(a) Describe any two devices in a flowering plant which prevent both autogamy and geitonogamy.
(b) Explain the events upto 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) Dichogamy (maturing of the stamen and pistil at different times to avoid self-pollination) and self-incompatibility (the inability to produce zygotes after self-pollination in a fertile hermaphrodite plant) helps in preventing autogamy and geitonogamy.
(b) 

OR
(a)
Figure showing menstrual cycle in human females.

(b) Ovulation occurs mostly at mid (14^th day) of every menstrual cycle where the chances of fertilization are very high. Couples can take contraceptive measures between the periods of 10 to 17 days of the cycle.