Contents
Developmental biology, a branch of Biology Topics, examines the processes of growth and differentiation.
What are Flowering Plants and Sexual Reproduction in Flowering plants?
The plants that have flowers are called flowering plants. Most of the flowering plants reproduce by sexual reproduction method involving the fusion of sex cells called gametes. This means that two sexes (male and female) are involved in reproduction in flowering plants. Flowers are the reproductive parts of a plant. The reproductive organs (or sex organs) of a plant are in its flowers (see Figure).
In other words, flowers contain the sexual reproductive organs of a plant. In most of the plants, the same flower contains the male organ as well as the female organ. The function of a flower is to make male and female gametes, and to ensure that fertilisation will take place to make seeds for growing new plants. In other words, flowers perform the function of reproduction in plants by producing seeds of the plant. Thus, flowers are for sexual reproduction in plants.
The sexual reproduction in plants involves the fusion (combination) of male and female gametes of the plant which leads to the formation of the seeds of the plant. These seeds can then be used to grow new plants. We will first describe the various parts of a flower including the sexual reproductive organs. And then we will describe how the flowers produce seeds by the sexual method which can then be used to grow new plants.
Structure of Flower
The main parts of a flower are: Receptacle, Sepals, Petals, Stamen and Pistil. These main parts of a flower are shown in Figure.
1. Receptacle. The base of a flower to which all the parts of a flower are attached is called receptacle (see Figure).
2. Sepals. The green, leaf-like parts in the outermost circle of a flower are called sepals (see Figure). All the sepals taken together are called ‘calyx’. The function of sepals (or calyx) is to protect the flower in its initial stages when it is in the form of a flower bud.
3. Petals. The colourful parts of a flower are called petals (see Figure). The petals lie inside the sepals. All the petals taken together are called ‘corolla’. The petals are usually scented. The function of petals is to attract insects (for pollination) and to protect the reproductive organs which are at the centre of the flower.
4. Stamen. The little stalks with swollen tops just inside the ring of petals in a flower are called stamens. Stamen is the male reproductive organ of the plant (see Figure). The stamen is made of two parts: a filament and an anther (see Figure). The stalk of stamen is called filament and the swollen top of stamen is called anther. Anther makes the pollen grains and stores them. Pollen grains are exposed when the anther ripens and splits. Pollen grains appear to be yellow, powder like substance to us.
Pollen grains contain the male gametes (or male sex cells) of the plant. Each pollen grain contains one male gamete of the plant. The male gamete (or male sex cell) of the plant which is present inside a pollen grain is called ‘male nucleus of pollen grain’.
Pollen grains generally have a tough protective coat which prevents them from drying up. It is clear from this discussion that the male gametes of a plant are made in the anther of stamen. Please note that a flower usually has a number of stamens in it.
5. Pistil. In the centre of a flower, there is a flask-shaped organ called pistil. Pistil is the female reproductive organ of the plant (see Figure). A pistil is made of three parts: stigma, style and ovary (see Figure). The top part of pistil is called stigma. Stigma is for receiving the pollen grains from the anther of stamen (during pollination). The middle.part of pistil is called style. Style is a tube which connects stigma to the ovary. The swollen part at the bottom of a pistil is called ovary (see Figure).
The ovary makes ovules and stores them. Ovules contain the female gametes (or female sex cells) of the plant. The ovary contains one or more ovules. Each ovule contains only one female gamete of the plant. The female gamete (or female sex cell) of the plant which is present inside the ovule is called ‘egg’. It is clear from this discussion that the female gametes of a plant are made in the ovary of pistil. Please note that the female reproductive organ of the plant called pistil is also known as carpel. Another point to be noted is that the female organs called pistils are surrounded by a number of male organs called stamens in the flower.
We have just learnt that stamen is the male reproductive organ of a plant whereas pistil is the female reproductive organ of the plant. Now, male and female reproductive organs of a plant may be present in the ‘same flower’ or in ‘different flowers’. Depending on this, a flower may be unisexual or bisexual.
(i) The flowers which contain only one reproductive organ (either male or female) are called unisexual flowers. This means that the unisexual flowers contain either male organ ‘stamen’ or female organ ‘pistil’. Both male and female unisexual flowers may be present in the same plant or in different plants. The plants such as papaya, watermelon, corn, and cucumber have unisexual flowers.
(ii) The flowers which contain both the reproductive organs (male as well as female) are called bisexual flowers. This means that the bisexual flowers contain both, the male organ “stamen’ as well as the female organ ‘pistil’. The plants such as rose, mustard, Hibiscus and petunia have bisexual flowers.
A new seed of the plant is formed when the male gamete present in a pollen grain unites with the female gamete present in the ovule. This happens in two steps : pollination and fertilisation.
Pollination
For the male gamete to be able to combine with the female gamete, it is necessary that first the pollen grains from the anther of stamen of a flower should be carried to the stigma of pistil. The transfer of pollen grains from the anther of a stamen to the stigma of a pistil is called pollination. Pollination can occur in two ways: self-pollination and cross-pollination.
- When the pollen grains from the anther of a flower are transferred to the stigma of the same flower (or another flower on the same plant), it is called self-pollination [see Figure (a)],
- When the pollen grains from the anther of a flower on one plant are transferred to the stigma of a flower on another similar plant, it is called cross-pollination [see Figure (b)], Pollination is done by insects, wind and water. This is described below.
(i) Pollination by Insects. Pollination is done, by insects such as bees and butterflies. The insects carry pollen from flower to flower and help in pollination. This happens as follows : Certain flowers have a nectary near their base which contains a sugary liquid called nectar. When an insect sits on the flower of a plant for sucking nectar, then the pollen grains from the anther of this flower stick to its body. And when this insect now sits on another flower to suck nectar, then the pollen grains sticking to its body are transferred to the stigma of this second flower.
In this way, the insects transfer pollen grains from flower to flower and carry out pollination. The insect pollinated flowers have large, bright coloured, and scented petals as well as nectar to attract insects for pollination. They have large pollen grains which can stick to the body of insects. The insect pollinated flowers have anthers and stigmas inside the flower so that the insect can brush against them when it is drinking nectar. Some of the insect pollinated flowers are of the plants such as : sunflower, marigold, Bougainvillea, jasmine, banana, and banyan, etc.
(ii) Pollination by Wind. The blowing wind carries pollen grains from one flower to other flowers and helps in pollination. Wind pollinated flowers do not have large, brightly coloured and scented petals or nectar because they do not have to attract insects. The wind pollinated flowers have anthers that hang outside the flowers to catch the wind. They produce large amount of light, small pollen grains which can blow in the wind. The wind-pollinated flowers have also spreading and feathery stigmas to catch the air-borne pollen grains. Some of the wind-pollinated flowers are in the plants such as : grass, maize, rice, sugarcane, bamboo and palms.
(iii) Pollination by Water. The pollination in aquatic plants (particularly those plants which are submerged in water) is done by water. Since pollen grains are light, they are easily carried by water from flower to flower for pollination. The pollination in aquatic plants such as Vallisneria and Hydrilla is carried out by water.
Fertilisation
After a pollen grain falls on the stigma of a flower, the next step is fertilisation. The process in which the male gamete present in pollen grain fuses (joins) with the female gamete present in ovule to form a new cell called zygote is called fertilisation. In other words, the fusion of male and female gametes to form zygote is called fertilisation. Fertilisation takes place as follows : When a pollen grain falls on the stigma of the pistil, the pollen grain grows a pollen tube downwards through the style towards the female gamete in the ovary [see Figure (a)]. A male gamete moves down the pollen tube. The pollen tube enters the ovule in the ovary [see Figure (b)], The tip of pollen tube bursts open and male gamete comes out of pollen tube. In ovary, the male
gamete of pollen combines (or fuses) with the female gamete present in ovule to form a ‘fertilised egg cell’ called ‘zygote’ (Actually, the male gamete fuses with the nucleus of the ‘female gamete’ or ‘egg cell’ to form zygote). Zygote develops into an embryo. Embryo is that part of a seed which develops into a new plant. An embryo is a kind of baby plant in dormant state (inactive state).
Formation of Fruits and Seeds
After fertilisation, the ovule present in ovary grows to become a seed. The seed contains embryo (baby plant) and food for developing new plant. There is a tough protective coating around the seed which is called ‘seed coat’ (see Figure). A seed is the sexual reproductive body of a plant which can be used to grow more plants. Actually, all the ovules present in the ovary of a flower get fertilised by pollen grains and grow to become seeds. Thus, the ovary of a flower may give rise to a number of seeds.
After fertilisation, the ovary of a flower develops and becomes a fruit (with seeds inside it). Thus, fruit is the ripened ovary of a flower. The other parts of flower dry up and fall off. Only the ovary is left behind. So, at the place on the
plant where we had a flower originally, we now have a fruit (which is the ovary of the flower containing seeds). A fruit protects the seeds. Apples, oranges, mangoes, plums, lemons, tomatoes, pea pods, almonds, peanuts, and walnuts, are all fruits. They all have seeds inside them (see Figure). Some fruits are soft, fleshy, sweet and juicy like apples, mangoes and oranges. But some fruits are hard and woody such as almonds and walnuts.
Germination
When a seed is sown in the moist soil, it begins to grow. The process in which a seed begins to grow is called germination. When a seed germinates, the seed coat splits, a tiny root grows downward and a shoot grows upwards. This produces a seedling of the plant (see Figure).
The seedling grows further and ultimately forms a new plant. After a certain period, the plant bears flowers. These flowers again produce fruits and seeds. Wheat grains, gram (charm), maize (makka), peas and beans, etc. are all seeds. All these seeds can germinate when sown in soil under suitable conditions to produce new plants.