The Female Reproductive Organ: The Carpel
The female parts of a flower consist of an ovary, which contains one or more ovules, a style and the stigma. The ovary is at the base of the flower. From the ovary, extends a tubular structure called the style and on the top of the style is a surface receptive to pollen called the stigma. The stigma can take many different forms, most of them designed to help trap pollen. There are many variations on this basic structural theme. After fertilization the ovule becomes the seed and the ovary becomes the fruit.
The Male Reproductive Organ: The Stamen
The male parts of a flower consist of one or more stamens. Each stamen is made up of paired anthers (sacs containing pollen) on a filament or stalk. The anthers are the orange/yellow structures often seen in the centre of a flower. Pollen from the anthers of one flower is transferred to the stigma of another usually either by wind, or by animals, especially insects.
Petals, Corolla, Sepals, Calyx
The reproductive structures in higher plants are contained within flowers. Flowers have more than one petal, and the flower petals are collectively called the corolla. A flower bud is protected by green leafy structures called sepals. Collectively, all of the sepals form the calyx. The corolla or petals are often brightly coloured with markings attractive to insects. The flowers may also be scented. For instance, Honeysuckle has showy, attractive flowers which attract insects by day. However, in the dark, their colourful show is not much use, and their heady scent then helps to attract night-flying moths. In insect-pollinated plants, there are also usually nectaries which secrete sugary nectar, located within the flower. These provide an incentive to insects to visit the flowers. In the search for nectar, the insects will often get pollen grains caught on their bodies. This may then brush off onto the stigma of the next flower visited and in this way the flowers are pollinated. The receptacle is the place on the stem where floral organs originate and attach.
Formation of Sex Cells
The sex cells of the flowering plant are called gametes. There are both male and female gametes thus the flower undergoes sexual reproduction.
A cross section of the developing anther displays four chambers. These chambers are called pollen sacs (see upper illustration). Each pollen sac is filled with cells containing large nuclei. As the anther grows, each of these cells goes through two meiotic divisions, forming a tetrad. These cells are called microspores. Each one of these microspores eventually becomes a pollen grain. Each pollen sac is enclosed by a protective epidermis and a fibrous layer. Inside the fibrous layer is the tapetum. This is a food store and will provide energy for future cell divisions.
Each pollen grain is surrounded by a tough protective wall called an exine. This is a tough covering that allows the pollen grain to survive harsh conditions for long periods of time. The intine is another thin protective coating.
Pollination and Fertilization
The pollen grains and ovules by themselves cannot travel from one place to another. There are other factors such as wind, water, or animals that enable the pollen grains to travel to reach the stigma of the same or another flower. The process by which pollen grains are transferred from the anthers of one flower to the stigma of the same or another flower is known as pollination.
Once the pollen grain carrying the male gamete reaches the stigma of the same or different flower, it is transported downwards via a pollen tube that arises from the pollen grain to reach the ovary. The ovary is present at the base of the flower and is lobed. These lobes contain the ovules carrying the female gamete.
The male gamete from the pollen now fuses with the female gamete in the ovule to form a zygote. This zygote undergoes cell division to form an embryo. Once the embryo starts developing, the ovary develops into the fruit, the other structures like the calyx and corolla fall off. The embryo becomes the seed which has the potential to give rise to a new plant.
Seed Formation
The fertilized becomes the seed. The integuments become the wall of the seed called the testa. The micropyle closes. The endosperm nucleus leads to the formation of triploid endosperm, a food tissue. The diploid zygote, by mitosis, develops into a plant embryo. The developing embryo draws nourishment from the endosperm. The embryo ceases development and goes dormant. The ovule becomes a seed, which contains a dormant plant embryo, food reserve, and the protective coat called the testa.
The Embryo
The embryo is made up of the radicle or future root and the plumule or future shoot. The endosperm cells divide many times and absorb the nucellus. This is the nutrition (mainly fats, oils and starch) for the embryo. There are 2 types of seeds. Some are endospermic while others are non-endospermic. In endospermic seeds the food reserve is the endosperm, which is outside the plant embryo. Examples of this type of seed are maize and wheat. Non-endospermic seeds have food reserve within the cotyledon(s) of the plant embryo. This occurs in broad beans.
Monocots and Dicots
Monocots have one cotyledon in the seed while dicots have two cotyledons. The cotyledons are food reserves for the young plant after it germinates from the soil. It uses these food reserves until it is capable of making its own food. In monocots the food is absorbed from the endosperm while in dicots the food is stored in the cotyledons.
Fruit Development
The ovary becomes a fruit. The wall of the ovary becomes the wall of the fruit called the pericarp. The fruit protects the developing seeds and plays an important role in seed dispersal. This process is controlled by auxins produced by the seeds. Once the fruit forms the rest of the flower parts die and fall away.
Fruit and Seed Dispersal
Seed dispersal is the scattering of offspring away from each other and from the parent plant. As a result of dispersal there is an improved chance of success by reducing competition and overcrowding. Dispersal also enables colonisation of new suitable habitats and thus, there is an increased chance of species survival.
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