REPRODUCTION in ANGIOSPERMS - EMBRYO SAC

EMBRYO SAC / FEMALE GAMETOPHYE

• Haploid (n)
• Develop from one functional haploid megaspore, so called, monosporic development.
• Cell division - mitotic nuclear division.

• Mature gametophyte has- 
• 7 cells= 3 antipodal cells + 2 synergids + 1 egg cell ( female gamete) + central large cell
• 8 nuclei = 3 antipodal nuclei + 2 synergid nuclei + 1 egg cell nuclei + 2 polar nuclei
• So, embryo sac is in 7 celled, 8 nucleated stage.
• 2 synegids + 1 egg cell = egg apparatus
• Synergids have special structure- filliform apparatus
• Filliform apparatus- it guides the pollen tube into embryo sac towards egg cell.

REPRODUCTION in ANGIOSPERMS - FEMALE PARTS

GYNOECIUM

• It is the female sexual organ of a flower.
• Unit- pistil or carpel.

Types- 

1. Monocarpellary - flower has only 1 pistil 

OR

Gynoecium has only 1 pistil.

2. Multicarpellary- multiple pistils.

Depending upon the fusion of these pistils, multicarpellary has two types-

A. Syncarpous- all carpels are fused together. Eg. Papever

B. Apocarpous- pistils are free.

Eg. Michella.

PARTS OF PISTIL / CARPEL

1. STIGMA

• It forms landing platform for pollen grains.
• Requirement- it should be rough to catch pollen grains.

2. STYLE

• Slender, elongated
• Tube like structure
• Connects stigma to ovary.
• Function- through the style, the pollen tube grow towards ovary.

3. OVARY

• Basal part
• Bulged part
• Cavity- ovarian cavity / locule.
• Inside ovary, placenta is present.
• From placenta, ovules (megasporangium) arise.

Number of ovules in a flower- variable

• Wheat, paddy, mango - one
• Papaya, water melon, orchids - many.
• Trick- fruits with multiple seeds, had multiple ovules in their flowers. 

OVULE / MEGASPORANGIUM

STRUCTURE-

1. Funicle/ stalk - connects ovule with the placenta.

2. Hilum- junction of funicle and body of ovule.

3. Nucellus- cell mass, which contains abundant food reserve.

4. Integument- covering of ovule, protects the ovule. Two in number, outer and inner.

5. Micropyle- opening in integument.

6. Chalaza- opposite of micropyle.

MEGASPOROGENESIS

Nucellus (2n) - near micropyler end - a cell differentiate --- megaspore mother cell (dense cytoplasm, big nucleus)

Megaspore mother cell (2n) --- meiosis --- 4 megaspores (n)

Out of there 4 megaspores-- 

3 degenerate

And 1 remain functional 

This one functional megaspore -- female gametophyte /  embryo sac (n)

REPRODUCTION in ANGIOSPERMS - POLLEN GRAIN

POLLEN GRAIN (MALE GAMETOPHYTE)

• Haploid (one set of chromosomes)

Structure-

1. Outer wall - EXINE

• Made up of sporopollenin.
• Sporopollenin- one of the most resistant organic material on earth. So it is Nonbiodegradable.
• It can withstand- high temperature, acid, alkali and also resistant to enzymes.
• Because of this sporopollenin, pollen grains form fossils.

Germ pore- 

• An opening/ aperture in Exine.
• Here sporopollenin is absent.
• Function- when the pollen grain germinates, pollen tube comes out from this germ pore.

2. Inner wall - INTINE

• Thin layer.
• Continuous layer.
• Made up of cellulose and pectin.

2 cells- 

1. Vegetative cell-

• Large
• Dense cytoplasm- abundant food reserve
• Function- provides nutrition to generative cell.

2. Generative cell-

• Small
• Spindle shaped
• Floats in the cytoplasm of vegetative cell
• Function- It later gives rise to male gamete.

Note-

• In 80% of angiosperms- pollen grains are released in 2 celled stage
• But, in rest of the angiosperms- generative cell divides by mitosis and give rise 2 male gametes. And thus released in 3 celled stage.

Facts about pollen grains-

1.Pollen grains are like dust. They can cause allergies and respiratory problems and bronchial diseases eg. Asthma, chronic obstructive pulmonary disease, bronchitis etc.

Eg. Parthenium grass- this produces huge amount of pollen dust.

2. Pollen grains are rich in nutrients. So they are used as food supplements as tablets and syrups.

They are known to increase the performance of athletes and horses.

Viability

• We do not know how much time it will take a pollen grain to reach to the stigma of a appropriate flower.
• This time period, from its release till the pollen grain remain alive is called viability.
• Viability of different plants are variable.
• Example- 1. Rice, wheat pollen grains viability= approx. 30 minutes.  
• 2. Some members of family, rosaceae, solanaceae, leguminoceae, pollen grain viability= for months

Pollen banks-

• Just like seed banks.
• Pollen grains are stored at very low temperature, -196℃ (in liquid nitrogen) 
• At this temperature in pollen banks, pollen can be stored for years.
• And after this period of storage they can be used to pollinate the flower.