Sexual Reproduction in Flowering Plants Notes PDF in English for Class 12, NEET, AIIMS and Medical Exams

Sexual Reproduction in Flowering Plants Notes PDF: Find below the important notes for the chapter, Sexual Reproduction in Flowering Plants as per the NEET Biology syllabus. This is helpful for aspirants of NEET and other exams during last-minute revision. Important notes for NEET Biology- Sexual Reproduction in Flowering Plants Notes PDF cover all the important topics and concepts useful for the exam.

Sexual Reproduction in Flowering Plants

Sexual Reproduction in Flowering Plants Notes PDF

Reproduction in plants includes both sexual and asexual means. Most of the flowering plants reproduce sexually ultimately producing flowers. Flowers are vital in the process of sexual reproduction. This is why flowers are also referred to as the reproductive part of a plant. 

Structure of Flower

Structure of flower

Stamen (Male Reproductive Unit)

A stamen is an angiosperm’s male reproductive unit. It is composed of an anther and a filament. The anther is bilobed, with each lobe containing four pollen sacs or microsporangia. A number of pollen grains are contained in each pollen sac. A dithecous anther’s four pollen sacs are located in the four corners. The tip of the anther is supported by the filament. The development of an anther begins with a mass of homogeneous meristematic cells surrounded by an epidermis. Four lobes are formed, as are four layers of archesporial cells.

Microsporangia and Microsporogenesis

The process of the formation and differentiation of microspores (pollen grains) from microspore mother cells by reductional division is called microsporogenesis. The cell wall is formed after meiosis –I and meiosis –II in successive types, resulting in an isobilateral pollen tetrad. Monocots have it as a distinctive trait.

Structure of Microsporangia

  • Tarentum: The cells are multinucleate and this layer is one of the innermost layers of the wall.
  • Middle layer: It is composed of three to four layers of thin-walled cells. They are located just beneath the endothelium.
  • Endothecium: This layer is inner to the epidermis. Some cell develops cells of the endothelium.
  • Epidermis: It is a protective outermost single-layer. Cells of the epidermis in Arceuthobium develop a fibrous thickening and the epidermis is designated as exothecium.

Gynoecium

It is the female reproductive organ and the last whorl of the flower. It is composed of pistil and occupies the central position of the thalamus. The stigma, style, and ovary are the components of the pistil. The ovary produces ovules internally. Through meiosis, ovules produce megaspores which in turn develops into female gametophytes. As a result, egg cells are produced. Gynoecium can be:

  • Monocarpellary: The gynoecium consists of a single pistil. For eg., peas and beans.
  • Multicarpellary: Here, gynoecium comprises more than one pistil.
  • Syncarpous: It is the gynoecium with combined pistil. For eg., Tomato, cucumber.
  • Apocarpous: It is the gynoecium with free pistil. For eg., Lotus Vinca.

A carpel or pistil is made up of three parts:

  • Stigma: It is a tubular structure that connects the ovary and the stigma. It is responsible for the transportation of pollen from the stigma to the ovary and holding the stigma in place.
  • Style: It is attached to the top of the carpel, where the pollen from other flowers lands.
  • Ovary: The ovary is a chamber where ovules (eggs) are stored, waiting for fertilization.

Megasporogenesis

Megasporogenesis is the process of formation of megaspores from the Megaspore Mother Cell (MMC). The MMC is a large cell with dense cytoplasm and prominent nucleus. It undergoes meiosis resulting in the production of four megaspores.

Calyx

It is the outermost whorl of a flower. It comprises units called sepals. In the bud stage, calyx encloses the rest of the flower. They usually exhibit green colouration, at some other instances, they may be a colour like petals. This state of Calyx is termed as petaloid. Calyx can either be prominent or absent.

Corolla

It consists of many numbers of petals and it is the second whorl of the flower. These petals are sometimes fragrant. They are coloured, thin and soft that would help in the process of pollination as they would attract animals and insects.

Pollination

Pollinations is a process of transferring the pollen grains across the anther and the stigma of the same flower of a plant or to a flower of different plants for the process of fertilization and the production of seeds. The agents involved in transferring the pollen grains are birds, wind, animals, and water.

There are two types of pollination:

  • Self-Pollination: This process occurs when the pollen grains from the anther is deposited on the stigma of the same flower, or another flower on the same plant.
  • Cross-Pollination: This process occurs when the pollen grains are transferred from the anther of one flower into the stigma of another flower of different plants of the same species.

Kinds Of Pollination

  • Autogamy – Pollination takes place within the same flower wherein the pollen grains from the anther are transferred to the stigma of the same flower
  • Geitonogamy – In this type of pollination, the pollen grains are transferred from the anther to the stigma of a different flower but of the same plant
  • Xenogamy – It is the transfer of pollen grains from the anther to the stigma of a different plant.

Double Fertilization 

Fertilization is defined as the process by which male and female gametes fuse to form the zygote. The zygote will eventually mature into an embryo. Two male gametes are released into the embryo sac by the pollen tube. The diploid zygote is formed when one of the male gametes fuses with the egg. This is known as syngamy or generative fertilization. The second male gamete joins the two polar nuclei. This leads to the formation of a triploid primary endosperm nucleus. This is known as triple fusion, and it is also referred to as vegetative fertilization. Two sexual fusions occur in an embryo sac, one in syngamy and the other in triple fusion. This is known as double fertilization.

Post Fertilization

  • Endosperms: Endosperm is a nutritive tissue that develops as a result of vegetative fertilization. The endosperm is intended to nourish the embryo. It is typically triploid. The effects of genes from the male gamete may be seen in the endosperm. The condition is known as xenia. This happens because the endosperm in a mature ovule is fully developed. Endosperm nutrients aid in early seedling growth in plants with albuminous seeds. Endosperm nourishes the developing embryo.
  • Embyrogeny (embryo formation): It is the formation of a mature embryo from a zygote or an oospore. Early development results in an axially symmetric pro-embryo. The embryo goes through the globular stage. Because of the presence of a suspensor, embryo development occurs on the inner side. As a result, embryo development is endoscopic.

Formation of Fruits and seeds

The ovules develop into seeds and ovary develops into a fruit. The ultimate product of sexual reproduction is seeds in angiosperms. Typically formation of seeds takes place within fruits. Seeds are advantageous structures in the following ways:

  • The process of seed formation is dependable as other reproductive processes(fertilization and pollination) are water-dependent.
  • For dispersal to newer habitats, seeds have a better adaptive strategy helping species to colonize other areas.
  • Young seedlings are self-nourished until  photosynthesis as they have enough food reserves.
  • Generates new genetic combinations resulting in variations.
  • Seeds (dormant and dehydrated) can be stored for usage throughout the year.

Apomixes

  • It is the formation of new individuals through asexual methods that mimic sexual reproduction, including seed formation, but do not involve gamete or sex cell fusion.
  • Amphimixis is a normal type of sexual reproduction with two regular features, namely meiosis and fertilization.
  • Apomicts are organisms that reproduce through apomixes.
  • Apomixis is controlled by genes, and individuals are genetically similar to the parent that produced them, i.e. they are clones, and members of a clone are known as ramets.

Parthenocarpy

  • It is the process of fruit formation that occurs without the event of fertilization.
  • Parthenocarpic fruits are seedless, such as apples, pears, bananas, and pineapples.
  • Parthenocarpic fruits also have seeds with an asexual embryo or pseudoseeds.
  • There are three types of parthenocarpy: genetic, environmental, and chemically induced.

Polyembryony

  • In 1719, Leeuwenhoek discovered polyembryony. Schnarf confirmed the same in 1929.
  • Polyembryony refers to the process of having more than one embryo.
  • Polyembryony caused by the fertilization of more than one egg cell is referred to as simple polyembryony.
  • Additional embryos can be formed from various parts of the ovule, such as synergids, antipodal, nucellus, integuments, and so on.
  • Citrus, groundnut, onion, Opuntia, Mangifera are some examples.
  • Polyembryony occurs more frequently in gymnosperms than in angiosperms.
  • Polyembryony can be either true or false embryony.
  • Polyembryony is important for practical reasons because nucellar embryos can produce genetically uniform parental type seedlings.

 

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