Plant Reproduction Test - 8

One or more thin areas present in the exine of pollen grains are known as germ pores. The germ pores are apertures in the exine layer of the pollen grain where the sporopollenin is absent. The germ pore helps in the formation of the pollen tube and the release of the male gametes during fertilisation. There are usually three germ pores in dicots (tricolpate) and one in monocots (monocolpate).

Tapetum secretes Ubisch bodies which get covered with sporopollenin and so increase thickness of exine of pollen grain wall. Tapetum provides nourishment to developing microspores. It also secretes callase enzyme which dissolves callose substances by which four pollens of a pollen tetrad are united, hence separating microspores or pollens of a tetrad.

• Autogamy (Gk. autos-self, gamos-marriage) is a type of self-pollination in which an intersexual or perfect flower is pollinated by its own pollen.
• Geitonogamy is a type of pollination in which pollen grains of one flower are transferred to the stigma of another flower belonging to either the same plant or a genetically similar plant. In geitonogamy, the flowers often show modifications similar to ones found in xenogamy or cross-pollination.
• Xenogamy or cross-pollination is the transfer of pollen grains from the anther of one flower to the stigma of a genetically different flower.

Each microspore mother cell or pollen mother cell (PMC) or reduction division(meiosis) gives rise to 4 pollen grains. Hence, to form 100 pollen grains, 25 meiotic divisions are required.

Megaspore mother cell (MMC) is the sporogenous cell that divides to give 4 haploid megaspores. Out of these 4 megaspores, only one is functional and other three degenerate. Hence 100 meiotic divisions are required for the formation of 100 functional megaspores.

After arriving at the wall of the ovary, the pollen tube enters the ovule either through the micropyle or by some other route. The entrance of the pollen tube-through the micropyle is the normal condition and is known as porogamy. In some cases the pollen tube enters the ovule through the chalaza (chalazogamy e.g. Casuarina), or through integuments (mesogamy e.g. Cucurbita).

Pollen is a fine to coarse powdery substance comprising pollen grains which are male microgametophytes of seed plants, which produce male gametes (sperm cells). The study of pollen is called palynology and is highly useful in paleoecology, palaeontology, archaeology, and forensics.

In the pollen grains of an insect pollinated plant, the exine is covered by a yellowish, viscous, sticky and oily layer called pollen kitt. It is made up of lipids and carotenoids. Pollen kitt is secreted by tapetum.

Given figure represents an anatropous ovule.

• In apocarpous condition the ovaries must be free, though the other parts maybe fused.
• Example Oleandar. In simple words, separate carpels.