(a) Describe any two devices in a flowering plant, which prevent both autogamy and geitonogamy.

(b) Explain the events up to double fertilisation after the pollen tube enters one of the synergids in an ovule of an angiosperm.

OR

(a) Explain menstrual cycle in human females.

(b) How can the scientific understanding of the menstrual cycle of human females help as a contraceptive measure?

(a) In several flowering plants, male and female flowers are borne on different plants that mean a flower is either male or female. This condition is known as Dioecy. It prevents both autogamy and geitonogamy. Example of dioecious plant – papaya.

In a few flowering plants, maturity of the stamens and carpels in a bisexual flower is not synchronised. That is the male and female flowers mature at different times. This condition is known as Dichogamy. Example of dichogamous plants – Cotton and Ficus.

(b) Double fertilization

After entering one of the synergids,

•The pollen tube releases the two male gametes into the cytoplasm of the synergids.

•One of the gametes moves towards the egg cell and fuses with its nucleus. This process completes the syngamy.

•This results in the formation of a diploid cell, the zygote.

•The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN).

•As this involves the fusion of three haploid nuclei, it is termed triple fusion.

•Since two types of fusions, syngamy and triple fusion take place in an embryo sac; the phenomenon is termed double fertilisation.

•The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

•Later the synergids also disintegrate.

OR

(a) The reproductive cycle in the female primates is called menstrual cycle. The first menstruation that begins at puberty is called menarche. In human females, menstruation is repeated at an average interval of about 28/29 days. Ovulation occurs during the middle of each menstrual cycle. There are 4 phases of menstrual cycle:

•Menstrual phase – menstrual flow (bleeding) occurs during this phase, which lasts for 3-5 days. It occurs because of breakdown of endometrial lining of the uterus. Menstruation occurs only if the released ovum is not fertilised. It usually occurs about 14 days after ovulation.

•Follicular phase – in this phase, primary follicles grow into mature Graafian follicles. The endometrium also proliferates. The levels of LH and FSH increase gradually. The growing follicles start to secrete estrogen. The combined effect is the thickening of endometrium. Both LH and FSH attain a peak during the middle of each cycle. Rapid secretion of LH leads to its maximum level during mid-cycle, called as LH surge. LH surge induces rupture of Graafian follicle.

•Ovulation – LH surge induces ovulation, which usually occurs on 14^th day of a cycle.

•Luteal/secretory phase – after ovulation, the egg is moved through the fallopian tubes where it awaits fertilization. The remaining cells of Graafian follicle forms Corpus luteum. It is responsible for thickening and maintaining the endometrium as it secrets large amounts of progesterone and small amounts of estrogen. LH hormone also stimulates the corpus luteum to secrete estrogen and progesterone, which inhibit the release of LH and FSH. As a result, development of new follicles is prevented. During this phase levels of estrogen and progesterone will rise, while levels of LH and FSH will drop. Low level of LH causes degeneration of corpus luteum leading to decline in progesterone level that causes menstruation.

If fertilization occurs, corpus luteum is maintained and is not degenerated.

(b) The scientific understanding of the menstrual cycle of human females can help as a contraceptive measure. Periodic abstinence is one method in which the couples avoid sexual intercourse from day 10 to 17 of the menstrual cycle because ovulation occurs at this time. As chances of fertilisation are very high during this period, it is called the fertile period. Therefore, by abstaining from coitus during this period, conception could be prevented.