(a): Longitudinal section of pistil of an angiosperm:

(b): The steps that occur after a poll lands on stigma are as follows:

1. Pollen-pistil interaction- once the pollen grain falls on stigma which is receptive, it recognises and accepts the pollen with the aid of chemical components interacting with pollen.

2. Germination of pollen grain- once it is recognised, it germinates on the stigma of flower.The tube cell of pollen grain protrude out through germ pores to form a pollen tube.The generative cells divides to form 2 male gametes and are released into the tube.

3. Growth of pollen tube- pollen tube grows down through the tissue of stigma and style and enters ovule, usually through micropyle. Inside ovule , the filiform apparatus guides the pollen tube, carrying gametes to the egg.

4. Double fertilisation- after releasing the two male gametes into the synergids , one of them fuses with egg to form a diploid zygote and the other male fuses with 2 polar nuclei to form triploid primary endosperm cell. Because of these two fusion occur it is known as double fertilisation.

OR

(a):

Oogenesis is the formation of female gamete or ova in the ovary.

1. It starts during embryonic stage in female

2. Millions of oogonia are formed in the ovary of female foetus.

3. The oogonial cells starts meiotic division, enter into prophase-I, and are temporarily arrested at that stage , called as Primary oocytes.

4. Each primary oocytes then get surrounded by a layer of granulose cell called Primary follicle.

5. A large no.of primary follicles degenerate during the phase from birth to puberty.

6. Then the primary follicles get surrounded by more layers of granulose cells and a new theca and are called Secondary follicles.

7. The thecae layer in secondary follicle become organised into an outer theca externa and inner theca interna. This stage is known as Tertiary follicle.

8. Tertiary follicle is characterised by fluid-filled cavity called Antrum.

9. At this stage, the primary oocyte within tertiary follicle grows in size and completes its 1st meiotic division.

10. The 1st meiotic division whah is unequal results in formation of haploid Secondary oocyte and 1st tiny polar body.

11. The tertiary follicle develops into a mature follicle or Graafian follicle.

12. The secondary oocyte forms a new membrane called Zona pellucida surrounding it.

13. The Graafian follicle ruptures to release secondary oocyte i.e ovum from the ovary by the process called Ovulation.

(b): FSH (follicle stimulating hormone) and LH (Leutinizing hormone) are Pituitary hormones.

Oestrogen and progesterone are Ovarian hormones.

The levels of FSH and LH secreted from the anterior pituitary gland increases during the follicular phase.

FSH secreted under the influence of RH( releasing hormone) from the hypothalamus stimulates the conversion of a primary follicle into a graafian follicle. The lives of LH increase gradually leading to the growth of follicle and secretion of oestrogen. The increasing level of LH causes the rupturing of graafian follicle and relate the ovum into the fallopian tube. The ruptured follicle changes to corpus luteum and starts secreting progesterone hormone during the luteal phase.

High levels of progesterone hormone in the blood decrease the secretion of LH and FSH , therefore inhibiting ovulation