FERTILIZATION

For many poeple, pollen might just be a trigger for sneezing and watery eyes. But for plants, it’s a vital ingredient in the reproductive process. Explore plant fertilization through this blog.

Plant fertilization is the union of male and female gametes (reproductive cells) to produce a zygote (fertilized egg).

So, how does fertilization occur? It’s a pretty straight-forward process that’s similar for both flowering plants (Angiosperms) and seed-bearing plants (Gymnosperms). Fertilization in flowering plants was discovered by Ralph B. Strassburger in the year 1884. 

Diagrammatic representation of ferlilization process

 Let’s break the fertilization process down into four generalized steps.

Step 1: Pollination

In general, male gametes are contained in pollen, which is carried by wind, water, or wildlife (both insects and animals) to reach female gametes. The pollen is deposited on a plant’s stigma, which is part of the pistil (the elongated part of a flower extending from the ovary). This process is called pollination.

Step 2: Germination of pollen

Within a few minutes, pollen tubes begin growing, or germinating, toward the egg cell. These tubes will provide a path for the sperm carried in the pollen to reach the egg.

Step 3: Penetration of the Ovule

The pollen tubes penetrate the ovule, which contains the female gametes.

Step 4: Fertilization

Sperm travel down the pollen tubes and fertilize an egg. Most angiosperms undergo double fertilization, where both an egg and the polar nuclei in the embryonic sac are fertilized.

CROSS-POLLINATION (Xenogamy/Allogamy)

“In this type of pollination, the pollen is transferred from the anthers of one flower to the stigma of another flower.”

 In this case, the two flowers are genetically different from each other. The agents of pollination include birds, animals,water wind, and insects. Agents of pollination Cross-pollination depends on insects visiting flowers of the same species in sequence. To help ensure that this happens, the plants have various characteristics that help pollinators locate the right flowers, including the colour, size, shape and scent of the flowers, as well as the food reward. Based on the agent of pollination, cross-pollination can be of different types:

Hydrophily –Here the pollination occurs via water means. The flowers are often very small and inconspicuous to other agents. They do not have any fragrance or too much color on their petals. The pollen is adapted to be able to float in water. Example– Vallisneria

Anemophily– These flowers are pollinated by the agency of wind. These flowers, like zoophilous flowers, are small and inconspicuous. Another important feature of flowers that are wind pollinated is that they are very light so that they are easily carried by the wind. The pollen grains are very light, non-sticky and sometimes winged. Example- Pine trees

Zoophily– In this type of pollination, the pollinating agents are animals. The flowers have pollen that is designed to stick on to the body of the animal so that they can be easily carried from one flower to another. It is of following types:

Entomophily– In this type of pollination by insects. These flowers are often attractive to look at with bright petals and are fragrant to attract the insect visitors to them. They often have broad stigmas or anthers to allow the insect to perch on it. Many of the insect-pollinated flowers also secrete nectar which attracts bees, butterflies or other similar insects to the flowers. The pollen grains in these flowers are often spiny or have extensions that help them to stick on to the body of the insects. Example– flowers of Asteraceae family

Ornithophily– These flowers are pollinated by birds. Very few flowers and birds show this form of pollination. Example– Coral trees

Chiropterophily– The pollination is carried out by Bats. Example– Kigelia, Adansonia trees

TYPES OF POLLINATION

Pollination is natural process but quite a along time we have been doing it through artificially induced method in orer to bring desired genetic variation. In this blog I would be writing about the natural methods of pollination in plants. It is of two types:-

1 Self Pollination

2. Cross Pollination

Self Pollination

“When the pollen is transferred from the anthers of a flower to the stigma of the same flower, it is called as self- pollination.“

 This form of pollination is common in hermaphrodite or dioecious plants which contain both male and female sexual parts on the same flower.

Some plants reproduces through self-pollination, which means the flower can actually pollinate itself so that all the offspring are identical. Living species are designed to ensure survival of their progeny; those that fail become extinct. Genetic diversity is therefore required so that in changing environmental or stress conditions, some of the progeny can survive. Self-pollination leads to the production of plants with less genetic diversity, since genetic material from the same plant is used to form gametes, and eventually, the zygote. 

Self  Pollination can be further divided into two types:-

  1. Autogamy
  2. Geitonogamy
  1. Autogamy– In this type of self-pollination, the pollen is transferred from the anthers of one flower to the stigma of the same flower. 

EXAMPLE– Sunflowers, orchids, peas and tridax are examples for autogamy.                       

Geitonogamy– It is a type of self- pollination, the pollen are transferred from the anthers of one flower to the stigma of another flower but on the same plant. Monoecious plants which contain both male and female flowers in the same plant undergoes geitonogamy. As mentioned earlier, flowers using geitonogamy depend on the external pollinating agents like wind, insects, and animals.

EXAMPLE– Corn is the most common example of geitonogamy flowers. Geitonogamy is involved in the production of genetically similar offspring to parent.

The main difference between autogamy and geitonogamy is that Autogamy occurs when the pollen grains from the anther of the flower are deposited on the stigma of the same flower, while Geitonogamy occurs when the pollen grains from the anther of one flower are deposited on another flower of the same plant.

Summary

POLLINATION

Have you ever wonder why bees are attracted to flowers? Flowers look beautiful and smell nice but there is a reason behind this – flowers actually help plants to reproduce. Flowering plants (Angiosperms) have seeds, carry the genetic information of the parents and develop into aant. So flowers are the tools that plants use to make their seeds

Lets explore the concept and process of Pollination.

“Pollination is the process by which pollen grains from the anther (male) of a flower transfers to the stigma(female) of the same or different flower.”

So one can ask how does pollen get from one flower get from one flower today another? The male pollen cannot move to the female part of a flower on its own, so wind and animals are used to carry it instead. All the colours, shapes, sizes and smells of flowers are their ways of making it easy for the wind or animals to pick up and carry the pollen to the right female flower part. Pollination is usually the unintended consequence of an animal’s activity on a flower. The Pollinator is often eating or collecting pollen for its protein and other nutritional characteristics like sipping nectar from the flower. During this act of pollinator pollen grains get attached to the animal’s body. When the animal visits another flower for the same reason, pollen can fall off onto the flower’s stigma and may result in successful reproduction of the flower. Flowers must rely on vectors to move pollen. These vectors can include wind, water, birds, insects, butterflies, bats, and other animals that visit flowers. We call animals or insects that transfer pollen from plant to plant “pollinators”. Flowers have male and female structures, and it is the process of  pollination that transfers pollen from the male part(androecium) to the female part(Gynoecium).

After pollination, pollen releases a male gamete that fertilizes a female gamete in  the ovule and mixes their genetic material. After this fertilisation, the ovule grows to form a seed.