🍃 Epidermal tissue
This covers the outside of the plant like a skin. The cells fit closely together to protect the tissues underneath. On a leaf, the epidermis is thin and see-through, so light can pass straight through it to the cells below.
Plant Tissues, Organs and Systems » Plant Tissues and the Leaf
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.2.3.1
- The main plant tissues and how each one's structure suits its job
- Where meristem tissue is found and what it does
- How the leaf is built as an organ, from top to bottom
- How guard cells surround stomata
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Unlock This CoursePlants are made of tissues too
Plants are organised in the same way as animals. Cells that do the same job group together to form tissues, and tissues work together to make organs. The leaf is a plant organ. It is built from several different tissues, each with a structure that fits its function.
This lesson looks at the main plant tissues first, then shows how they fit together inside a leaf.
Key terms:
- Mesophyll: the tissue in the middle of a leaf, where most photosynthesis happens. It comes in two types: palisade and spongy.
- Guard cells: a pair of cells that surround each stoma and control whether it is open or closed.
- Stoma (plural stomata): a tiny hole in the leaf surface that lets gases move in and out.
The main plant tissues
☀ Palisade mesophyll
These tall, thin cells are packed with chloroplasts. They sit near the top of the leaf, where they catch the most light. Their shape lets lots of cells fit in a small space, so lots of photosynthesis can happen.
🌿 Spongy mesophyll
These cells are rounder and loosely packed, with large air spaces between them. The air spaces let gases such as carbon dioxide and oxygen move through the leaf by diffusion. The cells have some chloroplasts, but fewer than the palisade cells.
🌱 Meristem tissue
Meristem tissue is found at the growing tips of shoots and roots. Its cells keep dividing to make new cells, which then differentiate. This is how a plant grows longer.
Xylem and phloem: the transport tissues
Xylem and phloem are the transport tissues of a plant. You met their cells earlier, so here is a quick reminder.
- Xylem carries water and mineral ions from the roots to the stem and leaves. Its hollow tubes with strong walls let water flow through easily.
- Phloem carries dissolved sugars to every part of the plant. Holes in the end walls of its cells let the dissolved sugars pass along.
In a leaf, xylem and phloem run together in bundles called veins. The veins bring in water for photosynthesis and carry the sugars away.
Remember the direction
Xylem brings water in from the roots. Phloem moves sugars to the parts of the plant that need them. The next lesson, Transpiration and Translocation, explains how the movement works.
The leaf as an organ
A leaf is an organ because it is made of different tissues working together for one overall job: photosynthesis. Imagine cutting a leaf in half across its width. This is called a transverse section. From top to bottom you would see these layers:
- Upper epidermis: a thin, transparent layer that lets light through.
- Palisade mesophyll: tightly packed, chloroplast-rich cells that do most of the photosynthesis.
- Spongy mesophyll: loosely packed cells with air spaces for gases to move through.
- Lower epidermis: contains the stomata, each one surrounded by two guard cells.
Running through the middle are the xylem and phloem in the veins.
Top of the leaf
Transparent epidermis and palisade cells sit at the top, so light reaches the chloroplasts.
Middle of the leaf
Spongy mesophyll has air spaces, so carbon dioxide can spread to every cell.
Veins
Xylem delivers water and phloem takes sugars away, so the leaf keeps working.
Guard cells and stomata
Stomata are the tiny holes in the epidermis. Most are on the underside of the leaf. Each stoma is surrounded by two guard cells, which are usually bean-shaped.
The guard cells can open and close the stoma. An open stoma lets carbon dioxide diffuse into the leaf and oxygen diffuse out. Guard cells are different from other epidermal cells because they contain chloroplasts. How stomata work in action is covered in the next lesson.
Worked example
Question: Explain why palisade cells are near the top of the leaf and spongy cells are below them.
Answer: The top of the leaf receives the most light. The palisade cells are packed with chloroplasts, so placing them near the top means they absorb the most light for photosynthesis. The spongy cells below have air spaces that let carbon dioxide diffuse in from the stomata on the underside.
Observing and drawing a leaf
The spec suggests you observe and draw a transverse section of a leaf under a microscope. When you draw it:
- Use a sharp pencil and draw clear, single lines. Do not shade.
- Draw the layers in proportion, with the right order from top to bottom.
- Label each tissue.
Common mistakes
Saying the palisade layer is at the bottom. It is at the top, where the light is. Another mistake is saying the whole leaf is made of one type of cell. A leaf is an organ made of several tissues. Students also forget that guard cells have chloroplasts, but ordinary epidermal cells do not.
Exam-style question
(a) Name the tissue in a leaf that carries water to the cells. [1 mark]
(b) Palisade mesophyll cells are tall and packed with chloroplasts. Explain how this helps the leaf. [2 marks]
(c) Name the cells that surround each stoma, and state one way they differ from other epidermal cells. [2 marks]
Model answer
(a) Xylem. (1)
(b) The chloroplasts contain chlorophyll, which absorbs light for photosynthesis (1). Tall cells near the top mean many cells fit in a small space, so more light is absorbed (1).
(c) Guard cells (1). They contain chloroplasts, but other epidermal cells do not (1).
Exam tip
In a "how" or "explain" question about a leaf, link the structure to the function. For example, write "air spaces, so gases can diffuse", not just "air spaces".