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

(a) Dioecy (unisexual flowers on separate plants) and self-incompatibility prevent both self- and geitonogamous pollination. (b) After the pollen tube discharges two male gametes in a synergid: one + egg → zygote (2n) (syngamy); the other + two polar nuclei → PEN (3n) (triple fusion) = double fertilisation. OR: menstrual cycle phases (menstrual/follicular/ovulatory/luteal) and its use for the rhythm (safe-period) contraceptive method.

Concept (main). Flowering plants have evolved devices to encourage cross-pollination and discourage self-pollination — preventing autogamy (self-pollination within a flower) and geitonogamy (pollen transfer to another flower of the same plant). After successful pollination, angiosperms show the unique event of double fertilisation.

(a) Two devices that prevent both autogamy AND geitonogamy.

1. Dioecy (unisexuality): producing male and female flowers on separate individual plants (dioecious plants, e.g. papaya). Since a plant bears only one sex of flower, pollen cannot fertilise the same flower (no autogamy) nor another flower of the same plant (no geitonogamy). (In monoecious plants like castor and maize, unisexual flowers prevent autogamy but not geitonogamy — hence dioecy is the device that prevents both.)
2. Self-incompatibility (self-sterility): a genetic mechanism in which the pistil rejects the pollen of the same plant — the pollen fails to germinate or the pollen tube fails to reach the ovule. As it acts against all pollen from the same plant, it prevents both autogamy and geitonogamy.

(b) Events up to double fertilisation. After the pollen tube grows through the style and enters the ovule (through the micropyle), it enters one of the synergids and releases its two male gametes into it.

1. Syngamy: one male gamete fuses with the egg cell (oosphere), forming a diploid zygote (2n).
2. Triple fusion: the other male gamete fuses with the two polar nuclei (secondary nucleus) in the central cell, forming the triploid Primary Endosperm Nucleus (PEN, 3n), which develops into endosperm. Since two fusion events (syngamy + triple fusion) occur in the same embryo sac, the phenomenon is called double fertilisation — a feature unique to angiosperms.

OR — Menstrual cycle in human females and its contraceptive use.

(a) The menstrual cycle is the cyclic change in the reproductive system (mainly the ovary and uterus) of primate (human) females, repeating about every 28 days from menarche to menopause. Phases:

• Menstrual phase (days 1-5): the uterine lining (endometrium) breaks down and is shed as menstrual flow, if fertilisation has not occurred.
• Follicular / proliferative phase (days 6-13): under FSH, the primary follicles mature into a Graafian follicle; the endometrium regenerates; estrogen rises. …