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Sexual Reproduction in Flowering Plants (Part II)

The document discusses sexual reproduction in flowering plants, focusing on the structure and function of the female reproductive part, the pistils, and the formation of ovules. It details the process of gametogenesis including microsporogenesis and megasporogenesis, explaining how male and female gametes are produced within the pollen grain and ovule respectively. Key aspects include the types of ovules, arrangement of pollen tetrads, and the development of the embryo sac in angiosperms.

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Sexual Reproduction in Flowering Plants Part - II Presented by: ShashankTripathi
Female reproductive part of Flower: The Pistils • The Gynoecium represents the female reproductive part of the flower. • Gynoecium may consist of a single Carpel (Monocarpellary) or may have more than one carpel (Multicarpellary). • If there are more than one carpels, we use term Pistil at the place of Carpel. • If, there are more than one carpels if they may be fused together (syncarpous) or free (apocarpous)
Female reproductive part of Flower: The Pistils • Each Pistil has following 3 parts: An ovary (where the ovules are produced;), a style (a tube on top of the ovary), and a stigma (which receives the pollen during fertilization).
Structure of a Megasporangium (Ovule): • An Ovule has following Parts: o Funiculus: It is the stalk to which the body of ovule is attached with the Placenta. o Integument: Each ovule has one or two protective envelopes called integuments. o Nucellus: The integument encloses a large parenchymatous tissue known as nucellus. This is the central part of an ovule, that encloses embryo sac. o Micropyle: There is a small opening at the apex of the integument called the micropyle. o Chalaza: This is the basal part of ovule found opposite to the micropyle. o Embryo Sac: In center of the nucellus is situated the female gametophyte known as embryo-sac which develops from a functional megaspore.
Types of Megasporangium (Ovule): • 1. Orthotropous or atropous ovule (ortho- straight, tropous - turn): The body of the ovule is erect or straight. The hilum, chalaza and the micropyle lie in a straight line e.g. Polygonum. • 2. Anatropous ovule (ana - backward or up, tropous - turn): The body of the ovule becomes completely inverted during the development, so, that the micropyle lies very close to the hilum (e.g.,) Gamopetalae members. • 3. Hemi-anatropous or hemitropous ovule: The body of the ovule is placed transversely at right angles to the funicle. The micropyle and chalaza lie in one straight line e.g. Ranunculus.
Types of Megasporangium (Ovule) Contd…: • 4. Campylotropous ovule (kampylos - curved): The body of the ovule is curved or bent round so that the micropyle and chalaza do not lie in the same straight line. e.g. Leguminosae. • 5. Amphitropous ovule: The curvature of the ovule is very much pronounced and the embryo sac also becomes curved e.g. Allismaceae and Butomaceae. • 6. Circinotropous ovule: The nucellus and the axis are in the same line in the beginning but due to rapid growth on one side, the ovule becomes anatropous. The curvature continues further and the micropyle again points upwards (e.g.) Opuntia.
Gametogenesis in Flowering Plants: • In flowering Plants Gametogenesis takes place in two stages: oSporogenesis oGametogenesis Gametogenesis In Male Plants Microsporogenesis Microgametogenesis In Female Plants Megasporogenesis Megagametogenesis
Microsporogenesis: • The formation of microspores inside the microsporangia (or pollen sacs) of seed plants from Microsporocytes or PMCs. • Cells of the sporogenous tissues of Anther are capable to give rise a Pollen Tetrad i.e., cluster of 4 pollens. So, they are called as Pollen Mother Cells (PMCs). • The nuclei of these Pollen Mother Cells undergoes Meiosis to form a Cluster of 4 Nuclei. These clusters of nuclei are called Pollen Tetrads as soon as they develop a cell wall around them. • The arrangement of pollen grain in a tetrad is affected by cytokinesis during meiosis. It is of following types: Simultaneous type, Where, cytokinesis occurs only at the end of meiosis II. This results in the production of four cells by cell wall formation between the four nuclei. Successive type, where, cytokinesis occurs twice – once at the end of meiosis I, forming two cells and then again at the end of meiosis II to form four cells.
Arrangement of Pollens in Pollen Tetrads: • In different species the arrangement of Pollens in a tetrads may be different. • It can be- Linear- E.g., T-Shaped- E.g., Decussate- E.g., Isobilateral- E.g., Tetrahedral-E.g., • Pollen tetrads of most of the plants are tetrahedral in arrangement. • As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains. • Inside each microsporangium several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther.
Dehiscence of Anther and Release of Pollen Grains: • It consists of the following stages:  Expansion of the epidermis and endothecium cells and deposition of U‐shaped wall thickenings inside the latter.  Enzymatic opening of the septum between two locules.  Mechanical rupture of the tapetum.  Mechanical opening of the stomium by the centripetal force of the highly turgescent epidermis and endothecium.  Outward bending of the locule walls by the centrifugal force of the dehydrating epidermis and endothecium.
How a Pollen looks like?? • Pollen grains are generally spherical. • Architecture, i.e., sizes, shapes, colors, designs of Pollens varies species to species. • They are about 25-50 micrometers in diameter.
How a Pollen looks like?? • A pollen is provided with two coats: Exine or Extine- It is outer tough, cutinized layer called exine or extine. The exine is often sculptured or provided with spines, warts, etc. › Exine is made up of Sporopollenin which is highly resistant to temperature and acids and alkalis. Intine- The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin. • The cytoplasm of pollen grain is surrounded by a plasma membrane.
How a Pollen looks like?? • When the pollen grain is mature it contains two cells-  Vegetative Cell Generative Cell • The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus. • The generative cell is small and floats in the cytoplasm of the vegetative cell. • It is spindle shaped with dense cytoplasm and a nucleus.
Microgametogensis : • Formation of gametes in the flowering plants take place inside the Pollen grain. • It takes place by the Mitotic division in Generative cells • In over 60 per cent of angiosperms, pollen grains are shed at this 2-celled stage. • In such plants the generative cells divide mitotically after shading on the stigma of the flower and become 3-celled in which two cells are male gametes while third one is vegetative cell. • In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
Megasporogenesis: • Megasporogenesis is formation of megaspore that is female gametophyte from the MMC. • In the case of Angiosperms it is known as Embryo sac. • Megasporogenesis take place inside the ovule. • During the Megasporogenesis one hypodermal cell of the nucellus becomes differentiated from the other by its bigger size, dense cytoplasm and conspicuous nucleus, called archesporial cell. • The archesporial cell functions as Megaspore mother cell (MMC). • The megaspore mother cell is diploid (2n), which undergoes meiosis I and then Meiosis II forms four haploid (n) megaspores
Megasporogenesis: • These Megaspore tetrads are arranged in a linearly one above other. • Unlike Microsporogenesis, Megaspore tetrads are mostly linear in arrangement of Megaspores. • Among these 4 Megaspore tetrads 3 gets degenerated and only one remain functional. • The functional megaspore gets nutrition from nucellus and enlarges to serve as the function of female gametophyte. • This type of embryo sac development is very common in angiosperms and is known as ordinary type or normal type or Polygonum type embryo sac development. • This type is also known as monosporic type, because, out of four megaspores, only one remains functional and forms the embryo sac.
Megagametogenesis: • So, inside the female gametophyte there is a single nucleus that undergoes a mitotic division to give rise 2 nuclei. • Among these two on emigrate to chalazal end and one towards micropylar end. • Again both the nuclei divides successively to give rise 4-4 cells at both the ends. • From these cells on from each ends migrates towards center again and fuses with each other, to give rise polar nuclei or central cell.
Megagametogenesis: • There is a characteristic distribution of the cells within the embryo sac. • Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell. • The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid. • Three cells are at the chalazal end and are called the antipodals. • The large central cell, as mentioned earlier, has two polar nuclei. • Thus, a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled.
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