a). Describe the structure of a pollen grain. (4 marks) (b) Give a detailed account of formation, transfer and germination of a pollen grain of a flowering plant up to the moment of fertilization. (16 marks

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a). Describe the structure of a pollen grain. (4 marks)

-It is the male gamete in plants which has two nuclei, generative nucleus and pollen tube nucleus and a rough sculptured wall.
-The outer wall is elaborate and is made of protein and sugars;
-The wall may be smooth or in some cases rough (spiky); depending on the agent of pollination;

(b) Give a detailed account of formation, transfer and germination of a pollen grain of a flowering plant up to the moment of fertilization. (16 marks)
-A mature anther has four pollen sacs with densely packed active dividing mother cell.
-The diploid mother cell undergoes meiosis to form a tetrad of haploid cells.
-The haploid cells separate as each of their nuclei divide mitotically to form two nuclei i.e. the pollen tube nucleus and the generative nucleus.
-Thickening and sculpturing of the pollen wall follows thereafter to form the mature pollen grain;
-The anther then dehisces to release pollen grains, which are transferred from the anther to the stigma by either agents such as wind, animals or insects.
-Once the pollen grains land on the stigma, the stigma produces a sticky chemical substance that provides a suitable medium for generation of the pollen tube from the pollen grains.
-As the pollen tube grows down the style, it receives nutrients from the surrounding style tissues;
-The tube nucleus takes its position just behind the tip of the pollen tube;
-The generative nucleus divides mitotically to form two male nuclei;
-The pollen tube enters the ovule through the micropyle and bursts open creating a clear way for the passage of the male nuclei;
-Once the pollen tube bursts, the pollen tube nucleus degenerate and one of the male nuclei fuse
with the two polar nuclei to form a triploid primary endosperm;
-The other one combines with the egg cell nucleus to form a diploid zygote; 

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