In plants, sexual reproduction occurs after the pollination process. Sexual reproduction is the fusion of the male gamete with the female gamete  to produce a diploid fertilised egg. Strasburger first discovered the fertilisation process in plants in 1884. The embryo in the seed is the final product of plant fertilisation. After sexual reproduction, the ovary develops into a fruit, whereas the ovule develops into a seed. Plants can reproduce sexually and asexually, but sexual reproduction is the most common phenomenon.

Features of sexual reproduction in plants

  • Both male and female, i.e., two parents contribute to the reproduction.
  • The initial step includes the formation of gametes and ends with fertilisation.
  • The complete process is steady and long-lasting.
  • This leads to variation, and thus all the offspring differ from the parents, both physically and genetically.

Pollination and Fertilisation

To complete the process of sexual reproduction, the male gamete has to fuse with the egg cell. And this is achieved by a process called pollination. Pollination can be self-pollination (between the same flower of a plant) or cross-pollination (between the different flowers of the same plant or another plant). It takes place with the help of pollinators like insects, birds, animals, water, wind.

When pollen grains come close to the stigma, the male gamete releases in stigma and unites with the egg cell of the ovary, and this process is known as fertilisation which gives rise to a diploid zygote.

During fertilisation, the male gamete enters into stigma, which is further carried by a pollen tube down the style, and it finally enters the ovary.

Changes occur after fertilisation:

  1. Various divisions in the diploid zygote give rise to embryo formation within the ovule.
  2. The ovule is then transformed into a seed by evolving a rigid coat around it. The source contains the baby plant within it.
  3. The remaining flower parts will vanish, such as stigma, petal, sepal, and style.
  4. Ovary gets converted into fruit with seeds into it.
  5. And so, the fruit is seen in the place of a flower. Plants may have soft or hard fruit, depending upon their characteristics.

Seed :

The formation of seed takes place when the fertilised ovule undergoes mitosis division. It stores necessary food and can develop into a new plant under favourable conditions. 

Structure of seed:

A seed has a seed coat, cotyledon, and an embryo axis.

Seed coat: The ovule envelope produces a tight seed coat covering the seed. It also contains micropyle as a tiny pore that helps in the germination process by allowing the passage of oxygen and water.

Cotyledons: they are known as the energy source of the seed as they have starch. Seed may be defined as monocotyledons or dicotyledons depending upon the number of cotyledons present. They are typically thick and straightforward structures with swollen sacs.

Embryo axis: It comprises two parts, epicotyl and hypocotyl. The epicotyl develops into the stem tip or plumule, whereas the hypocotyl develops into the root tip or radicle.

Types of seed:

Based on the number of cotyledons, the source can be classified into two types.

Monocotyledon seeds: which are made up of a single cotyledon. Examples of such plants are corn, rice, orchids etc.

Dicotyledon seeds: which are made up of two cotyledons. Examples of such plants are pea, mango, bean, mustard etc.

Based on the endosperm, sources can be albuminous and non- albuminous.

Albuminous (endospermic) seeds contain a part of the endosperm within them. This endosperm helps in the nourishment of the developing seedlings. Examples of such plants are custard, apple, poppy etc.

Non-albuminous (non- endospermic) seeds: they do not contain endosperm. The endosperm in such sources is wholly consumed during embryo enlargement. The foods are stored in cotyledons in such types of seeds. Examples of such plants are gram, bean, pea etc.

Apomixis: The plants which develop seeds without fertilisation are apomixis. The diploid egg transforms into an embryo irrespective of reduced divisions. Apomixis is an essential phenomenon of producing hybrid forms of vegetables and fruits.

Polyembryony: Single fertilised seed gives rise to more than one embryo, known as polyembryony. Example: mango, citrus etc.

Functions of seed:

  1. Seed is served to plant an entirely new plant. They are capable of photosynthesis on their own.
  2. They lead to variation as they result from sexual reproduction. New genetic combinations can be made.
  3. Seeds can be stored for months to years. Suitable conditions lead to the germination of seeds. A record of oldest origin is lupine, which has an estimated history of 10,000 years.
  4. Various animals and humans depend on seeds such as wheat, corn, rice, soybean, walnut, almond, peanuts, etc.

Conclusion:

When pollen grains come close to the stigma, the male gamete releases in stigma and unites with the egg cell of the ovary, and this process is known as fertilisation which gives rise to a diploid zygote.after division of zygote ,embryo change into the  seed.

During fertilisation, the male gamete enters into stigma, which is further carried by a pollen tube down the style, and it finally enters the ovary.