Detailed Notes – 2 – Chapter 1 : Sexual Reproduction in Flowering Plants

1.3 DOUBLE FERTILISATION

1. Process Overview:

   • After the pollen tube enters one of the synergids in the embryo sac, it releases two male gametes.
   • One male gamete fuses with the egg cell, resulting in the formation of a diploid zygote through syngamy.
   • Simultaneously, the other male gamete fuses with the two polar nuclei in the central cell, forming a triploid primary endosperm nucleus (PEN) through triple fusion.
   • Double fertilization involves the occurrence of both syngamy and triple fusion in the same embryo sac.

2. Outcome:

   • The zygote develops into an embryo, which will give rise to the future plant.
   • The central cell, now called the primary endosperm cell (PEC), develops into the endosperm, a tissue that provides nourishment to the developing embryo.

1.4 POST-FERTILISATION : STRUCTURES AND EVENTS

1. Endosperm Development:

   • The primary endosperm nucleus (PEN) formed by triple fusion undergoes cell division to form the endosperm.
   • Endosperm development involves multiple rounds of mitotic divisions, leading to the formation of a triploid tissue.
   • The endosperm serves as a nutrient-rich tissue that nourishes the developing embryo.

2. Embryo Development:

   • The zygote, formed by the fusion of the sperm cell and egg cell, undergoes cell division and differentiation to develop into an embryo.
   • Embryo development includes the formation of various embryonic structures, such as the cotyledons, embryonic axis, and seed coat.
   • The embryo contains the future plant, including the shoot, root, and embryonic leaves (cotyledons).

3. Maturation of Ovule into Seed:

   • The fertilized ovule develops into a seed, which contains the mature embryo, endosperm, and protective seed coat.
   • During seed maturation, the endosperm accumulates reserves of nutrients, such as starch, proteins, and lipids, to support seed germination and early seedling growth.
   • The seed coat develops from the integuments of the ovule and provides protection to the embryo and endosperm.

4. Ovary Development into Fruit:

   • Following fertilization, the ovary undergoes development into a fruit, which encloses and protects the developing seeds.
   • Fruit development involves changes in the ovary wall, such as enlargement, differentiation, and ripening.
   • Ripening of the fruit may involve changes in color, texture, and flavor, making the fruit attractive to animals for seed dispersal.

1.4.1 Endosperm

1. Timing of Development:

   • Endosperm development precedes embryo development in flowering plants.
   • It begins shortly after fertilization and continues to provide nutrients to the developing embryo throughout its growth.

2. Nutrient Supply:

   • The endosperm serves as a nutritive tissue, supplying essential nutrients to support embryo growth and development.
   • These nutrients include carbohydrates, proteins, and lipids, which are vital for the embryo’s metabolic processes.

3. Protection and Support:

   • The endosperm surrounds and protects the developing embryo, providing a supportive environment for its growth.
   • By forming early in seed development, the endosperm ensures that the embryo is surrounded by a continuous source of nutrients.

4. Storage of Reserve Food:

   • Endosperm tissue acts as a storage organ for reserve food materials.
   • These reserves, such as starch, proteins, and oils, are deposited in the endosperm and are essential for the germination and early growth of the seedling after germination.

5. Types of Endosperm Persistence:

   • In some plants like peas, beans, and groundnuts, the endosperm is completely consumed by the developing embryo before seed maturation.
   • Seeds of these plants typically have cotyledons that store nutrients absorbed from the endosperm during embryo development.

6. Persistence in Mature Seeds:

   • In other plants such as castor beans and coconuts, the endosperm persists in the mature seed.
   • It serves as a nutrient reserve for the developing seedling during germination and remains intact even in the mature seed.

7. Role in Cereal Grains:

   • Cereal grains like wheat, rice, and maize typically retain the endosperm in the mature seed.
   • The endosperm in cereal grains serves as a storage tissue for carbohydrates and contributes significantly to the nutritional value of these staple food crops.

1.4.2 Embryo

1. Development Location:

   • The embryo develops at the micropylar end of the embryo sac, where the zygote is located.
   • Zygotes in most cases begin dividing only after a certain amount of endosperm is formed, ensuring adequate nutrition for the developing embryo.

2. Embryogeny Stages:

   • Early stages of embryo development, known as embryogeny, are similar in both monocotyledons and dicotyledons.
   • The stages include the formation of the proembryo, followed by the development of the globular, heart-shaped, and mature embryo.

3. Dicotyledonous Embryo Structure:

   • A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
   • The embryonal axis has an epicotyl (above cotyledon level) ending with the plumule (stem tip), and a hypocotyl (below cotyledon level) ending with the radicle (root tip) covered by a root cap.

4. Monocotyledonous Embryo Structure:

   • Monocotyledonous embryos possess only one cotyledon, known as the scutellum, situated laterally on the embryonal axis.
   • The embryonal axis includes the radicle and root cap enclosed in a sheath called the coleorrhiza.
   • Above the attachment of the scutellum, the embryonal axis forms the epicotyl, which has a shoot apex and leaf primordia enclosed in a structure called the coleoptile.

5. Observing Embryo Parts:

   • To observe the various parts of embryos, seeds such as wheat, maize, peas, chickpeas, and groundnuts can be soaked in water overnight and then split open.
   • This allows for the examination of the embryonic axis, cotyledons, epicotyl, hypocotyl, radicle, root cap, and other structures present in the developing embryo.

1.4.3 Seed

1. Seed Description:

   • Seeds are the final product of sexual reproduction in angiosperms, formed inside fruits.
   • A typical seed consists of seed coat(s), cotyledon(s), and an embryo axis.

2. Cotyledons:

   • Cotyledons are simple structures in the embryo, usually thick and swollen due to the storage of food reserves.
   • Seeds may be non-albuminous (completely consuming endosperm during development) or albuminous (retaining part of the endosperm).
   • Some seeds, like black pepper and beet, may have persistent remnants of the nucellus known as perisperm.

3. Seed Coat:

   • Integuments of ovules harden into tough, protective seed coats, with the micropyle remaining as a small pore for entry of oxygen and water during germination.

4. Seed Characteristics:

   • As seeds mature, their water content reduces, and they become relatively dry, with about 10-15% moisture by mass.
   • Mature seeds may enter a state of dormancy or germinate depending on environmental conditions.

5. Fruit Development:

   • Ovary transforms into a fruit simultaneously as ovules develop into seeds, with the ovary wall becoming the pericarp.
   • Fruits may be fleshy or dry and may have mechanisms for seed dispersal.

6. True and False Fruits:

   • Fruits can develop solely from the ovary (true fruits) or involve other floral parts like thalamus (false fruits).

7. Parthenocarpy:

   • Some fruits develop without fertilization through parthenocarpy, such as bananas, which can be induced by growth hormones to produce seedless fruits.

8. Advantages of Seeds:

   • Seeds offer advantages such as dependable reproduction independent of water, better adaptive strategies for dispersal, and protection for young seedlings.

9. Seed Viability and Longevity:

   • Seed viability varies greatly among species, with some seeds losing viability within months while others remain alive for hundreds or even thousands of years.

10. Reproductive Capacity:

    • Some plants, like orchids and parasitic species, produce fruits containing a very large number of seeds.
    • Examples like Ficus trees demonstrate how a tiny seed can produce a large biomass over time, highlighting the enormous reproductive capacity of certain flowering plants.

1.5 APOMIXIS AND POLYEMBRYONY

1. Apomixis:

   • Apomixis is a mechanism in certain flowering plants, like some species of Asteraceae and grasses, where seeds are produced without fertilization.
   • Fruit production without fertilization is called parthenocarpy.
   • Apomixis mimics sexual reproduction but involves asexual reproduction processes.
   • It can occur in various ways, such as the development of diploid egg cells without reduction division or the formation of embryos from nucellar cells surrounding the embryo sac.

2. Polyembryony:

   • Polyembryony is the occurrence of more than one embryo in a seed.
   • In species like Citrus and Mango, some nucellar cells divide, protrude into the embryo sac, and develop into multiple embryos.
   • Seeds from such plants may contain many embryos of different sizes and shapes.
   • Apomictic embryos are genetically identical and can be considered clones.

3. Significance of Apomixis:

   • Apomixis is crucial in the hybrid seed industry as it allows for the production of hybrid seeds without the need for costly hybrid seed production each year.
   • Hybrid seeds produced through apomixis maintain their hybrid characters in subsequent generations without segregation.
   • Active research is ongoing worldwide to understand the genetics of apomixis and transfer apomictic genes into hybrid varieties to enhance productivity and reduce costs for farmers.