicot leaf called dorsiventral?” Its mesophyll is divided into palisade parenchyma (upper side) and spongy parenchyma (lower side), so the two surfaces differ.
4 marks: “Describe the internal structure of a dicot leaf.” Upper epidermis with cuticle; palisade parenchyma; spongy parenchyma with air spaces; vascular bundles (xylem up, phloem down, bundle sheath); lower epidermis with stomata; add a labelled diagram.
Leaf sandwich, top to bottom: 'Eat Paneer, Skip Eggs': Epidermis (upper), Palisade, Spongy, Epidermis (lower, with stomata).