Anatomy of the leaf

A leaf is a food factory

The leaf is the main organ of photosynthesis. Its anatomy shows how it does the job. It must catch light, take in carbon dioxide, let out oxygen and water vapour, and bring in water. A thin leaf can do all this.

A vertical section of a leaf shows three parts: the epidermis, the mesophyll and the vascular system.

The dorsiventral leaf of a dicot

A dicot leaf has a clear upper side and a lower side. So it is called dorsiventral.

• The epidermis covers both surfaces. The upper one is the adaxial epidermis. The lower one is the abaxial epidermis. Both are covered by a cuticle.
• The abaxial epidermis usually has more stomata than the adaxial one. The adaxial epidermis may even have none.
• The mesophyll lies between the two epidermal layers. It is made of chloroplast-bearing parenchyma. It has two kinds of cells.
• The palisade parenchyma lies on the upper side. Its cells are long and stand side by side, at right angles to the surface. Each has many chloroplasts.
• The spongy parenchyma lies on the lower side. Its cells are oval or round and are packed loosely. They leave large air spaces and air cavities, which open to the stomata.

The palisade cells face the light, so most of the food is made there. The air spaces of the spongy layer let gases move between the stomata and all the cells.

The vascular system. The veins and the midrib have vascular bundles. Their size depends on the size of the vein. A dicot leaf has reticulate venation: the veins form a net. In each bundle, the xylem lies towards the upper side, and the phloem lies towards the lower side. A ring of thick-walled bundle sheath cells surrounds the bundle.

The isobilateral leaf of a monocot

A grass or a maize leaf stands almost upright. Both of its faces meet the same light. So it is isobilateral.

• The stomata are present on both surfaces, in about equal numbers.
• The mesophyll is not differentiated into palisade and spongy parenchyma.
• The leaf has parallel venation. The vascular bundles are of nearly equal size.
• Some cells of the upper epidermis, lying in rows along the veins, are large, empty and colourless. These are the bulliform cells.

Bulliform cells. When a grass has plenty of water, the bulliform cells are full and firm, and the leaf lies flat. In a dry spell they lose water and go limp. The leaf curls inwards, and this cuts down the loss of water.

Dicot leaf and monocot leaf side by side

• Shape of the section: dorsiventral in a dicot, isobilateral in a monocot.
• Stomata: more on the lower surface in a dicot, about equal on both in a monocot.
• Mesophyll: palisade and spongy parenchyma in a dicot, undivided in a monocot.
• Veins: a net in a dicot, parallel in a monocot.
• Bulliform cells: absent in a dicot, present in a grass.

You should find that a leaf of $175$ micrometres with $150$ micrometres of mesophyll is $85.7$ percent mesophyll. Nearly all of the thickness is the tissue that makes food.

Try it in Python

The program reads the stomata counts and the mesophyll of three leaves, and names the kind of leaf.

You should see leaf B, with $50$ percent below, called isobilateral. Leaf C with an undivided mesophyll becomes isobilateral too, as it should, since its stomata are on both faces.

Practise with new numbers

Common mistakes

• Putting the palisade cells on the lower side. They lie under the upper epidermis, where the light is.
• Saying that spongy cells are tightly packed. They are loose, with air spaces.
• Turning the bundle upside down. In a dicot leaf, the xylem faces the upper side and the phloem faces the lower side.
• Saying that a monocot leaf has palisade cells. Its mesophyll is not divided.
• Forgetting where the bulliform cells lie. They are on the upper epidermis of a grass leaf, along the veins.

Remember these

• A dicot leaf is dorsiventral: palisade above, spongy below, more stomata below.
• In a dicot vein, xylem is on the upper side and phloem is on the lower side.
• A monocot leaf is isobilateral: undivided mesophyll, stomata on both faces, nearly equal bundles.
• Bulliform cells help a grass leaf to curl in dry weather.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which layer of a dicot leaf has the most chloroplasts, and where does it lie?

2. Why are there large air spaces in the spongy parenchyma?

3. On which face of a dicot leaf are the stomata usually more numerous?

4. What are bulliform cells, and what do they do in dry weather?

5. Name two features that tell a monocot leaf from a dicot leaf in a section.