Module 11 – Fruits, Seeds, Pollination & Plant Reproduction

Fruit: Definition & Origin

Mature, ripened ovary enclosing seeds
Functions
- Protect seeds
- Aid in seed dispersal
Classification criteria
- By origin
- Simple – derived from one ovary of one flower
- Aggregate – from one flower with many free pistils/carpels
- Multiple – from fused ovaries of several flowers on one axis (e.g., pineapple)
- By pericarp (fruit wall) texture at maturity
- Fleshy
- Dry
  - Dehiscent – split open to release seeds
  - Indehiscent – remain closed

Simple Fleshy Fruits

Berry
- Entire pericarp soft
- Ex.: tomato
Pome
- Fleshy part mainly from enlarged receptacle (fused perianth base)
- Ex.: apple
Drupe (listed under dry in transcript but botanically fleshy)
- Inner pericarp (endocarp) hard “stone” protecting seed
- Ex.: peach, olive, mango
Pepo
- Thick, leathery rind; inferior ovary
- Ex.: cucumber, squash
Hesperidium
- Leathery, oil-gland-dotted rind; juice-sac carpels
- Ex.: orange, lemon

Simple Dry Fruits – Dehiscent

Follicle
- From single carpel; splits along one suture
- Ex.: milkweed
Legume/Pod
- From single carpel; splits along two sutures
- Ex.: beans, peas, peanuts (geocarpic)
Capsule – most common; from fused $\ge 2$ carpels; several split types
- Loculicidal – splits lengthwise through locule middles
- Septicidal – splits along septa
- Circumscissile – lid-like top (pyxidium)
- Poricidal – pores (poppy)
- Ex.: cotton, iris, orchid, horse-chestnut
Silique
- From two carpels; long and slender; seeds on central membrane
- Ex.: cabbage, mustard, radish
Silicle
- Like silique but < $3\times$ longer than wide

Simple Dry Fruits – Indehiscent

Achene – single seed loosely attached at one point (sunflower “seed”)
Cypsela – achene of Asteraceae with pappus
Caryopsis/Grain – pericarp completely fused to seed coat (wheat, corn)
Nut – large, stony pericarp, one seed (acorn, chestnut, hazelnut)
Samara – winged achene (maple, ash)
Schizocarp – splits into single-seeded segments (carrot family)
Utricle – bladder-like inflated pericarp larger than seed

Aggregate & Multiple Fruits

Aggregate
- One flower, many free carpels; each unit = small fruitlet
- Ex.: strawberry (achenes on receptacle), raspberry, blackberry
Multiple
- Several flowers on common inflorescence axis fuse
- Ex.: pineapple (central stem runs through core), mulberry

Fruit Ripening

Biochemical & physiological shifts
- Softening: cell-wall-digesting enzymes
- Color: chloroplast $\to$ chromoplast (green $\to$ red/yellow/orange)
- Flavor/aroma: organic acids & starch $\to$ sugars
Regulated by ethylene (gaseous plant hormone)
- “One bad fruit spoils the bunch” via ethylene diffusion
Climacteric vs. Non-climacteric
- Climacteric: surge in respiration + high ethylene; can ripen post-harvest (banana, tomato)
- Non-climacteric: ripen only on plant (citrus, grape)

Seed Dispersal

Diaspore = dispersal unit (seed ± fruit)
Atelochory = no special dispersal adaptation
Advantages
- Colonize new habitats
- Reduce parent-offspring competition
- Escape seed predators/pathogens concentrated near parent
Four principal mechanisms
1. Self/Autochory – forceful ejection (explosive fruits, squirting cucumber); geocarpy in peanut (ovary grows into soil)
2. Wind/Anemochory – wings, plumes, small mass
3. Water/Hydrochory – buoyant tissues, air spaces, corky pericarp (coconut)
4. Animal/Zoochory
- External (epizoochory): hooks, burrs
- Internal (endozoochory): fleshy fruits eaten, seeds pass gut

Seed Structure

General

Embryo + food supply + protective seed coat (testa)

Dicot (Eudicot) Example – Bean

Two thick cotyledons storing reserves
Embryonic axis
- Radicle – embryonic root
- Hypocotyl – stem below cotyledon node
- Epicotyl – stem above cotyledon node

Monocot Example – Corn

One cotyledon (scutellum)
Protective sheaths
- Coleoptile – covers plumule/epicotyl
- Coleorhiza – covers radicle

Germination Patterns

Monocot: Hypogeal/Cryptocotylar
- Cotyledon remains underground; coleoptile emerges, then shoot
Dicot: Epigeal/Phanerocotylar
- Hypocotyl hook lifts cotyledons above soil; they photosynthesize then wither

Seedless (Parthenocarpic) Fruit Formation

Causes
- Pollination failure
- Chromosomal imbalance (triploids, e.g., banana)
- Exogenous auxin application to unpollinated flowers (tomato, cucumber)

Pollination Basics

Transfer of pollen from anther $\to$ stigma; precedes fertilization
Agents
- Abiotic $\approx$ $20\%$
- Wind $\approx$ $98\%$ of abiotic
- Water $\approx$ $2\%$ of abiotic
- Biotic $\approx$ $80\%$

Coevolution

Reciprocal adaptive changes between flowering plants & pollinators
Examples
- Honeycreeper long curved beak $\leftrightarrow$ tubular corolla
- Specialized scents, colors, nectar guides, night opening, etc.

Wind Pollination (Anemophily)

Small, non-showy flowers; copious light pollen; large feathery stigmas; wasteful but feasible in open habitats

Water Pollination (Hydrophily)

Submerged or surface; pollen filamentous or rafted

Animal Pollination (Zoophily)

Insects, birds, mammals, occasionally reptiles/amphibians

Insect Syndromes

Bees
- Colors: yellow, blue, UV; sweet scent; nectar guides; landing platform
Butterflies
- Colors: red, orange; scented; long tubes for proboscis
Moths (nocturnal)
- Pale/white, strong sweet odor at night
Flies & Beetles
- Flies: carrion/dung odor; deceptive oviposition sites
- Beetles: dull color, strong scents; often eat pollen/parts

Vertebrate Syndromes

Birds (hummingbirds, honeycreepers)
- Bright reds/yellows; little or no odor; copious fluid nectar; sticky pollen
Bats (nocturnal)
- White/dusky flowers, musky/fruity odor; large sturdy blossoms
Mice (ground-level, inconspicuous)

Pollinator Rewards

Nectar – sugar concentration matches energy needs (higher for birds)
Pollen – protein source; some species produce edible sterile pollen plus fertile pollen

Flower Anatomy (Ex.: Hibiscus/Gumamela)

Perfect (bisexual) & complete (all 4 whorls)
Pistil (Gynoecium)
- Stigma | Style | Ovary
Stamen (Androecium)
- Anther (microsporangia) | Filament
Accessory
- Petals (corolla)
- Sepals (calyx; usually green)
- Receptacle & pedicel

Formation of Gametes

Male Side

In anther microsporangium, microsporocytes $2n$ undergo meiosis $\Rightarrow$ $4$ microspores $n$ each
Each microspore $\xrightarrow{mitosis}$ pollen grain (male gametophyte) containing two sperm cells

Female Side

In ovule megasporangium, one megasporocyte $2n$ $\xrightarrow{meiosis}$ $4$ megaspores $n$
Only one survives; undergoes $3$ mitotic divisions $\Rightarrow$ 8-nucleate embryo sac (female gametophyte)
- Components: egg cell, $2$ synergids, $3$ antipodals, $2$ polar nuclei (central cell)

Double Fertilization (Unique to Angiosperms)

Pollen tube delivers two sperm to embryo sac
1. Sperm $+$ egg $\Rightarrow$ zygote $2n$
2. Sperm $+$ two polar nuclei $\Rightarrow$ primary endosperm nucleus $3n$
Endosperm becomes nutritive tissue for embryo
Outcome summary
- Zygote $\rightarrow$ embryo
- Endosperm $\rightarrow$ food reserve
- Ovule $\rightarrow$ seed; ovary $\rightarrow$ fruit; ovary wall $\rightarrow$ pericarp

Alternation of Generations Recap

Sporophyte $2n$ (dominant in vascular plants)
- Produces haploid spores by meiosis
Gametophyte $n$
- Produces gametes by mitosis
In flowering plants, gametophytes highly reduced (pollen grain & embryo sac)

Life Cycle of Angiosperms (Sequential Overview)

Zygote $2n$ ➔ embryo in seed
Seed germination ➔ mature sporophyte (flowering plant)
Flower forms microsporangia (anther) & megasporangia (ovule)
Microsporogenesis & megasporogenesis produce microspores & megaspores (both $n$ )
Gametophyte development ➔ pollen & embryo sac
Pollination delivers sperm; double fertilization
Ovule becomes seed; ovary becomes fruit; cycle repeats

Numerical / Statistical References (LaTeX)

Biotic pollination: $80\%$ of flowering plants
Abiotic pollination: $20\%$ (wind $=98\%$ , water $=2\%$ of abiotic)
Microsporocyte meiosis: $4$ microspores produced
Megasporocyte meiosis: $4$ megaspores (only $1$ functional)
Ploidy symbols: haploid $n$ , diploid $2n$ , triploid $3n$

Ethical & Practical Implications

Crop breeding for parthenocarpy (seedless fruits) improves consumer appeal
Understanding dispersal aids habitat restoration & invasive-species management
Knowledge of climacteric behavior informs post-harvest storage to reduce food waste
Pollinator conservation essential for $\approx$ $80\%$ of plant biodiversity & global food security