Let’s first understand what is Pollen grain. Pollen Plant refers to the mass of the microspores, which is the powdery product that is synthesized by the seed-bearing plants, and these are responsible for the production of male gametes as given in Pollen definition biology. The Pollens are transported by air, wind, water, insects, birds, and animals to the female plants for the purpose of fertilization.
Wondering what is pollen in flowers? Pollens in flowers are also called microgametophytes, and they have a coating called sporopollenin. This coating is there to protect the gametophytes during transportation from stamens (male plant) to pistil (female plant). In Pollen definition biology, after Pollen reaches the pistil, a certain type of tube is formed, which transports the stamen to the ovule. This phenomenon is also termed as Pollination, which is the reproductive system of plants.
In simple words, Pollination is defined as the transfer of Pollens from one flower to another, from the anther to the stigma. There are two more types of fertilization that occurs in plants - Cross-Pollination and Self-Pollination. Cross-Pollination is the transfer of pollen from the stigma of one flower to another. Self-Pollination is the transfer of pollen from the stigma of the same flower.
In pollen definition flowers, all Pollen Plant grains are not the same. They vary in size, shape, and characteristics, and this depends on the species of the plant. The structure of a Pollen is very simple. The grains have a double wall, and the thin inner wall is termed as endospore and is composed of cellulose. The thick outer wall is termed the exospore and consists of sporopollenin. Since the shape and size of endospore and exospore are variable, this is used by biologists to determine the species of the plant they belong to.
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The purpose of this simple pollen plant structure is to protect the genetic material from the external environment (radiation, water, ultraviolet rays) during the transportation of the pollen from the anther to the stigma. Not only the internal structure but the outer surface also contains wax and protein, which repels moisture and helps the pollen to mix with the stigma properly.
Pollen grains consist of three parts. The centre cytoplasmic part is the one that is responsible for fertilization and is the source of the nuclei. The inner layer of the wall is called the intine, is composed of cellulose and hemicellulose. The wall’s outer layer is the most durable and is called the exine, and is exceptionally resistant. Even the treatment of the Pollen Plant with heat, strong winds, acids does not affect the outer layer. While the inner wall can break down easily, the exine layer is often preserved; however, the quality varies with different environments.
In the male Pollen parts, the pollen is formed in the microsporangia. In the female plant, the pollen is produced in the anthers, also called angiosperms. The microsporocyte is diploid, and it forms four haploid cells, which are called microspores. This process is called microsporogenesis. These four microspores then form the walls of the pollen grains using a material called callose. This callose is digested by callase, and only after this, the pollen grain completes the formation of endospore and exospore.
Pollen grains have high resistance. They are dispersed by winds and are produced in abundance by plants; therefore, they are common constituents of geological sediments. Due to this high resistance to decay and long-lasting nature, Pollen grains have provided a lot of information about the origin and the history of terrestrial plant life. Pollen grains also have a symmetrical structure that is easily recognizable under the microscope, and this is a helpful taxonomic tool. The pollen grains’ structure is very distinctive, and species can be identified by the pollen grains alone. The study of pollen grains is called palynology, and all angiosperm pollen and gymnosperm families can be identified by the pollen grains.
Plants often use tricks to spread their Pollen grains: Some plants produce a rotten smell to attract flies, and some male plants have plants that resemble the female flowers so that male plants pollinate them.
Plants use toxins to prevent self-pollination: A toxin called SRNase poisons the tube to avoid inbreeding if the pollen and pistil are closely related.
Pollens are of varied colours: Pollen grains come in vibrant colours to attract different insects and birds.
1. What are the Stages Involved During the Growth of Microspore into a Pollen Grain?
The stages involved are microsporogenesis, arrangement into tetrad, mitosis and rounding off of angiosperm Pollen. In microsporogenesis, the reductional division occurs and leads to the formation of four microspores. Next, these microspores arrange themselves in a tetrad shape, and then mitosis occurs inside the microsporangium to form microspores. These microspores are now known as Pollen grains, and they get a circular shape and then released into the atmosphere in the final rounding off stage, which is known as dehiscence. These pollen parts form a thick wall around them which differentiates them into individual grains. These are the stages involved in the maturity of a microspore into a pollen grain.
2. What is the Interaction Between the Pollen and the Pistil?
It is a sequence of events that starts when the pollen is deposited over the stigma and ends when the pollen tube is inside the ovule. The plant has inbuilt checks to promote and inhibit pollen growth. Even in incompatible cases, the plant breeders can manipulate pollen growth, and they can also ensure that cross-breeding does not occur. A fully-developed embryo sac consists of one egg cell and nucleus, two synergids, and three antipodal cells. The pollen tube penetrates the sac’s wall, and the tip of the tube disintegrates, and the two male gametes are liberated in the embryo sac.