- Structure of a Plant
- Leaf Structure
- Plant Nutrition and Growth
- Parts of a Flower
- Plant Reproductiontion
- Fertilisation and Seed Germination
Structure of a Plant
The four main plant organs are the roots, the stem, leaves and flowers.
The roots anchor the plant in the soil and also take in water and dissolved minerals, especially nitrates, from the soil. The roots have tiny root hairs which increase the surface area to take in water and minerals. The root hair cell has a really thin cell wall to absorb minerals. The root is an important part of the nutrition process.
The water and dissolved minerals travel through hollow tubes in the stem to the leaves where the process of photosynthesis produces food for the plant. The stem also holds the plant upright and supports the nutrition, growth and excretion processes.
Leaves are the organs of photosynthesis and contain chlorophyll which uses light to change carbon dioxide and water into glucose. Photosynthesis involves the reaction of water and carbon dioxide to produce carbohydrates and oxygen which is returned to the atomsphere. The process requires the presence of sunlight and the green pigment, chlorophyll, which is the catalyst:
carbon dioxide + water --(sunlight + chlorophyll)--> carbohydrate (glucose) + oxygen
Leaves usually have a large area to absorb the maximum amount of light. They are thin so that the carbon dioxide does not have to travel far through the leaf. The veins in a leaf give the leaf some support and provide the leaf with a supply of water. Leaves also have tiny little pores to pass the gases in and out. Thus leaves are involve in the nutrition and excretion processes.
The flower contains the reproductive organs. It attracts insects which carry pollen between plants for pollination. The flower contribute to the reproduction process.
Leaf Structure
The cross-section of a leaf shows a layered structure, with cuticle on the top, followed by upper epidermis, palisade cells, spongy layer and finally the lower epidermis. The underside of the leaf contains stomata (pores) that open and close (control by guard cells) to allow carbon dixoide enter and oxygen and water to escape through. The stomata close at night to prevent too much water loss.
Cuticle is the waxy layer on the surface of the leaf to prevent evaporation of water from the surface. Below the cuticle is a single layer of tightly fitting cells called the epidermis. Still near the surface are the palisade cells below the epidermis, containing a large number of chloroplasts with chlorophyll and it is here that the photosynthesis takes place.
The cells in the spongy layer below the palisade cells contains irregularly shaped cells and large air gaps exist between the cells. The oxygen and carbon dioxide gases are stored here.
While photosynthesis only occurs in sunlight, aerobic respiration occurs in plants all the time. The equation summarising aerobic respiration is the reverse of photosynthesis and energy is given out:
carbonhydrate + oxygen --> carbon dioxide + water
Plant Nutrition and Growth
Photosynthesis is the chemical reaction which takes place in every green plant. It happens in all the green bits of the plant but mainly in the leaves to produce food in the form of glucose for the plant to grow. From the photosynthesis equation, plant needs sunlight, chlorophyll, water and carbon dioxide for the photosynthesis to happen. Chlorophyll use the sunlight energy to convert water and carbon dioxide into glucose and oxygen. Water is taken from the roots travel up the stem and into the veins and carbon dioxide diffuses throught the pores (stomata) of the leaves. Glucose is stored as starch or taken in the veins to other parts of the plant. Oxygen diffuses out through the leaf's stomata.
Photosynthesis (hence plant growth) is affected by four parameters: light, water, temperature and CO2 levels. The rate of photosynthesis increases with the amount of light, water and CO2. However, there is an optimal temperature around 30°C where photosynthesis works best. Photosynthesis can take place between 30-40°C but slow down significantly above 40°C.
Plants also need essential minerals for growth. These are nitrates, phosphates and potassium (NPK) and absorbed by the root hairs from the soil together with water. The root hairs provide a large surface area and have a thin cell membrane for effective absorption. The plant will show a deficiency symptom from lack of these minerals:
- nitrates (硝酸鹽) provide nitrogen for making proteins - lack of nitrates affect growth (small plant) with yellow older leaves.
- phosphates (磷酸鹽) provides phosphorous for photosynthesis and respiration - lack of phosphates affect root growth and purple younger leaves
- potassium is needed by enzymes involved in photosynthesis and respiration - lack of potassium result in yellow leaves with dead bits
Parts of a Flower
A flower contains both the male and female parts. The male parts are called stamens (雄蕊)which are made up of anthers (花藥) and filament (the stalk to hold the anther up), and they make pollen grains inside four-chambered anthers. The female parts, called carpels (心皮), consist of stigma, style (tube leading to the ovary) and ovary (子房), are at the centre of the flower. The ovary is hollow which contains ovules (胚珠). When an ovule receives pollen the flower starts to form a seed.
Many flowers have brightly coloured petals surrounding the carpels and stamens. Sometimes nectar is produced at the nectary at the base of the petals. Both nectar and the petals (also scent) attract bees/insects for pollination. Below the flower there is usually an outer ring of green sepals which protect the flower when it is a bud.
Plant Reproduction
Plants can reproduce asexually or sexually. An asexual reproduction involves only one parent which produces offspring which are identical to the parent. Asexual reproduction can be achieved by stem/leaf cuttings which will root and produce plants that are all identical. variation does not occur with asexual reproduction, and any disease, or fault in the parent, will be pass-on and present in the offspring.
In sexual reproduction, new plants are produced by the flowers. This involves pollination where a pollen grain lands on its stigma. Self-pollination occurs when the pollen comes from the same plant. Crsos-pollination occurs when the pollen comes from another plant. There are two main pollination mechanisms: insect and wind. With insect pollination, plants have bright coloured petals, scented flowers with nectaries, small amount of pollen and sticky stigma to take the pollen off the inscet. With wind pollination, plants have small dull petals with no scent ornectaries, lots of pollen, long filaments hang the anthers outside the flower for pollen to be blown away and feathery stigmas to catch pollen in the air.
Fertilisation and Seed Germination
Pollination is followed by fertilisation where the male sex cells and the female sex cells join up. The entire process include pollen grains containing male sex cells landing on a stigma (from insect or wind), each pollen grain grows a pollen tube down to the ovule and, once a pollen tube reaches the ovule, the nucleus from the pollen joins with the nucleus in the female sex cell from the ovum. The fertilised ovule then turns into a seed containing an embryo plant. The embryo has a hard seed coat, embryo root, embryo shoot and a food store which it uses when it starts to grow or germinate. The seeds produced contain different genetic material and will grow into different new varieties of plants.
There are three ways in which seeds can be dispersed so they can grow with less competition from each other. In wind dispersal, the seeds have features that allow them to be carried by the wind. Examples are dandelion fruit with parachutes to catch the wind and sycamore fruit with wings to help it fly away. In animal dispersal, the fruits are either eaten by an animal (e.g tomato) and the seeds come out in the droppings or stick to an animal (e,g. burdock has hooks). Explosion dispersal is where the seeds are flick out as in peas.
The first stage in growing a plant from seed is germination (i.e. seed starts to grow). The conditions needed for germination are enough water, oxygen and the right temperature (usually between 5°C and 45°C). When the conditions are right, the seed takes in water and starts to grow using the energy store, the embryo root develops and grows downward to take in water whereas the embryo shoot begins to grow and, once out of the soil, leaves develop.