Plants are divided into flowers, stems, leaves and roots with root hairs. A generalised plant is shown in the illustration.
The stem provides support for the leaves and flowers. It also allows water and food to travel both up and down the plant.
The leaves make the food for the plant. Photosynthesis takes place in the leaves.
The roots anchor the plant in the soil and take up water and salts (mineral ions) from the soil. The root hairs provide a large surface area for water and salt uptake.
The flowers are reproductive organs. They attract insects that carry pollen from one plant to another. This process of transferring pollen from plant to plant is known as pollination.
The structure of the leaf
Leaves produce the food for the plant. The structure of the leaf is shown in the illustration. The leaf has prominent vascular bundles that contain two types of tubes, the xylem tubes and the phloem tubes. The leaf has the following parts
- The leaf has a waxy cuticle to stop it losing water and drying out.
- The epidermis is a protective layer of cells and contains no chloroplasts.
- The palisade layer contains the most chloroplasts as it is near the top of the leaf. The chloroplasts contain the pigment chlorophyll.
- The palisade cells are arranged upright. This means the light has to pass through the cell lengthways and so increases the chance of light hitting a chloroplast and being absorbed.
- The spongy layer contains fewer chloroplasts, enough to catch what the palisade layer cannot absorb.
- The spongy layer has air spaces to make it easier for gases to circulate in the leaf.
- The vascular bundle "vein" provides the leaf with water via the xylem vessels. Food, such as sugar, made in the leaf is transported in the phloem vessels to the rest of the leaf.
- The stomata (stoma - singular) are tiny pores that allow carbon dioxide to enter the leaf while oxygen leaves the leaf.
- Guard cells can open or close the stomatal pores to regulate how much gas can enter or leave the leaf. At night the pores close, opening in the daytime.
- Leaves are green because they contain the green pigment called chlorophyll. Chlorophyll is used in photosynthesis.
[[..|GSCE Science/Photosynthesis]] for more details
The structure of flowers
(Note: flowers are not covered by many GCSE boards)
- Sepals - these are arranged underneath the flower and are typically green.
- Petals - often brightly coloured to attract insects.
- Stamens - stalk-like filaments that have anthers at the top which produce pollen. Pollen contains the male gametes.
- Pistil - contains one or several carpels that contain the ovaries with ovules, the female gametes. Sometimes the carpels are merged. A stalk called the style leads upwards from each pistil and is topped by a sticky stigma that receives the pollen.
The pistil is the bottle shaped structure. A pistil can be composed of one or many carpels and a flower can have several pistils.
The structure of the stem
Plant growth requires glucose produced by photosynthesis and energy produced by respiration. It also requires minerals obtained from the soil. Plant growth is controlled by plant hormones called auxins.
Minerals needed for plant growth
There are three minerals that are essential for plant growth: phosphates, nitrates and potassium. Small quantities of iron and magnesium are also needed, especially for the production of chlorophyll.
Phosphates: used in photosynthesis and respiration. Phosphate deficiency: purple leaves and small roots.
Nitrates: used in the production of amino acids. Amino acids are combined to make proteins. Nitrate deficiency: yellowing of leaves and poor, stunted growth.
Potassium: maintains electrical potentials and helps enzyme action. Potassium deficiency: leaves become yellow with spotty, brown, dead areas.