Test: Pre-Fertilization (Structures & Events) - NEET MCQ

(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. 

Topic in NCERT: Structure of microsporangium

Line in NCERT: "it is generally surrounded by four wall layers (Figure 1.3b)- the epidermis, endothecium, middle layers and the tapetum. The outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus."

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).

Topic in NCERT: Pollen grain structure

Line in NCERT: "The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell."

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

Topic in NCERT: Anther

Line in NCERT: "A typical angiosperm anther is bilobed with each lobe having two theca, i.e., they are dithecous."

A megasporangium is a minute structure attached to the placenta with the help of a stalk known as a funicle. A megasporangium is also usually known as the ovule. The ovule attaches with the funicle in a region called the hilum. Each ovule may be covered by a couple of envelopes known as integuments that surround the ovule from all around except at its tip. The opening at the tip of the ovule is called the micropyle. The end where micropyle is present is the micropylar end. Opposite to the micropylar end is a structure known as a chalaza. It represents the basal end of the ovule. The protective integuments enclose a mass of cells called nucellus. The cells of the nucellus have abundant nourishment materials. The nucellus covers the embryo sac or the female gametophyte. A meiotic division in the megaspore mother cell generally gives rise to four haploid megaspores, out of which a single functional megaspore matures into an embryo sac inside the ovule.

Topic in NCERT: The megasporangium (ovule)

Line in NCERT: "The ovule is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule fuses with funicle in the region called 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.

Topic in NCERT: Kinds of pollination

Line in NCERT: "Geitonogamy - Transfer of pollen grains from the anther to the stigma of another flower of the same plant. Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant." "(iii) Xenogamy - Transfer of pollen grains from anther to the stigma of a different plant." "Autogamy: In this type, pollination is achieved within the same flower. Transfer of pollen grains from the anther to the stigma of the same 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.

Topic in NCERT: Agents of pollination

Line in NCERT: "Wind pollination also requires that the pollen grains are light and non-sticky so that they can be transported in wind currents. They often possess well-exposed stamens (so that the pollens are easily dispersed into wind currents) and large often-feathery stigma to easily trap air-borne pollen grains."

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 castor, 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.

Topic in NCERT: Monoecious and dioecious plants

Line in NCERT: "male and female flowers are present on different plants, that is each plant is either male or female (dioecy)."

• Pollen grain protoplasts are uninucleate at the beginning but at the time of liberation it becomes 2 or 3 celled.
• Pollen grains are the male gametes.
• They are present inside the pollen sacs.

Topic in NCERT: Pollen grain structure

Line in NCERT: "The cytoplasm of pollen grain is surrounded by a plasma membrane. When the pollen grain is mature it contains two cells, the vegetative cell and generative cell."

• The ovule bearing parenchymatous tissue is known as the placenta.
• It is also the part of the ovary where the funiculus attaches itself.
• Just like in humans, the placenta provides nutrition to the growing ovules.

Topic in NCERT: The placenta is located inside the ovarian cavity.

Line in NCERT: "The placenta is located inside the ovarian cavity."

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.

Topic in NCERT: Generative cell

Line in NCERT: "In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage)."

• Pollen allergies are usually caused by plants that produce pollens in a large number.
• Because of this they might float in air as well causing them to enter respiratory tracts.
• Carrot grass, Parthenium are a major contributor to pollen allergies.

Topic in NCERT: Pollen allergy causes

Line in NCERT: "Parthenium or carrot grass that came into India as a contaminant with imported wheat, has become ubiquitous in occurrence and causes pollen allergy."

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

Topic in NCERT: Filiform apparatus

Line in NCERT: "Many recent studies have shown that filiform apparatus present at the micropylar part of the synergids guides the entry of pollen tube."

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.

Topic in NCERT: Pollen-pistil interaction

Line in NCERT: "The stigma serves as a landing platform for 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.

Topic in NCERT: Pollen grain structure

Line in NCERT: "Pollen grains may have two cells (a vegetative cell and generative cell) or three cells (a vegetative cell and two male gametes) at the time of shedding."

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.

Topic in NCERT: Microsporogenesis

Line in NCERT: "Each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad."

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.

Topic in NCERT: Role of tapetum in pollen formation

Line in NCERT: "The innermost wall layer is the tapetum. It nourishes the developing pollen grains."

• Pollens are spiny because they make it easy for the pollens to attach themselves to the insects that come to suck the nectar.
• This makes cross pollination possible.

• Gynoecium represents the female part of the flower.
• It consists of a stigma, style and ovaries.
• The gynoecium is non motile.
• It’s the pollens that are motile.
• They fall on the stigma which starts the fertilization process.

Topic in NCERT: The gynoecium represents the female reproductive part of the flower.

Line in NCERT: "The gynoecium represents the female reproductive part of the flower."

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.

Topic in NCERT: Pollen grain structure

Line in NCERT: "The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known."

• Pistil is the free unit of a gynoecium.
• A pistil has three parts-style, stigma and ovaries.
• Stigma acts as a landing platform for the pollens.
• Style is the long tube-like structure that carries the male gamete to the ovaries.

Topic in NCERT: The pistil, megasporangium (ovule) and embryo sac

Line in NCERT: "The gynoecium represents the female reproductive part of the flower."

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.

Topic in NCERT: Animal-pollinated flowers

Line in NCERT: "Majority of insect-pollinated flowers are large, colourful, fragrant and rich in nectar."

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.

Topic in NCERT: Role of tapetum in pollen grain formation

Line in NCERT: "The innermost wall layer is the tapetum. It nourishes the developing pollen grains."

• 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.