Fertilisation in Plants
Plants substantiate their name as primary or universal producers. Providing us, humans the life giving essentials such as clothing, shelter, food and oxygen, they are indeed nature’s gift. As in the case of all other living beings, plants respire, vegetate, germinate, defecate and produce. In 1884, it was Ralph B. Strassburger who first discovered the fertilization process in flowering plants. Here, let us talk in detail regarding the fertilisation process in plants.
What is Fertilization?
Fertilization in plants is a process in which they undergo sexual reproduction in consequence to pollination and germination. In a physicochemical process, the male gametes (known as pollen) in fusion with the female gametes (known as ovum) form diploid zygote after carpal pollinates. The entire process happens in zygote which later germinates into a seed. During the process, what happens is male gametes are transferred into female reproductive organs through pollinators (butterflies, birds, honey bees, bats and flower beetles). This results in an embryo being formed in a seed. Flowers are the reproductive organs of angiosperms and reproduce in an entirely different method.
The process of fertilization in plants can be categorised into four major stages.
Pollination: In this, the male gametes covered in a pollen are carried away by water, wind or through other agents such as animals and insects. Then, the pollen is made to land on the stigma of the plant. This process is called pollination.
Germination: This is a process in which the pollen tube matures in a matter of minutes and enters the egg cell. The tube serves as a passage for the pollen grain to reach the egg.
Ovule Penetration: The pollen tube which contains the sperm penetrates into the embryo sag or ovule.
Fertilization: After successfully penetrating into the ovule, the sperms travel through the tube to fertilize the egg. Most of the flower bearing plants undergo double fertilization in which the polar nuclei and egg in the ovule are fertilized.
Post fertilization, the zygote cells divide and develop into an embryo which can also be termed as a developing plant. The embryo enveloped in a seed capsule lay dormant till favourable environmental conditions prevail to help it develop into a plant.
Fertilization in Seed Plants
During the process of fertilization in gymnosperms or seed bearing plants, the seeds grow on both sides of the leaves or scales or develop cones or prevail at the end of short stalks. The seed remains confined in the ovary.
Fertilization in Flower Bearing Plants
In flower bearing plants or angiosperms, an additional fertilization process occurs in the central cell. Two sperm cells get released from either one of the pollen grain. The first cell fuses with the zygote for egg fertilization and the second fuses with the twin polar nuclei to produce endosperm.
Process of Fertilization
As we said above, fertilization happens as a result of fusion of female and male gametes. For fusion to take place, both the female and male pollen grains reach stigma’s surface. This process is termed as pollination. Once the pollen is transferred to the stigma, it undergoes germination, passes through the style to enter the ovule. The pollen or the microspores has two cells namely generative cell and pollen tube cell. This pollen tube cell develops a pollen tube by which generative cells navigate. For pollen tubes to germinate, it requires oxygen, water and several other chemical signals. While pollen passes through style to reach the ovule or embryo sac, it is the tissue of the style which supports the growth of the pollen tube. During the entire process, in case the generative cell has not yet ruptured into two cells, it breaks down into two sperm cells. The pollen is aided by the chemical secretion of synergids in embryo sac as it penetrates into the ovule through micropyle.
One sperm cell from the sperm cells germinate the egg cell to form a diploid zygote. The remaining sperm cells undergo fusion with the polar nuclei to form a triploid cell which produces an endosperm. These twin fertilization events occurring in angiosperms or flowering plants are termed as double fertilization. Once the fertilization process is over, it is not possible for another sperm to enter. The fertilized embryo sac produces the seed while the ovary’s tissues become the fruit which envelops the seed.
The process in which gametes of two different species – male and female cross fertilize is known as allogamy. There is also autogamy or self-fertilization when dual gametes from a single individual fuse. This is commonly seen in hermaphrodites such as certain plants and flatworms.
Types of Fertilization
There are three types of fertilization, categorised mainly on the way in which pollen tubes enter the ovule or embryo sac.
The most common form of fertilization which happens in flowering plants or angiosperms. In this form, a pollen tube reaches the ovule through a micropyle. A common example is lily.
This form of fertilization happens in a species of plants namely casuarina. Here, the pollen tube permeates into ovule through chalaza rather than entering through micropyle. This is not very common. Examples would be casuarina, Betula and Juglans.
This form of fertilization happens in cucurbit plants including bitter gourd, ridge gourd, various other gourd plants and pumpkin. In this form, a pollen tube passes through integuments or its middle part to penetrate into the ovule.
Double Fertilization happens when a single female gamete undergoes fusion with two male gametes. Here, one of the sperm cells undergo fusion with the zygote while the other fuse with the two nuclei to produce endosperm. The entire angiosperm plants go through the process of double fertilization.
What Makes Double Fertilization Significant is
In double fertilization, two products are formed
Increases the feasibility of the seeds of the angiosperm
The plant will have a greater chance to survive and polyembryony (the chance to form more than a single embryo from one seed or a single fertile ovum)
Double fertilization results in an endosperm which nourishes the growing embryo
The two male gametes which are produced by pollen grains into effective use.