1. Reproduction in Lower and Higher Plants
Reproduction in Lower and Higher Plants
Formation of Embryo Sac
B. Biotic Agents
Reproduction
Reproduction is the process of producing young ones similar to their parents.
Reproduction is an essential life process because it: ● Ensures the continuation of species.
● Maintains the continuity of life.
● Each organism has its own specific method of reproduction.
Types of Reproduction
All methods of reproduction are broadly classified into two categories:
1. Asexual Reproduction
2. Sexual Reproduction
Asexual Reproduction
Asexual reproduction is reproduction that occurs without fusion of two compatible gametes/sex cells.
Key features
Involves a single parent/organism.
Produces genetically identical progeny.
These genetically and morphologically identical individuals are called clones.
The offspring inherit the genes of the parent.
Modes of Asexual Reproduction
i. Fragmentation
A multicellular organism breaks into fragments.
Each fragment grows into a new individual.
Example: Spirogyra
Fragment → Growth → New individual
ii. Budding
Common method in unicellular Protosiphon and yeast.
Usually occurs under favourable conditions.
One or more outgrowths called buds are produced.
Bud separates and develops into a new individual.
Parent → Bud formation → Bud separates → New individual
iii. Spore Formation
In Chlamydomonas, asexual reproduction occurs through flagellated, motile zoospores.
Zoospores can move and grow independently into new individuals.
Other methods
Binary fission :- Chlorella, Diatoms, Chlamydomonas
Conidia formation :- Penicillium
Gemma formation :- Marchantia
Vegetative Reproduction
Plants reproduce asexually through their vegetative parts, producing new plants that are genetically identical to the parent.
Some methods of vegetative reproduction do not occur naturally.
Agriculture and horticulture exploit vegetative reproduction to multiply fresh stocks of plants.
Artificial methods are used to propagate desired varieties according to human requirements.
a. Cutting
A small piece of any vegetative part having one or more buds is used for propagation.
Stem cutting: Rose, Bougainvillea
Leaf cutting: Sansevieria
Root cutting: Blackberry
b. Grafting
Parts of two plants are joined so that they grow as one plant.
Scion: Part of stem containing more than one bud.
Stock: Rooted plant onto which the scion is joined.
Joining scion with stock = Grafting.
Budding / Bud grafting: Only one bud is joined onto the stock.
Examples: Apple, Pear, Rose.
C. Tissue Culture
Tissue culture is a method in which a small amount of plant tissue is carefully grown to produce many plantlets.
Micropropagation is also used nowadays for rapid propagation of plants.
1.2 Sexual Reproduction
Sexual reproduction involves the fusion of two compatible gametes/sex cells.
Organisms attain maturity before reproducing sexually.
In plants, the end of the juvenile/vegetative phase marks the beginning of the reproductive phase, which is easily observed in higher plants during flowering.
Flower – Reproductive Structure
The flower is the specialized reproductive structure of a plant where sexual reproduction takes place.
Functions:
Produces haploid gametes
Ensures fertilization
A typical flower has four whorls: Calyx → Corolla → Androecium → Gynoecium
Major Events - Sexual reproduction involves:
1. Meiosis
2. Fusion of gametes → formation of diploid zygote
It produces genetically dissimilar offspring. These variations are important for the survival and evolution of species over time.
Characteristic Features
Sexual reproduction is characterized by: Fusion of male + female gametes (fertilization) → Zygote formation → Embryogenesis
Three Stages
The sequential events are grouped into:
1. Pre-fertilization → 2. Fertilization → 3. Post-fertilization
Life Cycle in Angiosperms
Diploid sporophyte (2n) is the predominant/main plant body in all angiosperms.
Meiosis occurs in the sporophyte to produce haploid spores (n).
These spores develop into gametophytes.
In angiosperms, gametophytes are considerably reduced and develop within the flower.
Gametophytes produce gametes.
Male Reproductive Whorl — Androecium
The male reproductive whorl of a flower is called androecium.
Individual member of androecium → Stamen
A typical stamen consists of: Filament + Connective + Anther
Structure at a glance
Stamen
Filament — stalk
Connective — tissue connecting the anther lobes
Anther — consists of two anther lobes (thecae).
Structure of Anther
Anther — Basic Structure
Anther is generally dithecous → has two lobes.
Each lobe contains two pollen sacs.
Therefore:
2 anther lobes × 2 pollen sacs = 4 pollen sacs
→ Hence, anther is tetrasporangiate.
Development of Anther
Immature anther consists of:
A group of parenchymatous cells
Surrounded by a single-layered epidermis
Differentiation begins when some hypodermal cells transform into archesporial cells.
T.S. of Anther
The archesporial cell divides into:
Outer primary parietal cell | Inner sporogenous cell
Sporogenous cell → Sporogenous tissue → Microspore tetrads
Each cell of sporogenous tissue can give rise to a microspore tetrad.
Primary parietal cell → Anther wall layers
Four Layers of Mature Anther Wall
Epidermis → Endothecium → Middle layers → Tapetum
🌼 1.3 Microsporogenesis
Formation of Microspores
Each microspore mother cell (MMC) undergoes meiosis to form a tetrad of four haploid microspores.
Microspore mother cell (2n)
↓ Meiosis
Tetrad → 4 haploid microspores (n)
↓
Pollen grains
Structure of Microspore / Pollen Grain
A typical pollen grain is:
Non-motile, Haploid (n), Unicellular, Contains a single nucleus.
It is surrounded by a two-layered wall called sporoderm.
Layer Structure & features
Exine Outer, thick layer; made of sporopollenin; complex and non-biodegradable; may be smooth or sculptured (species-specific); resistant to chemicals
Germ-pore
At certain places, the exine is very thin, forming thin areas called germ-pores.
Germ-pores allow the pollen tube to emerge during pollen germination.
Intine Inner layer; made of cellulose and pectin.
Pollen Viability
Viability = functional ability of a pollen grain to germinate and develop into the male gametophyte.
Pollen viability depends mainly on environmental conditions, especially: Temperature, Humidity,
It is 30 minutes in rice and wheat. But in some members of family
Solanaceae, Rosaceae, Leguminosae, it
lasts even for months.
Development of Male Gametophyte
Pollen grain marks the beginning of the male gametophyte.
It undergoes the first mitotic division to form:
Vegetative cell — larger, naked, rich in food and contains an irregularly shaped nucleus.
Generative cell — smaller, thin-walled and floats in the cytoplasm of the vegetative cell.
The second mitotic division occurs only in the generative cell, producing two non-motile male gametes.
This division may occur either:
Inside the pollen grain, or Inside the pollen tube.
In most angiosperms, pollen grains are shed from the anther at the two-celled stage.
Flow:
Pollen grain → First mitosis → Vegetative cell + Generative cell → Second mitosis of generative cell → 2 male gametes
Gynoecium (Female Reproductive Whorl)
Gynoecium (Pistil) = female reproductive whorl of a flower.
Individual member of gynoecium = carpel (megasporophyll).
Apocarpous - Many carpels are free from each other - Michelia
Syncarpous - Many carpels are fused together - Brinjal
Structure of a Typical Carpel
A typical carpel consists of three parts:
Stigma → Style → Ovary
Stigma — upper part
Style — middle part
Ovary — basal part containing ovules
Number of Ovules
Uniovulate = ovary contains one ovule
Paddy, Wheat, Mango
Multiovulate = ovary contains many ovules
Tomato, Lady's finger (Okra)
1.4 Structure of Anatropous Ovule
Ovule develops inside the ovary and is attached to the placenta by a small stalk called the funiculus (funicle).
Hilum — point of attachment of the funiculus to the main body of the ovule.
Anatropous ovule — the most common type of ovule in angiosperms, in which the micropyle is directed downwards and lies close to the funiculus.
Nucellus — central parenchymatous tissue of the ovule.
Integuments — usually two protective coverings surrounding the nucellus: Outer integument
Inner integument
Micropyle — narrow opening at the apex of the ovule.
Chalaza — basal region of the ovule, directly opposite to the micropyle.
Embryo sac (female gametophyte) — oval, multicellular structure embedded in the nucellus.
1.5 Megasporogenesis
Megasporogenesis is the process of formation of haploid megaspores (n) from a diploid Megaspore Mother Cell (MMC, 2n).
The Megaspore Mother Cell (MMC) becomes differentiated in the nucellus, generally near the centre but towards the micropylar end of the ovule.
The MMC undergoes meiosis and forms a linear tetrad of four haploid megaspores.
Development of Female Gametophyte
Of the four megaspores:
Upper three megaspores degenerate/abort.
The lowest megaspore, towards the centre of the nucellus, remains functional.
This functional megaspore is the first cell of the female gametophyte.
It undergoes three successive free-nuclear mitotic divisions, producing 8 haploid nuclei.
Four nuclei are present at each pole.
One nucleus from each pole migrates towards the centre → these are the two polar nuclei.
Egg apparatus:
Egg cell — large, central, haploid.
Two synergids — supporting haploid cells.
Each synergid has filiform apparatus (hair-like projections), which helps guide the pollen tube towards the egg.
The two polar nuclei fuse just before fertilization to form the diploid secondary nucleus (definitive nucleus).
Thus, the mature embryo sac is:
7-celled + 8-nucleated
Structure:
3 antipodals + 2 synergids + 1 egg cell + 1 central cell = 7 cells
The central cell contains 2 polar nuclei, making a total of 8 nuclei.
Key Terms
Development of embryo sac from one megaspore → Monosporic development
Female gametophyte develops within the megaspore → Endosporous development
Female gametophyte is colourless, endosporic and remains concealed within the ovule, which is enclosed by the ovary.
1.6 Pollination
Pollen grains are non-motile, therefore angiosperms depend on external agents for their transfer.
Pollinating agents may be:
Abiotic agents → Wind, Water
Biotic agents → Birds, Insects, Snails
Flowers may feed/attract visitors and exploit their mobility for pollination and seed dispersal.
Pollen grains are usually carried from one flower to another by these external agents.
Pollination is the transfer of pollen grains from the anther to the stigma of a flower.
Pollination is a prerequisite for fertilization because:
● Both male and female gametes are non-motile.
● Male and female gametes are produced at two different sites.
1. Self-pollination
Occurs within a single flower or between two flowers of the same plant.
Results in inbreeding / selfing.
2. Cross-pollination
Transfer of pollen from the anther of one flower to the stigma of a flower on another plant of the same species.
Types of Pollination Based on Source
a. Autogamy (Self-pollination)
Autogamy is the transfer of pollen grains from the anther to the stigma of the same bisexual flower.
Pollen grains are produced and received by the same flower.
Offspring are genetically identical to their parents. Examples: Pea, Clitoria
b. Geitonogamy
Geitonogamy is the transfer of pollen grains to the stigma of a different flower on the same plant.
It is functionally similar to cross-pollination because it generally involves a pollinating agent.
However, it is genetically similar to autogamy, because pollen comes from the same plant.
Therefore, it does not produce genetic variations and is mainly of ecological significance.
Example: Cucurbita maxima
c. Xenogamy (Cross-pollination / Outbreeding)
Xenogamy is the transfer of pollen grains from the anther of a flower on one plant to the stigma of a flower on a different plant of the same species.
It occurs with the help of a pollinating agency.
It produces genetically varied offspring.
A. Abiotic Agents
Abiotic agents are non-living agents involved in pollination, mainly wind and water.
1. Pollination by Wind — Anemophily
Most important crop plants are wind-pollinated, e.g. wheat, rice, corn, rye, barley and oats. Palms are also wind-pollinated.
Adaptations of anemophilous flowers:
HYDROPHILY — Pollination by Water
Hydrophily = pollination in which water acts as the pollen-transfer agent.
📌 Occurrence: Only ~30 genera of aquatic monocots
Examples: Vallisneria, Zostera, Ceratophyllum
Adaptations
( Small, inconspicuous flowers → water does the pollination, so showy flowers are unnecessary. )
Perianth + floral parts → unwettable → prevents water from damaging/interfering with pollen transfer.
Pollen → long + unwettable due to mucilage.
No nectar / fragrance → no need to attract animal pollinators.
🌊 Two Types
1. HYPOhydrophily = BELOW water ↓
Zostera (sea grass)
Pollen → sinks → reaches stigma underwater
Pollen is heavier than water.
Pollen grains are long, ribbon-like and without exine.
2. EPIhydrophily = ON water surface ↑
Vallisneria
Male flower → floats on surface → pollen floats → female flower/stigma
Dioecious + submerged freshwater plant
Female flower temporarily reaches water surface.
Male flowers float on water surface.
Pollen has specific gravity ≈ water → therefore floats.
Not all aquatic plants are hydrophilous
Anemophilous: Potamogeton, Halogaris
Entomophilous: Lotus, water hyacinth, waterlily.
Biotic agents = living organisms that help in pollination.
About 80% of plants depend on living, mobile organisms such as insects, birds, bats and snails to transfer pollen from one flower to another.
→ They also help maintain ecosystems and provide natural resources by supporting plant reproduction.
1. Pollination by Insects — Entomophily
Entomophily = pollination by insects 🐝
Examples: Rose, Jasmine, Cestrum
Adaptations of Entomophilous Flowers
To Attract the insect: Flowers are large, showy and often brightly coloured.
Produce sweet odour and have nectar glands → attract insects.
To Catch the pollen :
Stigma: rough due to hairs OR sticky due to mucilaginous secretion → helps pollen stick.
Pollen grains are spiny.
Surrounded by a yellow, sticky substance called pollen-kit → helps pollen adhere to insects.
Some plants have special adaptations that help insect visitors transfer pollen and promote cross-pollination.
Example: Salvia → lever mechanism / turnpipe mechanism.
2. Pollination by Birds — Ornithophily
Ornithophily = pollination by birds
Adaptations of Ornithophilous Flowers
● Flowers are brightly coloured, large and showy.
● Secrete profuse (abundant), dilute nectar.
● Pollen grains are sticky and spiny → easily attach to bird's body.
● Flowers are usually without fragrance, because birds have a poor sense of smell.
3. Pollination by Bats — Chiropterophily 🦇
Chiropterophily = pollination by bats
● Bats can carry pollen over long distances, sometimes several kilometres.
Adaptations of Chiropterophilous Flowers
● Flowers are large and dull coloured.
● Produce strong fragrance to attract bats at night.
● Secrete abundant nectar.
● Produce large quantities of edible pollen grains.
Examples:
Anthocephalus (Kadamb tree)
Adansonia (Baobab / Gorakh Chinch)
Kigelia (Sausage tree)
1.7 Outbreeding Devices (Contrivances)
Outbreeding devices = mechanisms that prevent/discourage self-pollination and promote cross-pollination.
Why needed?
Continuous self-pollination → inbreeding depression → reduced genetic diversity.
Cross-pollination → genetic diversity → important for evolution by natural selection.
1. Unisexuality
Male and female flowers are separate, so self-pollination is prevented.
Monoecious: male + female flowers on the same plant → Maize
Dioecious: male and female flowers on different plants → Mulberry, Papaya
2. Dichogamy
Anther and stigma mature at different times in a bisexual flower.
Protandry: anther/androecium matures first → Sunflower (disc florets)
Protogyny: stigma/gynoecium matures first → Gloriosa
3. Prepotency
Pollen from another flower germinates faster on the stigma than pollen from the same flower.
Example: Apple
4. Heterostyly (Heteromorphy)
Different forms of flowers have anthers and stigmas at different heights, reducing self-pollination. (heterostyly and heteroanthy)
Pollen from anther generally reaches a stigma at the same level.
Example: Primula
5. Herkogamy
A physical/mechanical barrier separates anther and stigma, preventing their contact.
Example: Calotropis — pentangular stigma lies above the anthers (pollinia).
6. Self-incompatibility (Self-sterility)
A genetic mechanism in which pollen from the same flower is prevented from germinating on its stigma.
Examples: Tobacco, Thea
Quick Revision :
Unisexuality → different sex flowers
Dichogamy → different maturity time
Prepotency → foreign pollen germinates faster
Heterostyly → different heights
Herkogamy → physical barrier
Self-incompatibility → genetic rejection
1.8 Pollen–Pistil Interaction
● Pollen–pistil interaction = interaction between pollen grain and pistil (stigma) from pollination up to fertilization.
● Pollination does not ensure that the correct pollen reaches the stigma; wrong/incompatible pollen may also land there.
● The pistil recognises compatible pollen of the same species and rejects incompatible pollen.
Special proteins determine pollen–pistil compatibility/incompatibility.
● Compatible pollen absorbs water + nutrients from the stigma → germinates → forms a pollen tube.
Pollen-tube growth through the style is controlled by specific chemicals.
● The stigma provides conditions required for pollen germination that are not present in the pollen itself.
● Pollen tube grows through the ovule, reaches the embryo sac, enters one synergid and ruptures to release its contents.
● Even compatible pollen grains show competition during interaction.
Importance
Pollen–pistil interaction → successful sexual reproduction + seed formation
In-vitro Pollen Germination
Synthetic medium:
Sucrose → induces pollen germination + pollen-tube growth.
Boric acid → facilitates & accelerates pollen germination.
Comments
Post a Comment