The slide shows transverse sections of three different types of leaf.
- Xerophyte – adapted to dry conditions
- Mesophyte – not particularly adapted to dry or wet conditions
- Hydrophyte – adapted to aquatic conditions
Leaves are made up of a series of layers
- upper epidermis
- palisade layer
- spongy mesophyll
- lower epidermis
The following three images show these layers in the three different types of leaf.
Leaves are efficient light catchers so that they can make food by photosynthesis. As well as light, they need carbon dioxide and water. The layer most involved in photosynthesis is the palisade layer near the upper surface of the leaf.
As well as light, plants need carbon dioxide and water in order to make food by the process of photosynthesis. Carbon dioxide is obtained from the atmosphere and water is brought up via xylem vessels from the roots. Carbon dioxide diffuses into the leaf via stomata (small pores in the epidermis). While stomata provide the means of entry of carbon dioxide they also provide the means of exit of water vapour. Water is pulled up through the xylem vessels by transpiration, which is the evaporation of water from the leaf. Without transpiration, the leaf would be unable to get water. So, while transpiration is an essential process,in the provision of water to tissues, it is important that the plant has the mechanisms to keep its loss to a minimum.
Plants need to be able to reduce water loss effectively without compromising their carbon dioxide uptake. Very often leaves have more stomata on the lower surface (which is cooler) than on the upper.
Stomata are opened and closed by the action of two guard cells surrounding each stoma. Stomata tend to be closed at night as photosynthesis does not take place then They will also close under extreme drying conditions.
Xerophytes have the biggest problem. Hot, dry conditions mean that water loss by evaporation needs to be kept to a minimum.
Xerophytic leaves often have a water proof cuticle together with thick epidermis which means that less water can evaporate from the upper surface.
The stomata on the lower epidermis of the xerophytic leaf are often found in hair lined pits. Moist air trapped here lengthens the diffusion pathway and reduces evaporation rate, but still enables carbon dioxide to diffuse in.
As the leaf is such a thin structure, mesophytes too need to keep water loss to a minimum. The epidermis is not as thick as that of the xerophyte and often there will be some stomata present allowing a shorter diffusion distance to the main photosynthesising tissue – the palisade layer (the closely packed cylindrical cells just beneath the epidermis).
Carbon dioxide diffuses from the air through the stomata and into the air spaces of the spongy mesophyll and up to the palisade layer. Water diffuses from the xylem into the air spaces and up to the palisade layer and also out through the stomata.
Aquatic plants have no problem with water loss, however they do need to ensure effective light collection. Large air spaces keep them buoyant and close to the surface so that carbon dioxide can diffuse in from the air. Many more stomata are present on the hydrophyte’s upper epidermis.