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➢ Structure of Microspore or Pollen Grain 

  • The pollen grain is the first cell of a male gametophyte. 

Structure of a Pollen GrainStructure of a Pollen Grain

  • The pollen grain is termed as an immature male gametophyte. Usually, they are in round shape. Pollen grain surrounded by two distinct layers. The outer layer (wall) is thick, rigid and ornamented, called exine. This layer is formed by cutin and sporopollenin. Sporopollenin is a highly resistant material. It is non-biodegradable. 
  • Due to the presence of sporopollenin, fossils of pollen grain are always found in good condition. The presence of fossils of pollen grains can forecast the presence of natural resources like petroleum, coals etc. in the earth.
  • The internal layer is thin, soft and elastic in nature. It is called the intine. It is made up of pectin and Cellulose or pecto-cellulose. 
  • Usually, at few places on the outer surface exine is absent or present in the form of a thin layer. These thin places are called germ pore. The intine comes out through any one germ pore during the germination of pollen grain in the form of a pollen tube.
  • The number of germ pore, structure and ornamentation of exine is a significant feature of taxonomy.
  • A detailed study of pollen grains is called Palynology. 
  • Three colpus type (slit type) of germ pore are present in the pollen grain of most of the dicots (Capsella). These type of pollen grains are called tricolpate. Only one germ pore is present in monocots and pollen grain is called monocolpate.
  • The plants in which pollination takes place by insects, their pollen grains having oily layer around the pollen grain. It is called pollen-kitt. It is composed of lipids and carotenoids.
  • Pollen grains of some plants present in the air cause allergy are called "aeroallergens".
    Example: Chenopodium, Parthenium, Sorghum, and Amaranthus. ("Hayfever" is caused by pollens of Ambrosia).
  • Pollen grains of many plants are rich in nutrients. It has become a fashion in recent years to use pollen tablets as food supplements. In western countries, a large number of pollen products in the form of tablets and syrups are available in the market. Pollen consumption has been claimed to increase the performance of athletes and racehorses.

Question for Pollen Grain & Microgametogenesis
Try yourself:Exine layer of pollen grain is made up of :
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➢ Functions of Pollen Kitt

(i) This oily layer protects the pollen grain from the harmful ultraviolet rays.An Insect on Pollen KitAn Insect on Pollen Kit

(ii) Its sticky surface helps to attach with the insects.
(iii) Its yellow colour attract insects. A pollen kitt is present on the pollens of Capsella.

➢ Dehiscence of Anther 

  • During the maturation of anther, various changes take place in the walls of the anther.
  • In the beginning, the middle layer degenerates due to absorption.
  • When the micropores are formed inside the pollen sacs, at the same time ubisch bodies are formed in cells of the tapetum, then after it degenerates. Ubisch bodies participate in the formation of exine of pollens.
  • In this way, in a mature anther, only two layers epidermis and endothecium are present in the form of an outer covering.
  • The sterile tissues are present between both the pollen sacs of each anther lobe degenerate. So both pollen sacs of each anther lobe fuse together to form a single pollen sac.
  • Therefore, in the T.S. of the mature anther, only two pollen sacs are present.
  • Dehiscence of anther takes place during the dry season. Due to the hygroscopic nature of endothecium, loss of water takes place from the cell of endothecium.
  • Walls of endothecial cells try to contract due to the loss of water but inner and radial walls do not contract due to the presence of fibrous thickening whereas outer thin-walled cells of endothecium contract and become concave or incurved.
  • Incurving of outer walls exert pulling force or tension over the entire surface of the anther. Due to tension, thin-walled stomial cells break off and the dehiscence of anther takes place and pollen grains present in pollen sacs are released into the atmosphere.

➢ Microgametogenesis or Development of Male Gametophyte

  • In flowering plants, pollen grain or microspore considered as the first cell of male gametophyte. Germination or development of pollen grain takes place before the dehiscence of the anther. So it is called precocious development
  • Development of pollen also takes place at mother place (inside pollen sac of anther) is called In-situ development.

(i) Pre-Pollination Development 

  • In the beginning of the process, only the nucleus of the pollen grain divided by unequal mitotic division, resulting in two unequal sized nuclei are formed. 
  • A small nucleus present near the wall is called the generative nucleus and a large nucleus present inside the cytoplasm is called Tube or Vegetative nucleus
  • Both the nucleus surrounded by cytoplasm and it becomes dense, then followed by unequal cytokinesis, resulting in two unequal sizes of the cells are formed. 
  • The larger cell in which a large nucleus is present known as a Vegetative cell and the smaller cell in which a small nucleus is present, called a generative cell
  • Now pollen grains come in the bi-celled and binucleated stage. In 60% of Angiosperms pollination of pollen grains take place in the bi-celled and binucleated stage and in 40% of angiosperms pollination occurs at the 3-celled stage. 
  • In this generative cell divides and form two male gametes. The development gametophyte takes place inside the sporangia is also known as endosporic development. This stage of the pollen grain is called immature or partially developed male gametophyte.Development of Male Gametophyte
    Development of Male Gametophyte
  • Generative cell detached from the wall and changed into the vermiform or spindle-shaped structure and enter inside the vegetative cell. 

Question for Pollen Grain & Microgametogenesis
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(ii) Post Pollination Development

  • Further development of pollen grain (Immature male gametophyte) takes place on the stigma of Carpel after pollination. Pollens absorb moisture and sugar content from the stigma. Due to this volume of internal contents of the cytoplasm increased. It exerts pressure on both outer layers. 
  • Because of this pressure intine comes out through any one germ pore in the form of a tube-like structure called a pollen tube. First of all vegetative nucleus enter into the pollen tube and assumes a terminal (tips) position. 
  • This spindle-shaped generative now enter the pollen tube. Inside the pollen tube, the generative cell divides mitotically and form two non-motile male gametes. Now male gametophyte comes in three celled structure in which one vegetative cell and two male gametes are present. 
  • This three celled stage represents the mature male gametophyte of Angiosperm (Capsellaalso). Male gametophyte is highly reduced and completely depends on the sporophyte. First of all, pollen tubes were discovered by G.B. Amici in the Portulaca plant. The longest pollen tube is found in Zea mays.

Additional Information

➢ Facts About Types of Pollen Grains

  • In Asclepiadaceae (Calotropis) and Orchidaceae family, all the pollen grains joined together to form "Pollinium". Pollinium of Calotropis is also called "Translatorapparatus".Pollen Grain & Microgametogenesis | Biology Class 12 - NEETPollen
  • More than four pollen grains are found inter-trade called "Polyspory".
    Example: Cuscuta.
  • In some plants, four pollens of tetrad join together permanently are called "Compound pollens".
    Example: Drosera, Typha, Drimys and Elodea. 
  • Compound pollens of the Mimosaceae (Mimosa) family contains 6-8 or 64 pollen grains that stick together to form a small unit that is called "Massullae".
    Example: Mimosa and Neottia plants.

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FAQs on Pollen Grain & Microgametogenesis - Biology Class 12 - NEET

1. What is a pollen grain?
Ans. A pollen grain is a structure produced by plants that contains the male reproductive cells. It is usually small and lightweight, allowing it to be easily dispersed by wind, water, or pollinators.
2. What is microgametogenesis?
Ans. Microgametogenesis refers to the process of the development of microspores into male gametophytes in plants. It involves the formation of pollen grains, which contain the male gametes necessary for fertilization.
3. How are pollen grains formed?
Ans. Pollen grains are formed through a process called microsporogenesis. It begins with the division of microspore mother cells within the anther of a flower. These mother cells undergo meiosis, producing haploid microspores. Each microspore then undergoes mitosis, resulting in the formation of a pollen grain.
4. How are pollen grains important in plant reproduction?
Ans. Pollen grains play a crucial role in plant reproduction as they contain the male gametes needed for fertilization. When pollen grains are transported to the stigma of a flower, they germinate and grow a pollen tube, allowing the male gametes to reach the ovary and fertilize the female gametes, leading to the formation of seeds.
5. How are pollen grains dispersed?
Ans. Pollen grains are dispersed through various means, including wind, water, and pollinators like insects, birds, or mammals. Wind-pollinated plants produce large amounts of lightweight pollen grains that can be easily carried by air currents. Water-pollinated plants release their pollen grains into the water, where they are transported to female reproductive organs. Pollinator-dependent plants produce pollen grains with structures that allow them to stick to the bodies of animals, ensuring transport to other flowers.
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