REPRODUCTION IN ANGIOSPERMS - FERTILIZATION

DOUBLE FERTILIZATION

• Pollen tube -- ovule
• Filliform apparatus guides pollen tube into one of the synergids.
• Both male gametes are released into cytoplasm of a synergid cell.

• One male gamete -- fuses with female gamete -- zygote (diploid) - syngamy
• Second male gamete -- fuses with two polar nuclei -- primary endosperm nucleus (PEN)  -- triple fusion
• Two types of fusion -- Double fertilization ( unique to angiosperms)
• Central cell become primary endosperm cell (triploid) -- later forms endosperm

POST FERTILIZATION EVENTS and STRUCTURES

• Zygote -- embryo
• Ovule -- seed
• Ovary -- fruit

ENDOSPERM

• Endosperm develop first then embryo development occurs.
• Primary endosperm cell --  divides repeatedly -- endosperm
• This nutrition helps to grow embryo
• Most common type of endosperm development-- Free nuclear endosperm (coconut water - liquid nuclear endosperm)
• Cell wall formation starts from periphery -- cellular endosperm ( white kernal part of coconut)

Two conditions-

1. Endosperm can be completely consumed by developing embryo

So, in mature seed, endosperm is absent.

Eg. Pea, beans, groundnut

2. Endosperm will not be consumed completely by developing embryo,

So, the endosperm persists in mature seeds.

Eg. Castor, coconut.

EMBRYO

• Develops near micropylar end (embryo sac was there, so was zygote)
• Zygote start divide after some part of endosperm is formed, so they can get uninterrupted nutrition.
• Early stages of embryo development is same in both monocots and dicot

Sequence

Zygote -- proembryo -- globular embryo -- heart shaped embryo

DICOT EMBRYO

• Embryonal axis + 2 cotyledons
• Part of embryonal axis above cotyledons -- plumule/ stem tip -- later form stem part
• Below cotyledons -- hypocotyl - radical/ root tip
• Root is covered with root cap

MONOCOT EMBRYO

• Embryonal axis + 1 cotyledons
• Cotyledon is called -- scutellum
• Radical / root cap is covered with -- coleorrhiza
• Part above cotyledon -- epicotyl
• Epicotyl - shoot apex + few leaf primordia
• Covered with coleoptile

SEED

• Fertilised ovule
• Ovule -- seed
• Funicle -- seed funicle
• Hilum  -- hilum of seed
• Micropyle -- micropyle of seed
• Integument -- seed coat
• Seed coat + cotyledons + embryo axis

DICOT SEED

• Non albuminous seed
• Cotyledons are swollen due to food storage ( since endosperm is absent)

MONOCOT SEED

• Albuminous seed 
• Have remaining endosperm

PERISPERM

Remaining part of nucellus in seed

Eg. Black pepper, beet etc

Seed -- mature -- water content - reduce -- dry ( only 10-15%) 

Metabolic activity -- very slow

Seed may become inactive -- stage -- Dormancy

Favourable condition -- seed -- germinate

FRUITS

• Develop from ovary-- true fruit
• Sometimes other parts also take part- false fruit
• Eg. Strawberry, apple - thalamus
• Ovary wall -- pericarp (fruit wall)
• Pericarp -- epicarp + mesocarp + endocarp
• Eg. Coconut - green epicarp, fibrous mesocarp, hard endocarp
• Mango -- outer coloured epicarp, pulp - mesocarp, very thin endocarp.

PARTHENOCARPY

• Fruits develop from ovary, after fertilization
• But in some plants, ovary develop into fruit without fertilization - parthenocarpic fruits
• No fertilization, no seeds
• It means, parthenocarpic fruits do not have seeds
• Eg. Banana
• Artificial parthenocarpy can be induced by plant harmones
• Eg. Auxins, if we give auxin in an unfertilized ovary, it will develop into a fruit.

ADVANTAGES OF SEEDS

• More dependable, as pollination and fertilization do not depend on water, unlike bryophytes and pteridophytes.
• Easy dispersal
• Can withstand adverse condition, like high temp, dry season -- dormancy
• Sufficient food reserve - new seedling is nourished till they become capable of photosynthesis
• Hard seed coat - protection

SEEDS IN AGRICULTURE

• Seeds are the basis of our agriculture
• Dehydration and Dormancy -- long storage

VIABILITY OF SEEDS

• Variable
• In majority of plants, seeds remain alive for years
• archeological group has found thousands of year old viable seeds.