Sexual Reproduction in Flowering Plants: Comprehensive Notes

Biology: The Story of Life on Earth

Biology is essentially the story of life on Earth. Species survive for millions of years through reproduction, either asexually or sexually. Sexual reproduction creates new variants, enhancing survival.

Unit Overview

This unit explains reproductive processes in flowering plants and humans. It also addresses human reproductive health and how to avoid reproductive ill health.

• Chapter 1: Sexual Reproduction in Flowering Plants
• Chapter 2: Human Reproduction
• Chapter 3: Reproductive Health

Panchanan Maheshwari (1904-1966)

• Distinguished botanist known worldwide.
• Inspired by Dr. W. Dudgeon to study Botany and morphology.
• Worked on embryological aspects and used embryological characters in taxonomy.
• Established the Department of Botany, University of Delhi.
• Emphasized artificial culture of immature embryos; tissue culture is now a landmark in science.
• Gained acclaim for work on test tube fertilization and intra-ovarian pollination.
• Honored with fellowship of Royal Society of London (FRS), Indian National Science Academy, and other institutions.
• Contributed to school education by leading the creation of Biology textbooks for Higher Secondary Schools published by NCERT in 1964.

Sexual Reproduction in Flowering Plants

Introduction

Flowering plants exhibit sexual reproduction, utilizing diverse adaptations in inflorescences, flowers, and floral parts to form fruits and seeds.

1.1 Flower – A Fascinating Organ of Angiosperms

Flowers hold aesthetic, ornamental, social, religious, and cultural value, symbolizing human emotions. To biologists, flowers are morphological and embryological marvels and sites of sexual reproduction.

• Floriculture: The cultivation of flowers.

1.2 Pre-fertilisation: Structures and Events

Before the appearance of flowers, hormonal and structural changes occur, leading to the development of the floral primordium. Inflorescences bear floral buds and flowers undergo differentiation to form the androecium (male reproductive structure) and gynoecium (female reproductive structure).

1.2.1 Stamen, Microsporangium, and Pollen Grain

• Stamen: Consists of a filament (stalk) and an anther (terminal structure).
• Anther: Typically bilobed with two theca (ditecous). A longitudinal groove separates the theca.
  • A typical angiosperm anther is bilobed (dithecous) and tetragonal, containing four microsporangia at the corners.
  • Microsporangia develop into pollen sacs filled with pollen grains.
• Microsporangium Structure:
  • Appears circular in outline in transverse section.
  • Surrounded by four wall layers: epidermis, endothecium, middle layers, and tapetum.
    • Epidermis, Endothecium & Middle Layers: Protection and dehiscence of anther for pollen release.
    • Tapetum: Innermost layer, nourishes developing pollen grains. Cells have dense cytoplasm and multiple nuclei.
• Sporogenous Tissue: Compactly arranged homogenous cells in the center of each microsporangium of a young anther.
• Microsporogenesis:
  • Cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads.
  • Each cell of the sporogenous tissue is a potential pollen or microspore mother cell.
  • The process involves meiosis, reducing the ploidy of the cells.
  • Microspores are arranged in a cluster of four cells (microspore tetrad).
  • As anthers mature and dehydrate, microspores dissociate and develop into pollen grains.
  • Thousands of microspores or pollen grains form in each microsporangium and are released via dehiscence.
• Pollen Grain: Represents the male gametophyte.
  • Spherical, measuring about $25-50$ micrometers in diameter.
  • Has a two-layered wall: exine and intine.
    • Exine: Hard outer layer made of sporopollenin, a resistant organic material that can withstand high temperatures, strong acids, and alkali. It has germ pores where sporopollenin is absent. Well-preserved as fossils.
    • Intine: Inner wall made of cellulose and pectin.
  • Cytoplasm is surrounded by a plasma membrane.
  • Mature pollen grain contains two cells: vegetative cell (larger, with abundant food reserve and an irregularly shaped nucleus) and generative cell (small, spindle-shaped, with dense cytoplasm and a nucleus).
    • In over 60% of angiosperms, pollen grains are shed at the 2-celled stage.
    • In the remaining species, the generative cell divides mitotically to form two male gametes (3-celled stage).
  • Pollen grains can cause severe allergies and respiratory disorders like asthma and bronchitis.
    • Parthenium (carrot grass) is a contaminant in imported wheat that causes pollen allergy.
  • Pollen grains have a viability period that varies depending on temperature and humidity.
    • In cereals like rice and wheat, viability is lost within 30 minutes.
    • In Rosaceae, Leguminoseae, and Solanaceae, viability can last for months.
  • Pollen grains can be stored for years in liquid nitrogen ($-196^{\circ}C$) and used in pollen banks for crop breeding programs.

1.2.2 The Pistil, Megasporangium (Ovule), and Embryo Sac

• Gynoecium: Represents the female reproductive part of the flower, consisting of a single pistil (monocarpellary) or multiple pistils (multicarpellary). Pistils can be fused (syncarpous) or free (apocarpous).
• Pistil: Consists of three parts: stigma, style, and ovary.
  • Stigma: Landing platform for pollen grains.
  • Style: Elongated, slender part beneath the stigma.
  • Ovary: Basal, bulged part containing the ovarian cavity (locule). The placenta is located inside the ovarian cavity, bearing the megasporangia (ovules).
• Ovule Number:
  • May be one (wheat, paddy, mango) or many (papaya, watermelon, orchids).
• Megasporangium (Ovule) Structure:
  • Attached to the placenta by a stalk called the funicle.
  • The body of the ovule fuses with the funicle at the hilum.
  • Has one or two protective envelopes called integuments.
  • Integuments encircle the nucellus, except at the micropyle (small opening).
  • The chalaza is opposite the micropylar end, representing the basal part of the ovule.
  • The nucellus is a mass of cells enclosed within the integuments, containing reserve food materials.
  • Located within the nucellus is the embryo sac (female gametophyte), typically formed from a single megaspore.
• Megasporogenesis:
  • The process of forming megaspores from the megaspore mother cell (MMC).
  • A single MMC differentiates in the micropylar region of the nucellus. It is a large cell with dense cytoplasm and a prominent nucleus.
  • The MMC undergoes meiotic division to produce four megaspores.
  • Meiosis reduces the ploidy of the cells.
• Female Gametophyte:
  • In most flowering plants, one megaspore is functional, while the other three degenerate.
  • The functional megaspore develops into the female gametophyte (embryo sac) through monosporic development.
  • What will be the ploidy of the cells of the nucellus, MMC, the functional megaspore and female gametophyte?
  • The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to opposite poles, forming the 2-nucleate embryo sac.
  • Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and then 8-nucleate stages of the embryo sac.
  • These mitotic divisions are free nuclear, meaning nuclear divisions are not immediately followed by cell wall formation.
  • After the 8-nucleate stage, cell walls are laid down, resulting in the organization of the typical female gametophyte or embryo sac.
  • Six of the eight nuclei are surrounded by cell walls and organized into cells. The remaining two nuclei, called polar nuclei, are situated below the egg apparatus in the large central cell.
  • Three cells are grouped together at the micropylar end, forming the egg apparatus, which consists of two synergids and one egg cell. Synergids have filiform apparatus (cellular thickenings) at the micropylar tip, guiding pollen tubes.
  • Three cells are at the chalazal end and are called the antipodals.
  • The large central cell has two polar nuclei.
  • A typical angiosperm embryo sac, at maturity, is 8-nucleate but 7-celled.

1.2.3 Pollination

Male and female gametes in flowering plants are produced in the pollen grain and embryo sac, respectively. Since both are non-motile, they must be brought together for fertilization through pollination. Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.

• Kinds of Pollination:
  • (i) Autogamy: Pollination occurs within the same flower.
    • Pollen grains are transferred from the anther to the stigma of the same flower.
    • Requires synchrony in pollen release and stigma receptivity.
    • Anthers and stigma should lie close to each other.
    • Some plants like Viola, Oxalis, and Commelina produce chasmogamous flowers (open anthers and stigma) and cleistogamous flowers (do not open).
    • Cleistogamous flowers are invariably autogamous, ensuring seed-set even without pollinators.
    • Do you think that cleistogamy is advantageous or disadvantageous to the plant? Why?
  • (ii) Geitonogamy: Pollen grains are transferred from the anther to the stigma of another flower on the same plant.
    • Functionally cross-pollination (involving a pollinating agent), but genetically similar to autogamy.
  • (iii) Xenogamy: Pollen grains are transferred from the anther to the stigma of a different plant.
    • Brings genetically different pollen grains to the stigma.
• Agents of Pollination:
  • Plants use abiotic (wind and water) and biotic (animals) agents.
    • Majority use biotic agents.
    • Only a small proportion use abiotic agents.
    • Pollen grains contacting the stigma is a chance factor in wind and water pollination.
    • Flowers produce enormous amounts of pollen to compensate for uncertainties and losses.
  • Wind Pollination:
    • Common amongst abiotic pollinations.
    • Pollen grains are light and non-sticky for wind transport.
    • Flowers have well-exposed stamens and large, feathery stigmas to trap airborne pollen grains.
    • Often have a single ovule in each ovary and numerous flowers packed into an inflorescence.
      • Example: corn cob (ears are stigma and style).
    • Common in grasses.
  • Water Pollination:
    • Rare in flowering plants (limited to about 30 genera, mostly monocotyledons).
    • Water is the mode of transport for male gametes in lower plant groups such as algae, bryophytes, and pteridophytes.
    • Some bryophytes and pteridophytes have limited distribution due to their need for water for fertilization.
    • Examples: Vallisneria and Hydrilla (freshwater), and Zostera (marine sea-grasses).
    • Not all aquatic plants use water for pollination. Some, like water hyacinth and water lily, have flowers that emerge above water and are pollinated by insects or wind.
    • In Vallisneria, female flowers reach the water surface via long stalks, and male flowers/pollen grains are released onto the surface and carried by water currents to the female flowers.
    • In seagrasses, female flowers remain submerged, and pollen grains are released inside the water. Pollen grains are long, ribbon-like, and carried passively to the stigma.
    • Pollen grains are protected from wetting by a mucilaginous covering.
    • Wind and water-pollinated flowers are not very colorful and do not produce nectar.
    • What would be the reason for this?
  • Animal Pollination:
    • Majority of flowering plants use animals as pollinating agents.
    • Common agents: bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and hummingbirds), and bats.
    • Insects, particularly bees, are the dominant biotic pollinating agents.
    • Larger animals such as primates (lemurs), arboreal rodents, and reptiles (gecko lizards and garden lizards) can also be pollinators.
    • Flowers of animal-pollinated plants are specifically adapted for particular animal species.
    • Insect-pollinated flowers are large, colorful, fragrant, and rich in nectar.
    • Small flowers are clustered into inflorescences to make them conspicuous.
    • Animals are attracted by color and/or fragrance.
    • Flowers pollinated by flies and beetles secrete foul odors.
    • Flowers provide rewards to sustain animal visits, typically nectar and pollen grains.
    • Animals come into contact with the anthers and stigma while harvesting rewards.
    • The animal's body gets coated with pollen grains, which are generally sticky in animal-pollinated flowers.
    • Pollination occurs when the animal carries pollen to the stigma.
    • Some species provide safe places to lay eggs.
      • Example: Amorphophallus (tallest flower, ~6 feet). A moth species and the plant Yucca have a relationship where the moth deposits eggs in the ovary, and the flower is pollinated. Moth larvae emerge as seeds develop.
    • Why don’t you observe some flowers of the following plants (or any others available to you): Cucumber, Mango, Peepal, Coriander, Papaya, Onion, Lobia, Cotton, Tobacco, Rose, Lemon, Eucalyptus, Banana? Try to find out which animals visit them and whether they could be pollinators.You’ll have to patiently observe the flowers over a few days and at different times of the day. You could also try to see whether there is any correlation in the characteristics of a flower to the animal that visits it. Carefully observe if any of the visitors come in contact with the anthers and the stigma as only such visitors can bring about pollination. Many insects may consume pollen or the nectar without bringing about pollination. Such floral visitors are referred to as pollen/nectar robbers.

Outbreeding Devices

Majority of flowering plants produce hermaphrodite flowers, and frequent self-pollination leads to inbreeding depression. Flowering plants have evolved devices to discourage self-pollination and encourage cross-pollination:

• Pollen release and stigma receptivity are not synchronized.
  • Pollen may be released before the stigma is receptive, or the stigma becomes receptive before pollen release.
• Anther and stigma are placed at different positions to prevent pollen from contacting the stigma of the same flower.
• Self-incompatibility: a genetic mechanism that prevents self-pollen (from the same flower or other flowers of the same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
• Production of unisexual flowers.
  • If both male and female flowers are present on the same plant (monoecious, e.g., castor and maize), it prevents autogamy but not geitonogamy.
  • In several species (e.g., papaya), male and female flowers are present on different plants (dioecy). This condition prevents both autogamy and geitonogamy.

Pollen-Pistil Interaction

Pollination does not guarantee that the right type of pollen (compatible, of the same species) will be transferred to the stigma. The pistil can recognize compatible and incompatible pollen.

• If compatible, the pistil accepts the pollen and promotes post-pollination events leading to fertilization.
• If incompatible, the pistil rejects the pollen by preventing pollen germination or pollen tube growth in the style.
• Recognition is the result of a continuous dialogue between the pollen grain and the pistil, mediated by chemical components.
• Following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores.
  • The contents of the pollen grain move into the pollen tube.
  • In plants where pollen grains are shed at the two-celled condition (vegetative and generative cell), the generative cell divides to form two male gametes during pollen tube growth.
  • In plants that shed pollen in the three-celled condition, pollen tubes carry the two male gametes from the beginning.
  • The pollen tube reaches the ovary, enters the ovule through the micropyle, and enters one of the synergids through the filiform apparatus.
  • The filiform apparatus guides the entry of the pollen tube.
• All events from pollen deposition on the stigma until the pollen tubes enter the ovule are referred to as pollen-pistil interaction.
• Knowledge gained in this area helps plant breeders manipulate pollen-pistil interaction to obtain desired hybrids.

Artificial Hybridisation

A breeder is interested in crossing different species and genera to combine desirable traits and produce commercially superior varieties. Artificial hybridization is a major approach used in crop improvement programs. It is important to use only desired pollen grains for pollination and to protect the stigma from contamination, which is achieved through emasculation and bagging techniques.

• Emasculation: Removal of anthers from the flower bud before dehiscence in bisexual flowers.
• Bagging: Covering emasculated flowers with a bag (usually butter paper) to prevent contamination of the stigma with unwanted pollen.
  • When the stigma of a bagged flower attains receptivity, mature pollen grains from the male parent are dusted on the stigma, and the flowers are rebagged to allow fruits to develop.
• If the female parent produces unisexual flowers, emasculation is not required; female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the desired pollen, and the flower is rebagged.

1.4 Post-Fertilisation: Structures and Events

Events following double fertilization, including endosperm and embryo development, maturation of ovules into seeds, and the ovary into fruit, are collectively termed post-fertilization events.

1.4.1 Endosperm

Endosperm development precedes embryo development to provide nutrition to the developing embryo. The primary endosperm cell divides repeatedly to form a triploid endosperm tissue. These cells are filled with reserve food materials.

• In the most common type of endosperm development, the primary endosperm nucleus (PEN) undergoes successive nuclear divisions, resulting in a free-nuclear endosperm. Cell wall formation occurs later, and the endosperm becomes cellular.
• The number of free nuclei formed before cellularization varies.
• Coconut water from tender coconut is free-nuclear endosperm, while the surrounding white kernel is cellular endosperm.
• The endosperm may be completely consumed by the developing embryo before seed maturation (e.g., pea, groundnut, beans) or may persist in the mature seed (e.g., castor and coconut) and be used during seed germination.
• Endosperm may either be completely consumed by the developing embryo (e.g., pea, groundnut, beans) before seed maturation or it may persist in the mature seed (e.g. castor and coconut) and be used up during seed germination.
• Split open some seeds of castor, peas, beans, groundnut, fruit of coconut and look for the endosperm in each case. Find out whether the endosperm is persistent in cereals – wheat, rice and maize.

1.4.2 Embryo

Embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after some endosperm is formed to ensure nutrition.

• Early stages of embryo development (embryogeny) are similar in monocotyledons and dicotyledons.
• In dicotyledons, the zygote gives rise to the proembryo, then to the globular, heart-shaped, and ultimately mature embryo.
• A typical dicotyledonous embryo has an embryonal axis and two cotyledons.
  • The portion of the embryonal axis above the level of cotyledons is the epicotyl, terminating with the plumule (stem tip).
  • The portion below the level of cotyledons is the hypocotyl, terminating at the radicle (root tip), which is covered by a root cap.
• Monocotyledon embryos possess only one cotyledon.
  • In the grass family, the cotyledon is called the scutellum and is situated laterally on the embryonal axis.
  • At its lower end, the embryonal axis has the radicle and root cap enclosed in an undifferentiated sheath called the coleorrhiza.
  • The portion of the embryonal axis above the attachment of the scutellum is the epicotyl, which has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure called the coleoptile.
• Soak a few seeds in water (say of wheat, maize, peas, chickpeas, ground nut) overnight. Then split the seeds and observe the various parts of the embryo and the seed.

1.4.3 Seed

In angiosperms, the seed is the final product of sexual reproduction, formed inside fruits. A seed typically consists of seed coat(s), cotyledon(s), and an embryo axis.

• The cotyledons of the embryo are simple structures, generally thick and swollen due to stored food reserves (as in legumes).
• Mature seeds may be non-albuminous (exalbuminous) or albuminous.
  • Non-albuminous seeds have no residual endosperm because it is completely consumed during embryo development (e.g., pea, groundnut).
  • Albuminous seeds retain a part of the endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor).
• Occasionally, remnants of the nucellus may persist as the perisperm (e.g., black pepper and beet).
• Integuments of ovules harden as tough protective seed coats (testa).
• The micropyle remains as a small pore in the seed coat, facilitating entry of oxygen and water during germination.
• As the seed matures, its water content is reduced (10-15% moisture by mass), and the general metabolic activity slows down. The embryo may enter dormancy.
• If favorable conditions are available, they germinate.

1.4.4 Fruit

As ovules mature into seeds, the ovary develops into a fruit. The wall of the ovary develops into the fruit wall called the pericarp. Fruits may be fleshy (guava, orange, mango) or dry (groundnut, mustard).

• Many fruits have evolved mechanisms for seed dispersal.
• Recall the classification of fruits and their dispersal mechanisms that you have studied in an earlier class.
  *Is there any relationship between number of ovules in an ovary and the number of seeds present in a fruit?
• Most floral parts degenerate and fall off as the fruit develops. However, in a few species like apple, strawberry, and cashew, the thalamus also contributes to fruit formation, forming false fruits. Most fruits develop only from the ovary and are called true fruits.
• Although most fruits are the results of fertilization, some develop without fertilization, resulting in parthenocarpic fruits (e.g., banana). Parthenocarpy can be induced by applying growth hormones, and such fruits are seedless.

Advantages of Seeds

• Reproductive processes are independent of water.
• Better adaptive strategies for dispersal to new habitats.
• Sufficient food reserves for young seedlings until they can photosynthesize.
• The hard seed coat provides protection to the young embryo.
• Generate new genetic combinations leading to variations (products of sexual reproduction).
• Dehydration and dormancy of mature seeds are crucial for storage, providing food throughout the year and enabling crop planting in the next season.
• Can you imagine agriculture in the absence of seeds, or in the presence of seeds which germinate straight away soon after formation and cannot be stored?

Seed Viability

The period for which seeds remain alive after dispersal varies greatly.

• Some seeds lose viability within a few months.
• Seeds of many species live for several years.
• Some seeds can remain alive for hundreds of years.
• The oldest seed was a lupine, Lupinus arcticus, excavated from the Arctic Tundra, which germinated and flowered after an estimated 10,000 years of dormancy.
• A recent record is a 2000-year-old viable seed of the date palm, Phoenix dactylifera, discovered at King Herod’s palace near the Dead Sea.

Reproductive Capacity

Attempt to comprehend the reproductive capacity of flowering plants:

• How many eggs are present in an embryo sac?
• How many embryo sacs are present in an ovule?
• How many ovules are present in an ovary?
• How many ovaries are present in a typical flower?
• How many flowers are present on a tree?
• Can you think of some plants in which fruits contain very large number of seeds. Orchid fruits are one such category and each fruit contain thousands of tiny seeds. Similar is the case in fruits of some parasitic species such as Orobanche and Striga. Have you seen a tiny seed of Ficus? How large is the tree of Ficus developed from that tiny seed. How many billions of seeds does each Ficus tree produce?
• Can you imagine any other example in which such a tiny structure can produce such a large biomass over the years?

1.5 Apomixis and Polyembryony

Although seeds are generally the products of fertilization, some flowering plants (e.g., Asteraceae, grasses) have evolved mechanisms to produce seeds without fertilization, called apomixis. Apomixis is a form of asexual reproduction that mimics sexual reproduction.

• What is fruit production without fertilisation called?
• In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilization.
• In many Citrus and Mango varieties, nucellar cells surrounding the embryo sac divide, protrude into the embryo sac, and develop into embryos. In such species, each ovule contains many embryos.
• Occurrence of more than one embryo in a seed is referred to as polyembryony.
• Take out some seeds of orange and squeeze them. Observe the many embryos of different sizes and shapes from each seed. Count the number of embryos in each seed.
• What would be the genetic nature of apomictic embryos? Can they be called clones?

Importance of Apomixis for Hybrid Seed Production

Hybrid varieties of food and vegetable crops are widely cultivated, increasing productivity significantly. However, hybrid seeds must be produced every year because seeds collected from hybrids segregate, losing hybrid characteristics. Production of hybrid seeds is costly.

• If hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny.
• Farmers can reuse hybrid seeds to raise new crops annually without purchasing new seeds.
• Active research seeks to understand the genetics of apomixis and transfer apomictic genes into hybrid varieties.

Summary

Flowers contain the male (stamens) and female (pistils) reproductive organs of angiosperms.

• A typical anther is bilobed, dithecous, and tetrasporangiate. Pollen grains develop inside the microsporangia, which are surrounded by four wall layers: epidermis, endothecium, middle layers, and tapetum.
• Cells of the sporogenous tissue undergo meiosis (microsporogenesis) to form tetrads of microspores, which mature into pollen grains. Pollen grains represent the male gametophytic generation and have a two-layered wall (exine and intine). The exine is made of sporopollenin and has germ pores. Pollen grains may be two-celled (a vegetative cell and generative cell) or three-celled (a vegetative cell and two male gametes) at the time of shedding.
• The pistil has three parts: the stigma, style, and the ovary. Ovules are present in the ovary. The ovules have a stalk called funicle, protective integument(s), and an opening called micropyle. The central tissue is the nucellus, in which the archesporium differentiates. A cell of the archesporium, the megaspore mother cell, divides meiotically, and one of the megaspores forms the embryo sac (female gametophyte). The mature embryo sac is 7-celled and 8-nucleate. There is the egg apparatus consisting of egg cell and synergids, antipodals and polar nuclei.
• Pollination is the mechanism to transfer pollen grains from the anther to the stigma. Pollinating agents are either abiotic (wind and water) or biotic (animals).
• Pollen-pistil interaction involves all events from the landing of pollen grains on the stigma until the pollen tube enters the embryo sac (when the pollen is compatible) or pollen inhibition (when the pollen is incompatible).
• Following compatible pollination, the pollen grain germinates on the stigma, and the resulting pollen tube grows through the style, enters the ovules, and finally discharges two male gametes in one of the synergids. Angiosperms exhibit double fertilization because two fusion events occur in each embryo sac: syngamy and triple fusion. The products of these fusions are the diploid zygote and the triploid primary endosperm nucleus. The zygote develops into the embryo, and the primary endosperm cell forms the endosperm tissue.
• Formation of endosperm always precedes development of the embryo. The developing embryo passes through different stages such as the proembryo, globular and heart-shaped stages before maturation. Mature dicotyledonous embryos have two cotyledons and an embryonal axis with epicotyl and hypocotyl. Embryos of monocotyledons have a single cotyledon. After fertilization, the ovary develops into the fruit, and ovules develop into seeds.
• A phenomenon called apomixis is found in some angiosperms, particularly in grasses, resulting in the formation of seeds without fertilization. Apomicts have several advantages in horticulture and agriculture.
• Some angiosperms produce more than one embryo in their seed. This phenomenon is called polyembryony.