Sexual Reproduction in Flowering Plants — NEET notes
Class 12 Biology · NCERT chapter 2 · 8 topics · free to read
Flower Structure
A flower is the reproductive unit of angiosperms. It has four whorls — calyx, corolla, androecium (male), and gynoecium (female). Flowers can be complete or incomplete, bisexual or unisexual, and show various symmetry patterns. Understanding flower anatomy is the foundation for all reproduction concepts.
Flower = Calyx + Corolla + Androecium + Gynoecium
Four Whorls
A typical flower has four whorls arranged on the thalamus: calyx, corolla, androecium, and gynoecium — from outermost to innermost.
A flower develops on a stalk called the pedicel, which swells at the tip into the thalamus (receptacle). Four whorls are arranged concentrically on the thalamus: 1. Calyx (sepals) — outermost, usually green, protects the bud 2. Corolla (petals) — colorful, attracts pollinators 3. Androecium (stamens) — male reproductive whorl 4. Gynoecium (carpels/pistil) — female reproductive whorl, innermost
Thalamus = swollen tip of pedicel where all whorls are attached
Calyx + Corolla = perianth (when sepals and petals look similar, each member is called a tepal — e.g., lily)
When calyx and corolla are not distinguishable → tepals (e.g., *Lilium*)
Stamen (Androecium)
Each stamen consists of a filament and an anther. The anther produces pollen grains (male gametophytes).
A stamen has two parts: a long filament and a bilobed anther at the top. Each anther lobe has two pollen sacs (microsporangia) — so one anther has 4 microsporangia total. This is where pollen grains are produced.
Stamen = filament + anther
Anther is bilobed — each lobe has 2 pollen sacs → 4 microsporangia per anther (dithecous)
A sterile stamen is called a staminode (e.g., in *Cassia*)
Stamens can be epipetalous (attached to petals — e.g., brinjal) or epiphyllous (attached to perianth — e.g., lily)
The gynoecium consists of one or more carpels. Each carpel has a stigma, style, and ovary containing ovules.
The pistil (carpel) has three parts from top to bottom: - Stigma — landing platform for pollen, often sticky or feathery - Style — elongated tube connecting stigma to ovary - Ovary — basal swollen part containing ovules attached to the placenta
Apocarpous = multiple free carpels (e.g., *Michelia*, lotus) — each carpel has its own stigma, style, and ovary
Syncarpous = carpels fused together (e.g., mustard, tomato) — more common in advanced flowers
Placentation = arrangement of ovules on the placenta inside the ovary
Flowers are classified by sex (bisexual/unisexual), symmetry (actinomorphic/zygomorphic), and ovary position (superior/inferior).
Flowers are classified by multiple criteria: By sex: Bisexual (both androecium + gynoecium — e.g., *Hibiscus*, mustard) vs Unisexual (only one — male or female). Plants with both unisexual flowers on same plant = monoecious (e.g., maize, coconut). On different plants = dioecious (e.g., papaya, date palm).
Actinomorphic = radial symmetry, can be divided into two equal halves by any vertical plane (e.g., mustard, *Datura*, chilli)
Zygomorphic = bilateral symmetry, only one plane divides it into two mirror halves (e.g., pea, bean, *Cassia*)
Asymmetric (irregular) = no plane of symmetry (e.g., *Canna*)
Ovary is above other whorls on thalamus
Ovary is below other whorls (embedded in thalamus)
Other parts arise below the ovary
Other parts appear to arise above the ovary
Example: Mustard, China rose, brinjal
Example: Guava, cucumber, sunflower
Ovary is superior
Ovary is inferior
Microsporogenesis & Pollen Grain
Microsporogenesis is the formation of microspores (pollen) inside the microsporangium of the anther. Each microspore mother cell (2n) undergoes meiosis to form a tetrad of microspores (n), which develop into pollen grains — the male gametophytes with a distinctive two-layered wall (exine + intine).
The microsporangium (pollen sac) has a four-layered wall protecting the sporogenous tissue that will produce pollen.
Each anther has 4 microsporangia (2 per lobe). A transverse section of a young anther shows each microsporangium surrounded by four wall layers (from outside to inside): 1. Epidermis — outermost protective layer 2. Endothecium — helps in anther dehiscence (splitting to release pollen) 3. Middle layers — 2-4 layers, mostly compressed and absorbed during development 4. Tapetum — innermost, nourishes developing pollen; the most important layer
Tapetum provides Ubisch bodies (orbicules) for sporopollenin deposition
Tapetum has polyploid cells (due to endomitosis) — increases metabolic output
Center of microsporangium = sporogenous tissue (diploid cells → future microspore mother cells)
At maturity, wall between two pollen sacs of each lobe degenerates → bilocular anther appears unilocular
Microsporogenesis
The process where each microspore mother cell (MMC) undergoes meiosis to form four haploid microspores arranged in a tetrad.
Microsporogenesis is the formation of microspores from the microspore mother cell (MMC). Each diploid (2n) MMC undergoes meiosis to produce a tetrad of 4 haploid (n) microspores. These microspores are initially held together by a callose wall.
Sporogenous cells (2n) → directly function as Microspore Mother Cells (MMCs)
Each MMC undergoes meiosis → 4 haploid microspores (tetrad)
Tetrads are initially enclosed in callose ( -1,3-glucan)
Enzyme callase (from tapetum) dissolves callose → free microspores released
Each microspore now develops into a pollen grain (male gametophyte)
A mature pollen grain has a hard outer exine (with sporopollenin) and inner intine (pectin + cellulose), containing a vegetative cell and a generative cell.
The pollen grain is the male gametophyte (partially developed). It has a two-layered wall: - Exine (outer) — made of sporopollenin, the most resistant organic material known. Has openings called germ pores where sporopollenin is absent. - Intine (inner) — made of pectin and cellulose, thin and uniform.
Mature pollen grain is either 2-celled (vegetative cell + generative cell — majority of angiosperms) or 3-celled (vegetative cell + 2 male gametes — about 60% of species shed at 2-celled stage)
Vegetative cell — large, has food reserves, irregular nucleus; forms the pollen tube
Generative cell — small, spindle-shaped, floats within vegetative cell cytoplasm; divides to form 2 male gametes
Germ pores — thin areas in exine where pollen tube emerges; number varies by species (1 in grasses, 3 in *Hibiscus*)
Sporopollenin = most resistant biological material — no enzyme can break it down
Examples of 3-celled pollen at shedding: grasses, legumes, *Brassica*
Quick recall
What is the fate of the vegetative cell and generative cell after pollination?
The vegetative cell forms the pollen tube (delivers male gametes to the embryo sac). The generative cell divides by mitosis to form 2 male gametes (either inside the pollen grain before shedding, or inside the pollen tube after germination).
Megasporogenesis & Embryo Sac
Megasporogenesis is the formation of megaspores inside the ovule. One megaspore mother cell (2n) undergoes meiosis to form 4 megaspores, of which typically only one survives and develops into the 7-celled, 8-nucleate embryo sac — the female gametophyte. This is the Polygonum type, seen in 80%+ of angiosperms.
The ovule (megasporangium) is attached to the placenta by a stalk and contains the nucellus, integuments, micropyle, and embryo sac.
The ovule is the megasporangium of angiosperms. It's attached to the placenta (inside the ovary wall) by a stalk called the funicle. The point where the funicle joins the ovule body is the hilum.
Funicle — stalk connecting ovule to placenta
Hilum — junction of funicle and ovule body
Integuments — 1 or 2 protective layers enclosing the ovule (unitegmic or bitegmic)
Micropyle — small opening in integuments at one end; pollen tube enters here
Chalaza — opposite end to micropyle, where integuments and nucellus merge
Nucellus — mass of parenchyma cells inside integuments; the megaspore mother cell develops within it
The embryo sac (female gametophyte) develops inside the nucellus
Megasporogenesis
A single megaspore mother cell in the nucellus undergoes meiosis to form 4 megaspores. In most angiosperms, 3 degenerate and only the chalazal megaspore survives.
Megasporogenesis is the formation of megaspores. In the nucellus, one cell differentiates as the megaspore mother cell (MMC) — it is diploid (2n). The MMC undergoes meiosis to form a linear tetrad of 4 haploid megaspores.
Only ONE MMC per ovule (unlike many MMCs per microsporangium)
MMC (2n) → meiosis → 4 megaspores (n) arranged in a linear tetrad
Usually 3 micropylar megaspores degenerate
Only the chalazal megaspore (the one farthest from micropyle) remains functional
This functional megaspore develops into the embryo sac (female gametophyte)
This pattern is called monosporic development (Polygonum type — ~70% of angiosperms)
Occurs in microsporangium (anther)
Occurs in megasporangium (ovule)
Many MMCs per microsporangium
Only 1 MMC per ovule
All 4 microspores are functional
Only 1 of 4 megaspores is functional
Products = pollen grains (millions)
Product = 1 embryo sac per ovule
Both involve meiosis of mother cell
Both involve meiosis of mother cell
Embryo Sac
The functional megaspore undergoes 3 mitotic divisions to form the 7-celled, 8-nucleate embryo sac (Polygonum type) with egg apparatus, central cell, and antipodals.
The surviving chalazal megaspore develops into the embryo sac through 3 successive mitotic divisions producing 8 nuclei. These 8 nuclei organize into 7 cells: - Micropylar end: 3 cells = egg apparatus (1 egg cell + 2 synergids) - Chalazal end: 3 cells = antipodals (3 antipodal cells) - Center: 1 large central cell with 2 polar nuclei
Egg cell — the female gamete, will fuse with one male gamete → zygote (2n)
Synergids — have filiform apparatus (finger-like projections) that guide the pollen tube; one synergid degenerates when pollen tube enters
Central cell — largest cell, has 2 polar nuclei that fuse with second male gamete → PEN (3n) → endosperm
Antipodals — 3 cells at chalazal end; may be involved in nutrition; degenerate before or after fertilization
This is the Polygonum type embryo sac — named after *Polygonum* where it was first described
Total: 7 cells, 8 nuclei (the central cell contributes 2 nuclei)
Quick recall
Why is the embryo sac called '7-celled, 8-nucleate'?
There are 7 cells total: 1 egg cell + 2 synergids + 3 antipodals + 1 central cell = 7 cells. But the central cell contains 2 polar nuclei, so total nuclei = 6 + 2 = 8 nuclei in 7 cells.
Pollination
Pollination is the transfer of pollen grains from the anther to the stigma. It can be self-pollination (autogamy, geitonogamy) or cross-pollination (xenogamy). Plants use various agents — wind, water, insects, birds — and have evolved clever devices to promote cross-pollination and prevent self-pollination.
Autogamy: same flower | Geitonogamy: different flower, same plant | Xenogamy: different plant
Three Types
Pollination is classified by the source of pollen: same flower (autogamy), different flower on same plant (geitonogamy), or different plant entirely (xenogamy).
Depending on the source of pollen, pollination is classified into: 1. Autogamy — pollen from the anther lands on the stigma of the same flower. Requires anther and stigma to be close together and mature simultaneously. 2. Geitonogamy — pollen from one flower lands on the stigma of another flower on the same plant. Functionally cross-pollination, but genetically self-pollination (same genotype). 3. Xenogamy (allogamy) — pollen from a flower on a different plant of the same species. True cross-pollination — brings in genetic variation.
Xenogamy — the only type that provides new genetic combinations and evolutionary advantage
Pollination Agents
Plants have co-evolved with their pollination agents, developing specific adaptations for anemophily (wind), hydrophily (water), entomophily (insects), and ornithophily (birds).
Pollination agents (vectors) include abiotic (wind, water) and biotic (insects, birds, bats) agents. Each agent has driven specific flower adaptations.
Ornithophily (birds): bright red/orange flowers, tubular shape, copious nectar; e.g., Bombax (red silk cotton), Butea (flame of the forest), Bignonia, pollinated by sunbirds, hummingbirds
Chiropterophily (bats): large, dull-colored, strong-scented flowers that open at night; e.g., Kigelia (sausage tree), Bauhinia
Outbreeding Devices
Plants have evolved multiple mechanisms to avoid self-pollination and promote cross-pollination: dichogamy, self-incompatibility, heterostyly, and more.
Continued self-pollination leads to inbreeding depression — reduced fitness over generations. To prevent this, plants have evolved outbreeding devices:
Unisexuality (dicliny) — separate male and female flowers; monoecious (maize) or dioecious (papaya) → cross-pollination guaranteed in dioecious plants
Dichogamy — anther and stigma mature at different times. Protandry = anther matures first (sunflower, Salvia). Protogyny = stigma matures first (Mirabilis, Plantago)
Herkogamy — physical barrier between anther and stigma in the same flower (e.g., *Calotropis* — pollinia must be removed by insects)
Self-incompatibility (SI) — genetic mechanism; pollen from the same plant cannot germinate on its own stigma due to matching S-alleles
Heterostyly — different style and filament lengths in different plants of the same species (e.g., *Primula* — pin-eyed vs thrum-eyed)
Production of unisexual flowers is the most effective device in dioecious plants
Pollen-Pistil Interaction
After pollination, the pistil recognizes compatible pollen, allows germination, and facilitates pollen tube growth through the style to the ovule.
Pollination doesn't guarantee fertilization. The pistil has the ability to recognize and accept or reject pollen. If pollen is compatible, the pistil promotes pollen germination and tube growth. If incompatible (wrong species or self-incompatible), it blocks the process.
Pollen lands on stigma → absorbs moisture and nutrients from stigma surface
Compatible pollen germinates → pollen tube emerges through the germ pore
Pollen tube grows through the style (using nutrients from stylar tissues)
The vegetative cell of pollen forms the tube; the generative cell divides into 2 male gametes inside the tube
Pollen tube enters the ovule usually through the micropyle (porogamy — most common)
Other entry routes: chalazogamy (through chalaza — *Casuarina*) or mesogamy (through integuments — *Cucurbita*)
Pollen tube enters through micropyle
Pollen tube enters through chalaza
Most common route in angiosperms
Rare; found in *Casuarina*, *Betula*
Direct and shortest path
Longer, roundabout path
Example: most flowering plants
Example: *Casuarina* (she-oak)
Double Fertilization
Double fertilization is unique to angiosperms. The pollen tube delivers two male gametes to the embryo sac. One fuses with the egg cell (syngamy → zygote, 2n) and the other fuses with the two polar nuclei (triple fusion → primary endosperm nucleus, 3n). Both events together = double fertilization, discovered by Nawaschin.
Male gamete_1 + Egg (n) arrow Zygote (2n) | Male gamete_2 + 2 Polar nuclei (n+n) arrow PEN (3n)
Pollen Tube Growth
After germination, the pollen tube grows through the style and enters the embryo sac through the micropyle, guided by chemical signals from the synergids.
After landing on a compatible stigma, pollen germinates. The vegetative cell forms the pollen tube, which grows through the style tissue. In 2-celled pollen, the generative cell divides by mitosis inside the growing tube to form 2 male gametes.
Pollen tube growth is guided by chemotropism — chemicals from synergids attract the tube
The filiform apparatus of synergids plays a key role in guiding pollen tube entry
Pollen tube enters the ovule through the micropyle (porogamy)
Inside the embryo sac, the tube enters through one of the synergids (which degenerates)
Tube tip bursts, releasing 2 male gametes into the embryo sac
The vegetative cell (tube cell nucleus) degenerates after tube growth
Syngamy & Triple Fusion
Syngamy is the fusion of one male gamete with the egg cell to form the zygote. Triple fusion is the fusion of the second male gamete with the two polar nuclei to form the primary endosperm nucleus.
The two male gametes released into the embryo sac carry out two simultaneous fusions: 1. Syngamy (fertilization): Male gamete _1 + Egg cell (n) → Zygote (2n) 2. Triple fusion: Male gamete _2 + 2 Polar nuclei (n + n) → Primary Endosperm Nucleus (PEN, 3n) Since two fusions occur in the same embryo sac, the event is called double fertilization.
Syngamy = male gamete + egg → zygote (2n) — this is true fertilization
Triple fusion = male gamete + 2 polar nuclei → PEN (3n) — called 'triple' because 3 nuclei fuse
Double fertilization = syngamy + triple fusion (two fusion events in one embryo sac)
PEN (3n) → develops into endosperm (triploid nutritive tissue for embryo)
Discovered by S.G. Nawaschin (1898) — Russian botanist
After fertilization: zygote divides only AFTER some endosperm has formed (endosperm development precedes embryo development)
Quick recall
What is the ploidy of the PEN and why?
PEN is triploid (3n): one male gamete (n) + two polar nuclei (n + n) = 3n. This is why endosperm tissue is triploid in most angiosperms. The coconut water you drink is liquid triploid endosperm!
Significance
Double fertilization ensures endosperm formation only when fertilization occurs, preventing waste of nutrients. It also produces triploid endosperm with hybrid vigor.
Double fertilization is not just a biological curiosity — it has profound evolutionary significance for angiosperms.
No fertilization → no endosperm — resources are not wasted on nutritive tissue unless an embryo is actually formed
Triploid endosperm has more vigor than diploid tissue — grows faster, stores more nutrients
Endosperm nourishes the developing embryo — acts as a nutritive tissue
Ensures the evolutionary success of angiosperms — efficient resource use vs. gymnosperms (where endosperm forms before fertilization and is wasted if fertilization fails)
The zygote waits for endosperm to develop first — this ensures nutrition is ready before embryo growth begins
Double fertilization occurs
Only syngamy (single fertilization)
Endosperm is triploid (3n) — product of triple fusion
Endosperm is haploid (n) — pre-existing female gametophyte tissue
Endosperm forms after fertilization
Endosperm forms before fertilization
No resource waste if unfertilized
Resources wasted if pollination fails
Post-fertilization endosperm
Pre-fertilization endosperm
Seed Development
After double fertilization, the ovule develops into a seed. The PEN (3n) develops into endosperm (nuclear, cellular, or helobial type). The zygote develops into the embryo (with radicle, plumule, and cotyledons). Dicot embryos have 2 cotyledons; monocot embryos have 1 cotyledon (scutellum) plus coleoptile and coleorhiza.
The triploid PEN divides to form endosperm tissue. Three developmental patterns exist: nuclear, cellular, and helobial. Endosperm is the primary nutrition source for the developing embryo.
The Primary Endosperm Nucleus (PEN, 3n) undergoes repeated divisions to form the endosperm — a nutritive tissue for the embryo. Importantly, endosperm development starts BEFORE the zygote divides — the embryo needs food ready from day one.
Nuclear type — PEN divides repeatedly WITHOUT cell wall formation → free nuclei in cytoplasm → walls form later. Most common type. Example: coconut (free-nuclear liquid endosperm = coconut water; cellular part = coconut meat)
Cellular type — every division is followed by immediate cell wall formation. Example: *Petunia*, *Datura*, *Adoxa*
Helobial type — intermediate; first division forms 2 unequal cells, then the larger cell shows nuclear type and smaller shows cellular type. Example: monocots (order Helobiales)
Endosperm is triploid (3n) in angiosperms
In endospermic (albuminous) seeds, endosperm persists in the mature seed (e.g., maize, wheat, castor, coconut)
In non-endospermic (exalbuminous) seeds, endosperm is consumed during embryo development (e.g., pea, bean, groundnut)
Endosperm persists in mature seed
Endosperm consumed during development
Food stored in endosperm
Food stored in cotyledons
Examples: maize, wheat, castor, coconut
Examples: pea, bean, gram, groundnut
Cotyledons thin (don't store food)
Cotyledons thick and fleshy (store food)
Dicot Embryo
The zygote undergoes successive divisions to form the embryo. In dicots, the mature embryo has an embryonal axis with a radicle and plumule, plus two large cotyledons.
The zygote divides only after some endosperm has formed. It first divides transversely into a basal cell (towards micropyle) and a terminal cell (towards inside). The basal cell forms the suspensor, and the terminal cell forms the embryo proper.
Suspensor — pushes embryo into endosperm; its first cell (closest to micropyle) is the haustorium — absorbs nutrients
Mature dicot embryo has: embryonal axis (with radicle at one end, plumule at other) + 2 cotyledons
Radicle = embryonic root (grows first during germination)
Plumule = embryonic shoot (grows upward)
Cotyledons = seed leaves; in non-endospermic seeds, they store food; in endospermic seeds, they are thin
The heart shape comes from the two cotyledon primordia bulging outward
Monocot Embryo
Monocot embryos have a single cotyledon (scutellum), plus special protective sheaths: coleoptile covering the plumule and coleorhiza covering the radicle.
The monocot embryo (e.g., grass/cereal) has unique structures not found in dicots: - Scutellum — the single cotyledon, shield-shaped, pressed against the endosperm to absorb nutrients - Coleoptile — sheath covering the plumule (shoot tip); protects it during soil emergence - Coleorhiza — sheath covering the radicle (root tip); undifferentiated sheath
Scutellum = single cotyledon, lateral in position, absorbs food from endosperm via epithelial layer
Coleoptile = hollow, protective sheath around the plumule; photosensitive
Coleorhiza = protective sheath around the radicle; radicle pierces through it during germination
Epiblast — a small outgrowth opposite to scutellum; present in some grasses; debated homology
Monocot embryonal axis has the plumule and radicle at opposite ends, like dicots
Monocot embryo is asymmetric (scutellum on one side, embryonal axis on the other)
Two cotyledons
One cotyledon (scutellum)
No coleoptile or coleorhiza
Has coleoptile (plumule sheath) and coleorhiza (radicle sheath)
Cotyledons may store food (pea) or be thin (castor)
Scutellum absorbs food from endosperm
Epicotyl and hypocotyl distinct
Covered by specialized sheaths
Example: gram, pea, bean
Example: maize, wheat, rice
Seed Structure
A mature seed has a seed coat (from integuments), embryo, and sometimes endosperm. The seed coat has two layers — testa (outer) and tegmen (inner).
After fertilization, the ovule matures into the seed. The key transformations: - Integuments → Seed coat (outer = testa, inner = tegmen) - Zygote → Embryo - PEN → Endosperm (may or may not persist) - Micropyle → persists as a small pore for water absorption during germination - Funicle → scar on seed = hilum
Testa = outer seed coat (hard, protective) from outer integument
Tegmen = inner seed coat (thin) from inner integument
Hilum = scar where seed was attached to fruit via funicle
Micropyle = small pore in seed coat; allows water entry during germination
Perisperm = remnant of nucellus that persists in some seeds (e.g., *black pepper, beet, coffee*)
Seeds can be albuminous (with endosperm: wheat, maize, castor) or exalbuminous (without: pea, bean)
Fruit & Seed Dispersal
After fertilization, the ovary develops into a fruit (true fruit) and ovules into seeds. Parthenocarpy produces seedless fruits without fertilization. Seeds have dormancy mechanisms, and dispersal occurs by wind, water, animals, or explosive mechanisms — ensuring species survival and colonization of new habitats.
A fruit develops from the ovary after fertilization. If other parts contribute, it's a false fruit. Parthenocarpy produces fruits without fertilization (seedless).
After fertilization, the ovary wall develops into the pericarp (fruit wall) and the ovules become seeds. The fruit encloses and protects seeds and aids in their dispersal.
True fruit — develops from the ovary alone (e.g., mango, tomato, pea)
False fruit (pseudocarp) — other floral parts (thalamus, calyx, etc.) contribute to fruit. Example: apple (fleshy part = thalamus), strawberry (fleshy part = thalamus, true fruits = tiny achenes on surface)
Parthenocarpy — fruit develops WITHOUT fertilization → seedless fruit. Example: banana (naturally), seedless grapes, seedless watermelon. Can be induced by auxin or gibberellin application
Pericarp layers: Epicarp (outer skin), Mesocarp (middle fleshy part), Endocarp (inner — hard in drupe like mango)
In mango: epicarp = skin, mesocarp = edible pulp, endocarp = hard stone enclosing the seed
Develops from ovary only
Other floral parts also contribute
Ovary wall → pericarp
Thalamus/calyx also forms fleshy part
Example: mango, tomato, grape
Example: apple, strawberry, pear
More common in nature
Less common
Seed Dormancy
Seeds don't germinate immediately — they enter dormancy, a period of suspended development. This ensures germination occurs under favorable conditions.
Seed dormancy is a condition where a viable seed fails to germinate even when given favorable conditions (water, oxygen, temperature). It's an adaptive strategy that prevents premature germination and allows seeds to survive unfavorable periods.
Dormancy is caused by: hard seed coat (impermeability to water/gases), immature embryo, presence of inhibitors (like ABA — abscisic acid), or light/temperature requirements
Advantages: Allows seed dispersal over time, prevents germination in unfavorable season, storage of food for later use
Seed viability = duration for which seeds remain capable of germination
Most seeds viable for a few years to decades
Oldest viable seed: *Lupinus arcticus* — germinated after being buried in Arctic permafrost for ~10,000 years
Seeds of some species (e.g., orchids) have NO dormancy and must germinate quickly
Seed Dispersal
Seeds and fruits are dispersed by wind, water, animals, and explosive mechanisms. Each agent has driven specific adaptations in seed/fruit morphology.
Seed dispersal prevents overcrowding and competition among seedlings, and helps plants colonize new habitats. The four main agents of dispersal:
Anemochory (wind): Seeds/fruits have wings (*Acer* — maple), pappus (*Asteraceae* — dandelion), hair tufts (*Calotropis*, cotton), or are very light (orchid seeds — dust-like, 0.2 mg). Censer mechanism in poppy
Hydrochory (water): Seeds/fruits have air spaces or fibrous outer coat for buoyancy. Example: coconut (fibrous mesocarp traps air — can float for months across oceans), *Nelumbo* (lotus)
Zoochory (animals): Fleshy, colorful fruits eaten by animals → seeds pass through digestive tract unharmed (endozoochory). Hooked/spiny fruits attach to animal fur (epizoochory — e.g., *Xanthium*, *Urena*). Myrmecochory = dispersal by ants
Autochory (self-dispersal / explosive mechanism): Fruit bursts open, flinging seeds away. Example: balsam (*Impatiens*), *Ecballium* (squirting cucumber — seeds ejected up to 6 meters!), castor, pea pods
Apomixis & Polyembryony
Apomixis is seed production WITHOUT fertilization — the embryo develops from a diploid cell instead of a zygote. Polyembryony is the occurrence of MORE than one embryo in a seed. Both phenomena have immense significance in agriculture for producing genetically uniform hybrid seed and clonal propagation.
Apomixis: seed formed WITHOUT meiosis + fertilization | Polyembryony: multiple embryos in one seed
Apomixis
In apomixis, seeds are produced without the normal sexual process (no meiosis + fertilization). The embryo develops from unreduced (diploid) cells, producing offspring genetically identical to the parent.
Apomixis (Greek: *apo* = away from, *mixis* = mixing) is the formation of seeds without fertilization. The embryo forms from diploid cells, so the offspring is a genetic clone of the mother plant. It mimics sexual reproduction (produces seeds) but is actually asexual.
Diplospory — the megaspore mother cell (MMC) itself develops into the embryo sac without meiosis → diploid embryo sac → embryo from unreduced egg cell
Apospory — a nucellar cell (somatic cell, 2n) develops into an embryo sac instead of the megaspore → diploid embryo sac → diploid embryo
Adventive embryony — embryos develop directly from nucellus or integument cells (no embryo sac involved at all). Example: Citrus, mango
All apomictic offspring are genetically identical to the mother plant
Apomixis occurs in about 400 species of angiosperms, including many grasses (*Pennisetum*, *Cenchrus*), *Astereaceae*, *Rutaceae*
Some species show facultative apomixis — both sexual and apomictic seeds produced
Polyembryony
Polyembryony is the presence of more than one embryo in a single seed. It can arise from splitting of the zygote, multiple embryo sacs, or adventive embryony from nucellar/integumentary cells.
Polyembryony = occurrence of more than one embryo in a single seed. This was first observed by Leeuwenhoek (1719) in *Citrus* seeds.
Cleavage polyembryony — the zygote splits into multiple embryos during early development (like identical twins in humans). Example: *Orchis*
Adventive polyembryony — extra embryos develop from nucellar cells or integumentary cells (somatic, 2n) alongside the sexual embryo. Most common type. Example: Citrus (orange, lemon), mango
Multiple embryo sacs — more than one embryo sac in the same ovule, each producing an embryo
In Citrus: the sexual embryo (from zygote) + multiple nucellar embryos coexist in one seed → when you plant a Citrus seed, most seedlings are nucellar (clonal)
Polyembryony is useful in plant breeding — nucellar seedlings are virus-free and genetically uniform
Quick recall
What is the difference between apomixis and polyembryony?
Apomixis = producing seeds WITHOUT fertilization (the entire reproductive process bypasses sex). Polyembryony = having MULTIPLE embryos in one seed (fertilization may or may not have occurred for some of them). They can overlap — adventive embryony in Citrus is both apomictic (nucellar embryos skip fertilization) and polyembryonic (multiple embryos per seed).
Agricultural Significance
Apomixis and polyembryony have tremendous agricultural potential for maintaining hybrid vigor, producing virus-free plants, and reducing seed costs.
Understanding apomixis and polyembryony isn't just academic — these phenomena could revolutionize agriculture if harnessed effectively.
Hybrid vigor maintenance — if hybrid plants could produce apomictic seeds, farmers wouldn't need to buy new hybrid seeds every season (currently F_1 hybrid seeds segregate in F_2 )
Virus-free clones — nucellar embryos (from polyembryony) are free from viral infections present in the parent's vascular system. Used extensively in Citrus propagation