a) Describe the process that occurs during the production of a pollen grain, in the anther of a flowering plant until the mature grain is exposed to a visiting pollinator. (14 marks) b) By what means may such grains be prevented from reaching the receptive stigma of the same flower?(6 mks).

a) Describe the process that occurs during the production of a pollen grain, in the anther of a flowering plant until the mature grain is exposed to a visiting pollinator. (14 marks)

- Pollen grains are the male gametes.
- Formation occurs in the anthers.
- The anthers contain four pollen sacs in which pollen grains are formed.
- Each pollen sac contains densely packed pollen mother cells, all of which are diploid.
- Each pollen mother cell undergoes meiosis to give a tetrad of 4 Haploid pollen grains, which later separate.
- Within each pollen grain the nucleus divides mitotically into- 2 nuclei, the generative nucleus and the pollen tube nucleus.
- This mitotic division is accompanied by thickening and sculpturing of the pollen grain wall, forming the mature pollen grain.
- Further elaboration of the pollen grain to form the male gamete occurs after pollination.

b) By what means may such grains be prevented from reaching the receptive stigma of the same flower?(6 mks)

Dichogamy / Protandry and protogyny;
Situation in which male and female parts in a plant mature at different times; when male parts 1/
androecium matures earlier this is protandry and when female parts mature earliest this is protogyny;
These process ensures that when male parts are mature and shedding pollen grains the female parts are still immature and thus cannot receive pollen Il when female parts are mature and ready to receive pollen, the male parts are still immature and cannot shed pollen;

Self sterility;
Refers to the inability of the pollen grains of a flower to effect fertilization in the ovules of the same flower; the pollen grain may fail to germinate to pollen tube; germination may occur but fusion of gamete nuclei fail etc; this ensures that the flower obtains pollen from another flower of the same species;
Dioecism;
Refers to the existence of exclusively male and female plants in a species; such that ail flowers in a
plant are either exclusively staminate or pistillate; for such plants pollination and hence fertilization can
only occur if pollen grains are shed from one plant to another of the same species; e.g. in Pawpaw-
plant (Carica papaya); Heterostyly;
Refers to the possession of styles of different lengths on different plants of the same species; Members of the species whose stigma occur above the anthers can only have fertilization by receiving
pollen grains from another plant of the same species hence cross pollination; Existence of features that attract pollinating agents.
Flowers with features that attract insects pollinators such as sweet scented nectar; brightly coloured
petals etc tend to avoid self pollination since the visiting insects will surely carry pollen grains to the next flower they visit.