Flowering Plant Reproduction

• Reproduction ensures species' continuity as older individuals senesce and die
• Flowering plants reproduce sexually using male and female reproductive units, along with accessory structures

Flower Structure

• A flower is a modified stem with reproductive organs producing ova and/or pollen
• Consists of four whorls of floral appendages attached to the receptacle: calyx, corolla, androecium (male part with stamens), and gynoecium (female part with ovary, style, and stigma)

Pre-fertilisation: Structures and Events

• Structural and hormonal changes lead to floral primordium, which forms inflorescence bearing floral buds that develop into flowers
• Male (androecium) and female (gynoecium) parts of flowers differentiate and produce male and female gametes

Stamen, Microsporangium, and Pollen Grain

• Stamen consists of a long, slender filament and a bilobed anther; each lobe contains two theca (dithecious)
• The anther is a four-sided structure with four microsporangia, two in each lobe
• Microsporangia develop into pollen sacs containing pollen grains
• Microsporangium is surrounded by four layers: epidermis, endothecium, middle layer, and tapetum; the innermost layer, tapetum, nourishes developing pollen grains
• Sporogenous tissues are compactly arranged homogenous cells at the center of the microsporangium when the anther is young

Microsporogenesis

• Involves the formation and differentiation of microspores (pollen grains) from microspore mother cells (MMCs) through reductional division
• Sporogenous tissue cells undergo meiotic division to form microspore tetrads
• As the anther matures and dehydrates, the microspores dissociate and develop into pollen grains

Pollen Grain Structure

• Pollen grain represents the male gametophyte and is made of a 2-layered wall
  • Exine: made of sporopollenin, a resistant organic matter that can withstand high temperatures, strong acids, and alkali with no enzyme to degrade it
  • Intine: thin, continuous layer made of cellulose and pectin
• Germ pores are apertures on the exine where sporopollenin is absent, forming the pollen tube
• Mature pollen contains 2 cells with nucleus (vegetative and generative)
  • Vegetative cell: bigger with abundant food reserve and a large irregular nucleus, responsible for pollen grain development
  • Generative cell: small and involved in syngamy (fuses with an egg

Pollen Grain Significance

• Can cause severe allergies and bronchial diseases like asthma and bronchitis in some people
• Rich in nutrients, used as pollen tablets for food supplements
• Viability varies by species; pollen grains of large number of species are stored in liquid nitrogen at -196℃, called pollen bank

Pistil, Megasporangium (Ovule), and Embryo Sac

• Gynoecium can consist of a single pistil (monocarpellary) or multiple pistils (polycarpellary), fused (syncarpous) or free (apocarpous)
• A pistil has three parts: stigma, style, and ovary
• The ovary contains an ovarian cavity (locule), where the placenta is located, and megasporangia (ovules) arise from placenta

Megasporangium (Ovule) Structure and Components

• Ovule is a small structure attached to the placenta
• Funicle: stalk attaching the ovule to the placenta
• Hilum: junction between the ovule and the funicle
• Integuments: protective envelopes
• Micropyle: small opening at the tip of the ovule where the pollen tube enters
• Chalaza: basal part of the ovule
• Nucellus (2n): mass of cells enclosed in the integuments, with abundant food reserve

Megasporogenesis

• Process of forming megaspores from a megaspore mother cell through meiotic division in the ovule
• MMC differentiates in the micropylar region of the nucellus, undergoes meiosis, and produces 4 megaspores

Female Gametophyte (Embryo Sac) Development

• In most flowering plants, three megaspores degenerate, and one develops into the female gametophyte (embryo sac)
• The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to opposite poles to form a 2-nucleate embryo sac
• Two more sequential mitotic divisions result in an 8-nucleate embryo sac
• Six of the eight nuclei are surrounded by a cell wall, and the remaining two (polar nuclei) are situated below the egg apparatus
• Three cells are grouped at the micropylar end to form the egg apparatus, and three cells at the chalazal end form antipodal cells
• At maturity, the embryosac is 8-nucleate and 7-celled

Pollination

• Transfer of pollen grains from anther to stigma

Autogamy

• Transfer of pollen from anther to stigma of the same flower

Cleistogamous flowers

• Do not open and are autogamous, ensuring seed-set even without pollinators
• Examples: Viola (common pansy), Oxalis, and Commelina

Chasmogamous flowers

• Have exposed anther and stigma

Geitonogamy

• Transfer of pollen from anther to stigma of a different flower on the same plant
• Functionally cross-pollination, genetically similar to autogamy

Xenogamy

• Transfer of pollen from anther to stigma of a different plant of same species

Agents Of Pollination

• Abiotic: water, wind
• Biotic: insects, butterfly, honey bee etc.
• Abiotic pollination produces large number of pollen grains because most are wasted during transfer.

Adaptations in Flowers: Wind Pollination

• Pollen grains: light, non-sticky, winged
• Anther: well-exposed
• Stigma: large and feathery
• Flower: one ovule, arranged as inflorescence
• Examples: corn cob, cotton, date palm

Adaptations in Flowers: Water Pollination

• Seen in Bryophytes, Pteridophytes, and Algae
• Pollen grains: protected by mucilaginous covering
• Examples: Vallisneria, Hydrilla, Zostera

Main Features of Wind and Water Pollinated Plants

• Produce pollen grains in large numbers
• Do not produce nectar

Adaptations in Flowers: Insect Pollination

• Flowers: large, colorful, fragrant, rich in nectar
• Pollen grains: sticky
• Stigma: sticky

Rewards To Pollinators

• Nectar and (edible) pollen grains as food
• Safe place for laying eggs
• Example: Amorphophallus, Yucca

Outbreeding Devices

• Mechanisms that discourage self-pollination and encourage cross-pollination to avoid inbreeding depression

Methods

• Pollen release and stigma receptivity not synchronized
• Anther and stigma placed at different positions
• Inhibiting pollen germination in pistil
• Production of unisexual flowers

Pollen-Pistil Interaction

• Pistil's ability to recognize compatible pollen to initiate post-pollination events for fertilization
• Pollen grain produces pollen tube through germ pores to facilitate male gamete transfer to the embryo sac

Artificial Hybridization

• Crossing different varieties of species to create hybrid individuals with desirable traits
• Pollen grains for pollination are desired and stigma is protected from contamination

Steps in Artificial Hybridization

• Emasculation: removal of anther
• Bagging: covering flower to prevent contamination of stigma from unwanted pollen, attains receptivity- mature pollen grains- dusted on the stigma - rebagged and fruit allowed to develop

Double Fertilisation

• After entering a synergid, each pollen grain releases two male gametes
• One male gamete fuses with an egg (syngamy), and the other fuses with two polar nuclei (triple fusion) to produce a triploid primary endosperm nucleus (PEN)
• The phenomenon is called double fertilisation
• PEN develops into the endosperm and zygote develops into the embryo

Post-Fertilisation Events

• Include endosperm and embryo development, maturation of ovules into seeds, and the ovary into fruits

Endosperm Development

• Primary endosperm cell divides to form a triploid endosperm tissue with reserved food materials

Two Types

• Free nuclear type (common method)
• Cellular type
• Non-albuminous: endosperm is completely used before seed maturation (e.g., pea, groundnut)
  • Albuminous: a portion of endosperm remains in mature seeds (e.g., wheat, maize, castor)

Embryo Development

• Embryo develops at the micropylar end of the embryo sac where the zygote is located

Embryogeny

• Early stages of embryo development: zygote forms the proembryo, which becomes a globular, heart-shaped, and mature embryo

Embryo Consists of

• Embryonal axis
• Cotyledons
• Plumule
• Radicle

Monocotyledonous Seed

• Scutellum = Cotyledon
• Coleorrhiza: undifferentiated sheath covering the radicle and root cap
• Coleoptile: sheath covering plumule

Seed

• Fertilized and mature ovule
• Seed consists of: cotyledon(s) and embryonal axis
• Seed coat: double-layered, formed by integuments
  • Testa: outer coat
  • Tegmen: inner coat
• Micropyle: small opening facilitating entry of H2O & O2 for germination
• Hilum: scar on the seed coat
• Seed: Albuminous or Non-Albuminous

Perisperm

• Remnants of nucellus that is persistent, e.g., black pepper
• Dormancy: state of inactivity
• The ovary wall develops into the fruit wall called the pericarp
• In true fruits, only the ovary contributes to fruit formation; in false fruits, the thalamus also contributes

Apomixis

• Asexual reproduction mimicking sexual reproduction, resulting in seed formation without fertilisation
• Involves diploid cells developing into embryos

Polyembryony

• Occurrence of more than one embryo in a seed
• Often associated with apomixis, e.g., citrus, groundnut