Most green parts of a plant can photosynthesise, but the leaves are the plant organs which are best adapted for this function. To be able to photosynthesise efficiently, leaves need to have a large surface area to absorb light, many chloroplasts containing the chlorophyll, a supply of water and carbon dioxide, and a system for carrying away the products of photosynthesis to other parts of the plant. They also need to release oxygen from the leaf cells. Most leaves are thin, flat structures supported by a leaf stalk which can grow to allow the blade of the leaf to be angled to receive the maximum amount of sunlight.
Inside the leaf are layers of cells with different functions:
The two outer layers of cells have few chloroplasts and are covered by a thin layer of a waxy material called the cuticle. This reduces water loss by evaporation, and acts as a barrier to the enrtry of disease causing microorganisms such as bacteria and fungi.
The lower epidermis has many pores called stomata. Usually the upper epidermis contains fewer or no stomata. The stomata allow carbon dioxide to diffuse into the leaf, to reach the photosynthestic tissues. They also allow oxygen and water vapour to diffuse out. Each stoma is formed as a gap between two highly specialised cells called guard cells, which can alter their shape to open or close the stoma.
In the middle of the leaf are two layers of photosynthetic cells call the mesophyll. Just below the upper epidermis is the palisade layer. This is a tissue made of elongated cells, each containing hundreds of chloroplasts, and is the main site of photosynthesis. The palisade cells are close to the source of light, and the upper epidermis is relatively transparent, allowing light to pass through to the enormous numbers of chloroplasts which lie below.
Below the palisade layer is a tissue made of more rounded, loosely packed cells, with air spaces between them, called the spongy layer. These cells also photosynthesise, but have fewer chloroplasts than the palisade cells. They form the main gas exchange surface of the leaf, absorbing carbon dioxide and releasing oxygen and water vapour. The air spaces allow these gases to diffuse in and out of the mesophyll.
Water and mineral ions are supplied to the leaf by vessels in a tissue called the xylem. This forms a continuous transport system throughout the plant. Water is absorbed by the roots and passes up through the stem and through veins in the leaves in the transpiration stream. In the leaves, the water leaves the xylem and supplies the mesophyll cells.
The products of photosynthesis, such as sugars, are carried away from the mesophyll cells by another transport system, the phloem. The phloem supplies all other parts of the plant, so that tissues and organs that can’t make their own food receive products of photosynthesis. The veins in the leaf contain both sylem and phloem tissue, and branch again and again to supply all parts of the leaf.
Published in: Gardening