Plants Cell Organisation

The leaf is an organ made up of several types of tissue:

• 1) The epidermal tissues are covered in a waxy cuticle, which helps reduce water loss by evaporation.
• 2) The upper epidermis is a transparent layer so that light can pass through it to the palisade layer.
• 3) The palisade layer has lots of chloroplasts, which means that they're near the top of the leaf where they can get the most light.
• 4) The xylem and phloem form a network of vascular bundles, which deliver water and other nutrients to the entire leaf and take away the glucose produced by photosynthesis. They also help support the structure.
• 5) The tissues of the leaves are also adapted for efficient gas exchange. 

STOMATA:

The lower epidermis is full of stomata, which let CO2 diffuse directly into the leaf. The opening and closing of stomata is controlled by guard cells in response to environmental conditions. The air spaces in the spongy mesophyll tissue increase the rate of diffusion of gases.

Phloem tubes transport food:

• 1) They allow cell sap to flow through and they transport food substances (mainly dissolved sugars) made in the leaves to the rest of the plant for immediate use or for storage.
• 2) The transport goes in both directions.
• 3) The process is called translocation

Xylem tubes take water up:

• 1) They're made of dead cells and are strengthened with a material called lignin.
• 2) They carry water and mineral ions from the roots to the stem and leaves.
• 3) The movement of water from the roots, through the xylem and out of the leaves is called the transpiration stream.

TRANSPIRATION IS THE LOSS OF WATER FROM THE PLANT

Guard cells are adapted to open and close stomata:

• 1) When the plant has lots of water the guard cells fill with it and go plump and turgid. This makes the stomata open so gases can be exhchanged for photosynthesis.
• 2) When the plant is short of water, the guard cells lose water and become flaccid, making the stomata close. This helps stop too much water vapour escaping.
• 3) Thin outer walls and thickened inner walls make the opening and closing work.
• 4) They're also sensitive to light and close at night to save water without losing out on photosynthesis.
• 5) More stomata is usually found on the undrsides of leaves. The lower surface is shaded and cooler- less water is lost through the stomata than if they were on the upper surface.