(a). Distinguish between cross-pollination and self-pollination. (2 marks) (b). Describe pollination, fertilization and seed and fruit formation in common plants. (18 marks)

(a). Distinguish between cross-pollination and self-pollination. (2 marks)

Self-pollination is the transfer of pollen grains from the anther to the stigma of the same flower; while cross-pollination is the transfer of pollen grains to the stigma of another flower either on the same plant or another but of the same species;

(b). Describe pollination, fertilization and seed and fruit formation in common plants. (18 marks)

-The pollen grains land and stick onto the stigma.
-The stigma secrete/produce a sticky chemical fluid that causes pollen grains to adhere onto it.
-The sticky secretions also stimulate the pollen grain to germinate into a pollen tube.
-The pollen tube grows into the stigma, and protrudes down the style tissues.
-The pollen tube derives its nutrition from the style tissues.
-As the tube grows downwards, the tube nucleus occupies the position just behind the tip.
-The generative nucleus then enters the tube; and divides mitotically to give a pair of male gamete nuclei, which follow the tube nucleus, as the pollen tube grows further down the style.
-On reaching the ovary the pollen tube enters the ovary via the micropyle.
-It then penetrates the embryo sac wall and bursts open.
-The tube (vegetative) nucleus then disintegrates, leaving a clear passage for the entry of the two male gamete nuclei.
-Fertilization takes place where one male gamete fuses with both polar nuclei to form a triploid nucleus called primary endosperm nucleus.
-The other male nucleus fuses with the functional egg cell to form a Diploid zygote.
-This is Double fertilization, a unique feature of flowering plants.
-After fertilization a number of changes (post fertilization changes) occur in the flower; Main changes include the following: -
-The zygote divides mitotically, growing and developing into an embryo.
-The embryo becomes differentiated into a radicle (embryonic root), plumule (embryonic shoot), and one or 2 cotyledons.
-The triploid primary endosperm nucleus divides mitotically to form endosperm, which surrounds the developing embryo, and provide it with nourishment.
-The embryo sac expands to accommodate the developing and growing embryo.
-The nucellus becomes crushed so that the embryo and endosperm come to fill the whole area inside the integuments.
-The ovules develop in to the seed. The integuments become the seed coat (The outer integument becomes the testa and the inner integuments become the tegmen).
-The final transformation of ovule into seed involves removal of water from an initial 90% to final 15% thus forming a dormant resistant structure that can withstand adverse conditions.
-The ovary develops into a fruit; this protects the seed and aids in dispersal.
-The ovary wall becomes the pericarp (wall of the fruit).
-Petals, stamen, style and stigma wither, die and fall off.