Sexual Reproduction in Flowering Plants Test - 10

(i) A microsporangium or future pollen sac is a cylindrical sac that appears circular in the transverse section. It consists of two parts, the outer wall, and central homogeneous sporogenous tissue. 
(ii) Microsporangial wall has four types of layers - epidermis, endothecium, 1−3 middle layers, and tapetum. 
(iii) The outer three layers perform the function of protection in the young anther and mechanism of dehiscence in the ripe anther. 

In a mature pollen grain, the generative cell is smaller and the tube (or vegetative) cell is larger. In the later stages of development of pollen grain, callose dissolves and the naked generative cell comes to lie freely in the cytoplasm of the tube cell. The tube cell has a cytoplasm which is rich in the food reserve (starch, protein, fat with mostly unsaturated fatty acids).

Xenogamy is the cross-pollination between the flowers of different plants. Cross pollination is the transfer of pollen grains from the anther of one flower to the stigma of a genetically different flower. Cross pollination is performed with the help of an external agency.

A stamen or male reproductive organ of a flower is made up of two parts- a stalk-like filament and a knob like terminal anther. Each anther has two lobes which are attached at the back by a sterile band called connective. When both the anther lobes are present, the stamen is called bithecous (or dithecous). A dithecous anther is tetrasporangiate having four microsporangia. Each lobe has two microsporangia separated by a strip of sterile tissue. When a single anther lobe is present, the stamen is termed as monothecous

Ovule is an integumented megasporangium found in spermatophytes which develops into seed after fertilization. An angiosperm ovule is typically an ovoid and whitish structure. It occurs inside the ovary where it is attached to a parenchymatous cushion called placenta either singly or in a cluster. The ovule is stalked. The stalk is called funiculus or funicle. The point of attachment of the body of the ovule with the funiculus is known as hilum.

The body of the ovule consists of a mass of parenchymatous cells named nucellus. It is equivalent to megasporangium (2n). Megaspore mother cell or megasporocyte is a diploid (2n) cell which undergoes meiosis to form a linear tetrad of four haploid megaspores (n). Normally, the chalazal megaspore is the functional megaspore while the other three degenerate.

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.

Anemophily is a mode of cross pollination or transfer of pollen grains from a mature anther to the stigma of a pistil which is accomplished through the agency of wind.
Example: coconut palm, data palm, maize, many grasses, Cannabis, etc.

Entomophily type of pollination takes place through the agency of insects. The entomophilous flowers are brightly coloured and fragranted to attract the insects. Their pollen grains are sticky or spiny to easily get attached with the body of pollinators. When the insects visit the flowers, their body gets dusted with pollen grains, and when they fly and visit other flowers, they brush against the stigma which being sticky, at once receives the pollen grains from their body. Thus, cross-pollination is achieved.

Monoecious plant species have separate male and female flowers on the same plant. Examples or monoecious plants are maize, birch, coconut and cucurbit. The plants in which male and female flowers are borne on separate individual are referred to as dioecious plants. e.g., papaya, date palm.

(i) Kigelia pinnata (sausage tree) is an example of chiropterophilous flower. In chiropterophily, cross pollination is performed by bats. Chiropterophilous flowers are dull-coloured with strong fermenting or fruity odour, abundant nectar and pollen grains.
(ii) Pollen grains are also produced in more abundance. Other examples of chiropterophilous plants are Adansonia (Baobab), Anthocephalus (Kadam), etc.

Clesitogamous flowers are intersexual. They remain closed causing self pollination. Cleistogamy occurs late in the flowering season in some plants, e.g., Commelina, balsam, Oxalis, Viola. These plants possess both chamogamous and cleistogamous flowers. in cleistogamous flowers, the anthers dehisce inside closed flowers. Growth of style brings the pollen grains in contact with stigma. cleistogamy ensures self-pollination.

The development of the male gametophyte in angiosperms is called as microgametogenesis. Pollen grain is the first cell of a male gametophyte. This cell undergoes only two divisions, with the result of first division two cells are formed - a large vegetative cell and a small generative cell. The second division is concerned with generative cell only. This division may take place either in pollen grain or in the pollen tube and gives rise to two male gametes.

The pollen tube eats its way through the solid part of the stigma and style by secreting pectinases and hydrolytic enzymes. Pollen tube travels intracellularly and chemotropically along the concentration gradient of calcium - boron - inositol sugar complex.

In the ovule, the pollen tube is attracted by secretions of synergids. Jensen (1965) suggested that the filiform apparatus may be aiding the synergid in the absorption and transportation of materials into the embryo sac from the nucellus. He held the opinion that the filiform apparatus

Stigma is the terminal receptive part of the pistil which functions as landing platform for the pollen grains. It also determines the compatibility of the pollen grains.

A pollen grain with pollen tube carrying male gametes represents mature male gametophyte. It is 3-celled (one tube cell +2 male gametes) and 3-nucleated (one tube nucleus + two nuclei of each male gamete) structure.

Each pollen mother cell (PMC), by a meiotic division, gives rise to a group of four haploid microspores. Thus, 16 pollen mother cells undergo meiotic divisions to produce 64 pollen grains.

The tapetal layer is of great physiological significance as all the food material entering into the sporogenous tissue diffuses through this layer. Ultimately the cells of tapetal layer disorganise. Thus, tapetum makes a nutritive layer for the developing microspores.

Depending upon the configuration and orientation of the body of ovule in relation to funiculus. There are six types of ovule - orthtropous (atropous, erect), anatropous (inverted) hemitropous (half inverted), campylotropous (body curved), amphitropous (both body and embryo sac curved) and cricinotropous (funiculus is coiled around the ovule). circinotropous (funiculus is coiled around the ovule). Anatropous or inverted ovule is the most common type of ovule found in angiosperms (in 82% of angiosperm families). Here, body of the ovule gets inverted and micropyle is aon lower side. Further micropyle and funiculus lie side and micropyle is close to helium.

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.