(a) (i) Explain the structure of a mature embryo sac of a typical flowering plant. (ii) How is triple fusion achieved in these plants?

Three cells are grouped together at the micropylar end of embryo sac ,constitute

the egg apparatus, the egg apparatus in turn consists of two synergids , one egg cell, the synergids have special cellular thickenings at the micropylar tip called filiform apparatus, three cells are at the chalazal end and are called the antipodals, the large central cell has two polar nuclei, thus a typical angiosperm embryo sac at maturity though $\displaystyle 8$-nucleate is $\displaystyle 7$-celled.

One of the two male gametes fuses with the two polar nuclei located in the

central cell to produce a triploid primary endosperm nucleus (PEN).

Phases of Menstrual

cycle

Pituitary

hormones

Ovarian

hormone

Ovary

Uterus

1) Menstrual

phase/day $\displaystyle 1$-$\displaystyle 5$

Low FSH

and LH

Low

Progesterone

and

Estrogen

Follicle start

to develop

Lining of

endometrium

shed and

expelled as

menstrual

flow

2)Follicular/Proliferat

ive phase/day $\displaystyle 6$-$\displaystyle 13$

FSH and

LH rises

Estrogen

rises and

low

progesterone

Follicle

mature

Endometrium

to thicken

3) Ovulatory

phase/day $\displaystyle 14$-$\displaystyle 15$

LH surge

(about 14th

Increase

level of

Graafian

follicle

Uterus lining

further

day)

estrogen and

low

progesterone

ruptures

thickens

4) Luteal phase/day $\displaystyle 16$-$\displaystyle 28$

FSH and

LH level

begins to

decline

High

progesterone

and low

Estrogen

Ruptured

follicle

transformed

into the

corpus

luteum

which

produce

progesterone

and estrogen

Uterus lining

continues to

thicken for

implantation