Sexual Reproduction in Flowering Plants

Flower

The reproductive organ of a plant.

Calyx

Group of sepals, usually green in colour.

Corolla

Group of petals, brightly coloured except green.

Androecium

Male reproductive part of a flower consisting of the stamen having filament and anther.

Gynoecium

Female reproductive part of a flower known as pistil or carpel consisting of stigma, style and ovary.

Filament

Sterile part which connects the anther to the thalamus or petals of the flower.

Anther

Fertile part, within which are formed the pollen grains.

Microsporangium

Structure formed in the anther of the flowering plants where the development and maturation of microspores into pollen grains take place.

Dithecous

Anther is usually bilobed, each lobe has two theca.

Tetrasporangiate

A typical angiosperm anther has four microsporangia.

Endothecium

The endothecial cells develop a fibrous thickening before the dehiscence of the anther.

Middle layers

May range from 1-6 or in some cases, several middle layers are present; they degenerate at maturity of the anther.

Tapetum

The innermost layer of the anther; provides nourishment to developing pollen grains.

Pollen grain

A microspore that develops into a male gamete.

Product of sexual reproduction

Generally generates new genetic combinations leading to variations, large biomass, longer viability of seeds, and prolonged dormancy.

Arceuthobium

Has one microsporangium per anther.

Moringa and Wolfia

Each anther lobe has one microsporangium.

Hypodermal archesporial cells

Differentiated cells in the anther that undergo periclinal division.

Anther wall

Formed by the primary parietal cell undergoing divisions to create 3-5 layered structure.

Pollenkitt

A substance formed by the middle layers that provides nutrition.

Dehiscence

The process that helps in the dispersal of pollen grains.

Hygroscopic

Nature of the thickening made-up of a-cellulose with a little pectin and lignin in some cases.

Amoeboid or periplasmodial tapetum

A type of tapetum where tapetal cells break up releasing the protoplast into the pollen chamber, forming periplasmodium.

Secretory or glandular tapetum

A type of tapetum where tapetal cells remain throughout the development of microspores and finally degenerate.

Microsporogenesis

The process through which haploid microspores are formed in the microsporangium.

Sporogenous Tissue

Tissue formed from the archesporial cell that divides to form pollen grains through microsporogenesis.

Microspore Mother Cell (PMC)

A cell that undergoes meiosis to produce a tetrad of microspores.

Tetrad of microspores

A group of four microspores formed from a single microspore mother cell.

Callose

A polysaccharide that gets deposited over microspores during their development.

Microspore arrangements

The ways microspores are arranged in a tetrad: tetrahedral, isobilateral, decussate, linear, T-shaped.

Vegetative cell

The larger cell in the pollen grain that forms the pollen tube.

Generative cell

The smaller cell in the pollen grain that forms the two sperm cells.

Two-celled stage

The stage of the pollen grain containing one vegetative cell and one generative cell.

Three-celled stage

The stage of the pollen grain containing one vegetative cell and two sperm cells.

Palynology

The study of pollen grains in living plants and fossils (geopalynology).

Intine

The inner layer of the pollen grain made up of pectin and cellulose.

Exine

The outer layer of the pollen grain made up of sporopollenin.

Sporopollenin

A highly resistant material that makes up the exine and is not degraded by alkali, acids, or other chemicals.

Germ pore

A region on the surface of the pollen grain where the exine is absent and the pollen tube emerges.

Tricolpate pollen grains

Pollen grains of dicots that have three germ pores.

Monocolpate pollen grains

Pollen grains of monocots that have one germ pore.

Microspores

Haploid spores that develop into pollen grains.

Meiosis

The process by which microspore mother cells divide to produce microspores.

Mitosis

The process by which primary sporogenous cells divide to form sporogenous tissue.

Male Germ Unit

The association between the vegetative nucleus and the generative cell.

Anther Dehiscence

The dehiscence of the anther occurs due to the endothecium layer; the point where the endothecium layer breaks is known as the stomium.

Cryopreservation

To increase the viability of pollen, they are maintained at lower temperature, i.e., -196°C in pollen banks.

Apocarpous

Multicarpellary condition in which the carpels are not fused together.

Syncarpous

Multicarpellary condition in which the carpels are fused together.

Stigma

Landing surface for the pollens.

Style

Passage for the pollen tube to the ovary.

Ovary

Embryo sac with the female gamete, i.e., egg is present.

Unicarpellary

Having a single carpel.

Multicarpellary

Condition which can be further divided into apocarpous or syncarpous.

Funicle

The stalk that attaches the ovule to the placenta in the ovary.

Integuments

One or two coats that protect the nucellar tissue in an ovule.

Micropyle

A small opening at one end of the ovule through which water is absorbed and oxygen is exchanged.

Chalazal End

The basal region of the ovule where the funiculus is attached.

Female Gametophyte

The embryo sac present in the nucellus.

Anatropous Ovule

An ovule in which the micropyle and the funicle lie close to each other.

Ategmic Ovule

An ovule in which integuments are absent, e.g., Santalum.

Orthotropous

Condition where micropyle and funicle lie in one line.

Pollen Grains

They can cause severe allergies and can be stored in liquid nitrogen for crop breeding programs.

Pollens in Honey

Provide honey with amino acids, vitamins, and minerals.

Viability of Pollen

In leguminous plants, the viability of the pollen is maintained for six months.

Stored Pollen Uses

Can be used later for plant breeding to improve certain characteristics or if the plants face extinction.

Circinotropous

The length of funicle increases to cover the whole ovule.

Hemianatropous

Funicle is at right angle to the nucellus and integuments.

Amphitropous

Micropyle lies close to the nucellus due to unilateral growth.

Campylotropous

The embryo sac is horseshoe shaped.

Megasporogenesis

It is the development of megaspores from the megaspore mother cell by meiosis.

Archesporium

A hypodermal cell in the nucellus that becomes large in size and consists of dense cytoplasm.

Megaspore Mother Cell

The primary sporogenous cell divides to form the megaspore mother cell.

Functional Megaspore

Only one megaspore is functional while the other three degenerate.

Embryo Sac Formation

The eight nucleate structure develops a wall and forms the embryo sac.

Oenothera Type Embryo Sac

Derived from the micropylar megaspore of the tetrad, consisting of the egg apparatus and a single central nucleus.

Antipodal Cells

Cells at the chalazal end that degenerate before or soon after fertilization.

Synergids

Two cells formed at the micropylar end, known as synergids, with projections called the filiform apparatus.

Pollination

The process of transfer of pollen grains from the anther of a flower to the stigma of another flower.

Self-Pollination

Transfer of pollen grains from the anther to the stigma of the same flower or flower on the same plant.

Autogamy

The transfer of pollen grains from the anther to the stigma of the same flower.

Geitonogamy

The transfer of pollen grains from the anther to the stigma of a different flower on the same plant.

Cross-Pollination

Transfer of pollen grains from the anther of a flower to the stigma of another flower on another plant.

Polygonum Type Embryo Sac

The most common type of embryo sac, usually found in 81% of the plants, develops from the chalazal megaspore.

Central Cell

The central cell consists of two nuclei.

Haploid Cells

The cells of the antipodals and the egg apparatus are haploid while the central cell is diploid.

First Mitotic Division

The functional megaspore undergoes first mitotic division and forms a binucleate structure without wall formation.

Second Mitotic Division

It undergoes second mitotic division and forms four nuclei without wall formation.

Third Mitotic Division

It undergoes third mitotic division to form eight nuclei.

Advantages of Self-Pollination

It is economical for the plants and produces pure lines.

Disadvantages of Self-Pollination

It cannot bring about new variations and thus survival of the offspring in changing environment is difficult.

Inbreeding Depression

It leads to inbreeding depression.

Genetic Recombination

It produces variations due to genetic recombination and thus helps in the development of new species.

Adaptability

It provides adaptability to the changing environment to the plants.

Cleistogamous Flowers

These are the flowers which are closed and thus the sex organs are not exposed; they will always undergo self-pollination.

Chasmogamous Flowers

These are the flowers which have well-exposed reproductive parts; they can undergo both self or cross-pollination.

Abiotic Agents of Pollination

Agents like air and water that help in pollination.

Biotic Agents of Pollination

Living organisms that assist in the transfer of pollen.

Anemophily

Pollination of plants by air.

Characteristics of Anemophily

Flowers are small, borne in pendant catkins, and pollen grains are small, light, smooth, and dry.