Let's have a look at our daily food intake. From breakfast cereals and fruits to the chapatis and vegetables we eat in our diet comes from plants. Thus, plants are the primary source of food for human beings.
But have you ever wondered how these plants produce grains like wheat, rice or the delicious fruits and vegetables we eat?
The answer to this question is very simple, it is all a result of sexual reproduction in plants. Let us understand the mechanism of sexual reproduction in plants and it's fruitful result.
Flower are the structural units that favour sexual reproduction in plants. A flower has the following parts-
Sepals- they are green coloured and are distinctively seen when the flower is in bud stage.
Petals- the beautiful coloured part of flowers.
Androecium - it is the male reproductive part and is composed of the anther (contains male gametes) and filament.
Gynoecium - it is the female reproductive part and comprises stigma, style and ovary (contains female gametes).
Some plants may have both male and female flowers on the same plant and are referred to as monoecious whereas some plants have male and female flowers on different plants and are referred to as dioecious.
Sexual reproduction involves the fusion of male and female gametes. For this event to occur firstly the male and female gametes must be brought to the vicinity, which is achieved by pollination.
Pollination: Pollination is the transfer of pollen grains (containing male gametes) from another to the stigma of flowers. This process is aided by various biotic (living organisms such as bees, wasps, ants, animals, and human beings) and abiotic factors (non-living factors such as wind, water).
Plants have evolved various mechanisms to favour pollination.
Anemophily - pollination by wind. The tassels of maize are evolved to capture pollen in wind. This type of pollination is common in cereal crops, grasses etc.
Hydrophily - pollination by water. Pollen grains are carried away by water currents and cause pollination in aquatic as well as many terrestrial plants.
Zoophily - pollination by animals and insects. The pollen gets adhered to the body of insects and animals and is transported from one place to another.
After pollination, the pollen grains on reaching the stigma of flowers germinate and give rise to pollen tubes that penetrate stigma and grow inside style to reach the ovary. This leads to the next step fertilization.
Fertilization - It involves the fusion of male and female gametes. The pollen tube on reaching the ovary releases 2 male gametes which then enter the ovary through micropyle opening and fuse with female gametes in the embryo sac. This results in the formation of zygote.
The zygote formed as a result of fertilization develops into a seed and the ovary develops into fruit both of which are important products for human beings. Wheat, Rice, Maize etc. are all seeds developed from the zygote and the fruits and vegetables we eat are ripened ovaries. Thus we are fortunate that plants reproduce sexually.
1. What is Double Fertilization?
Ans. The two male gametes released from the pollen tube enter the embryo sac. One male gamete fertilizes with the egg in the embryo sac and results in the formation of the zygote, which is termed as fertilization. The other male gamete fuses with the two polar nuclei and results in the formation of a three celled structure called primary endosperm nucleus (PEN); this process is called triple fusion. The simultaneous occurrence of fertilization and triple fusion is termed as double fertilization.
2. Describe the Structure of Embryo Sac
Ans. The embryo sac consists of:
Three antipodal cells which degenerate after fertilization.
Two polar nuclei which fuse with male gamete to form the primary endosperm nucleus.
An egg apparatus with two synergies and one egg cell which forms zygote after fusion.
The opening part from where male gametes enter is called micropyle and the other is called chalaza.
3. What Do You Mean by Self-Incompatibility?
Ans. When pollen from the same flower fails to germinate on the stigma due to underlying genetic mechanisms, it is called self-incompatibility. It favours cross pollination and increases diversity in nature.
4. What are the Advantages and Disadvantages of Pollen Grains of Flowering Plants?
Pollen grains are rich in many important nutrients. It has become a trend in recent years to use pollen tablets as food supplements.
Pollen grains of many flowering plants cause severe allergies and bronchial afflictions in some people, which causes chronic respiratory disorders like asthma, bronchitis, etc.
One of the common examples of such flowers is Parthenium or carrot grass that came into India as a contaminant with imported wheat, has become ubiquitous in occurrence and causes pollen allergy.