Sexual Reproduction in Flowering Plants Test 11

Orthotropous or atropous ovule is the most primitive and simplest type of ovule in angiosperms. It is erect (i.e., micropyle at upper end) and here micropyle, chalaza and funiculus lie in a same striaght line, e.g., families polygonaceae and Piperaceae.

Outer layer (exine) of pollen grain is made of a highly resistant substance called sporopollenin. Sporopollenin is not degraded by any enzyme. It is not affected by high temperature, strong acid or strong alkali. Because of sporopollenin, pollen grains are well preserved as fossils.

Meiosis takes place in reproductive organs of plants j and young anthers are the best material to study meiosis.

Wheat is an anemophilous plant. Its flowers are not very showy. They do not have petals or sepals. Each female flower consists of an ovary from which two styles emerge, finished with two feathery sticky stigmas to catch pollen.

Entomophilous flowers produce an odour which may be pleasant (e.g., jasmine) or foul (e.g., Aristolochia, Arum, Rafflesia) Foul smell attracts flies and beetles. Malacophily is the kind of pollination in which snails perform pollination e.g. in Arisaema (cobra plant). Hydrophily refers to water and anemophily refers to wind pollination.

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.

Heterostyly is the presence of 2−3 types of flower with different heights of styles and stamens. In diheterostyly (dimorphic heterostyly), there are two types of flower, pin eyed (long style and short stamens) and thrum eyed (short style and long stamens).
Example: Primula vulgaris (primrose), jasmine.