A leaf is attached to the stem by a leaf stalk, which continues into the leaf as a midrib supporting the whole leaf structure. A network of veins branches from the midrib delivering water and salts to the leaf cells and also carrying away the food made by the leaves.
The epidermis is a cell layer on the upper and lower surfaces of the leaf. It is covered by a thin waxy layer called the cuticle.
Functions of the epidermis:
- helps to maintain the leaf’s shape.
- helps to reduce evaporation from the leaf.
- prevents bacteria and fungi from getting to the internal parts of the leaf.
- The epidermal cells produce the cuticle, which is a waxy layer helps to reduce water loss.
In the leaf epidermis there are very small openings called stomata (plural). A stoma (singular) is surrounded by a pair of guard. In some plants, the stomata occur only in the lower epidermis whilst in some the stomata are equally distributed on both sides of the leaf.
The region between the upper and lower epidermis is called mesophyll layer. It is made up of two parts: the palisade mesophyll on the top and the spongy mesophyll on the bottom.
The palisade mesophyll layer consists of palisade cells which are usually long and contain many chloroplasts. Chloroplasts are green organelles which contain the chlorophyll for photosynthesis. They are found in the cytoplasm of the palisade cells.
The spongy mesophyll cells vary in shape and have air spaces between them to allow carbon dioxide to diffuse into the leaf and oxygen out of the leaf.
Vascular bundles (veins) are made up of two different types of vessels, called xylem and phloem.
The xylem vessels are thick walled long thin tubes empty of cell contents when mature. Their role is to carry water and dissolved salts to cells of the leaf.
The phloem vessels are like sieve tubes, the ends of which are perforated like sieve plates. They transport food made in the leaf, such as sugars to other parts of the plant.
Adaptation of leaves for photosynthesis
When something is said to be adapted, it means that its structure is suited for its function. The leaf has the following adaptations for the process of photosynthesis:
- broad, flat shape to increase the surface area for absorption of sunlight and carbon dioxide.
- thin in order for carbon dioxide to diffuse only a short distance to reach the palisade cells.
- large spaces between the cells in the spongy layer to allow Carbon dioxide to diffuse easily into the leaf.
- many stomata on the under surface of the leaf to allow the exchange of carbon dioxide and oxygen with the external environment.
- more chloroplasts in the palisade cells to trap sunlight and make food.
- a large network of veins to provide a good water supply to the photosynthesising cells.
- thin cuticle to allow light through.
- palisade cells on the upper surface to maximise the absorption of sunlight.
- elongated palisade cells to increase the surface area for the absorption of light.
Summary of the parts of a leaf
|Part of leaf||Details|
|Cuticle||Waxy layer that waterproofs the leaf. It is produced by cells of the upper epidermis.|
|Upper epidermis||Thin transparent cells on the upper surface of the leaf. Act as a barrier against external conditions.|
|Palisade mesophyll||The main region where photosynthesis takes place. Cells are elongated and contain chloroplasts to trap light energy.|
|Spongy mesophyll||Air spaces between the cells to allow gaseous exchange – carbon dioxide into the cells, oxygen out of the cells during photosynthesis.|
|Vascular bundle||Made up of xylem and phloem vessels. Xylem vessels bring water and minerals into the leaf. Phloem vessels transport sugars away from the leaf (the process is called translocation).|
|Lower epidermis||This is a protective layer of cells on the bottom part of the leaf. Contains stomata to regulate the loss of water vapour through transpiration.|