The Leaf The Leaf is the Main photosynthetic organ in a plant. Controls gas exchange in plants. Controls the amount of water loss in plants when it is hot and dry by: – closing their stomates. – produce a thick cuticle layer. – reduce the surface area of their leaves.

Fig 30.10

Leaf Cross Section

Waxy Cuticle & Upper Epidermis Waxy cuticle Upper Epidermis Upper Epidermis cells contain no chloroplasts – not true of the stoma cells. They form layers on the upper and lower surfaces of the leaf. Their function is to prevent water getting out and stopping unwanted substances/organisms getting in. The waxy cuticle is a thin layer atop the epidermis. Its function is to reduce/prevent water loss from the leaf. In arid conditions (desert) this cuticle layer can be quite thick.

Palisade Mesophyll Layer Palisade mesophyll layer The palisade mesophyll layer is where most of the photosynthesis occurs in the leaf. The palisade cells contain a lot of chloroplasts to help them perform this photosynthesis. The cells of this layer are closely packed together to maximize the absorption of light. In the leaf cross-section we can see the palisade cells are only found in the upper part of the leaf.

Spongy Mesophyll Layer Spongy mesophyll layer The cells in the spongy mesophyll layer are not as closely packed as the cells in the palisade mesophyll layer. This creates air spaces inside the leaf to enable gases to move in and out. Bringing in CO 2 and releasing 0 2 and Water Vapor There are not as many chloroplasts in the spongy mesophyll cells as there are in the palisade mesophyll cells – but photosynthesis still occurs in the spongy mesophyll layer. Air space

Lower Epidermis and Stomata Stomata Lower epidermis is the bottom layer of the leaf, and is one cell thick May or may not contain a cuticle Within the Lower Epidermis, there are some holes found in leaves called stoma. These holes allows gases to diffuse in and out of the leaves. The stoma are formed by two highly specialized epidermis cells, called guard cells. Guard cells are the only epidermis cells that contain chloroplasts. The stoma open and close (using turgor pressure) depending upon the requirements of the plant. It is through these stoma that water leaves the leaf, the process that powers transpiration. Guard Cell

Transpiration Process by which evaporation of water from leaf surface draws water from roots upward

Stomata and Guard Cells

Vein- Leaf Transport The veins provide the transport of materials in the leaf – Carries water, minerals, and food through the leaf and rest of the plant Has two parts – Xylem- water and mineral transport – Phloem- sugar and food transport

Xylem and Phloem

Exercise 4: Leaf Structure 1. ___________ 2. ___________ 3. ______________________ 5. _____________________ 7. __________ 6. ________ 4. __________ Waxy cuticle Epidermis Palisade mesophyll layer Spongy mesophyll layer Epidermis Stomata Air space