Pollen grain - class-XII

Structure of anther contains four wall layers:-

• Pollen grains are a mass of microspores in a seeded plant which appears in the form of fine dust. 
• Each pollen grain is a minute body with varying shape and structure which are formed in the anthers of angiosperms.
• It consists of two layers:-

1. The pollen grain is the male gametophyte that produces two mature male gametes vegetative and generative cell.
2. Embryo sac is the female gametophyte that contains a thin sac within the nucellus that contains egg which is female gamete.

Most of the pollen grains consists of three distinct parts. The central cytoplasmic part is the source of nuclei responsible for fertilization. The other parts constituting the wall of the grains are an inner layer, the intine, and an outer layer, the exine. the outer and the most durable layer, the exine, is very resistant to disintegration. The constituents of the exine have been termed as the sporopollenin.

The study of the mature outer morphology of pollen grains and other spores is known as palynology mainly includes pollen that is found in archaeological and geological deposits. These are used for radiocarbon dating and helps to study the ancient climate and environment by knowing the type of plant that grows at that time.

Endothecium of anther is one of the four wall layers and it helps anther by secreting certain substances that lead to the maturation of pollens.

Pollen kitt is a sticky covering found on the surface of pollen grains. It is common in plants that are pollinated by insects. It is composed of lipids and carotenoids and is formed by tapetum. Pollen kitt helps in pollination. Hence, chemically pollen kitt is made up of lipids and carotenoids.

Pollen exine is formed of sporopollenin. It is resistant to acids and alkali and cannot be degraded by any enzyme. So, the correct option is 'Pollen exine'.

Seed plants are referred to angiosperms, in which seeds are present and are covered. They have male and female gametes that are responsible for fertilization. The male gametes are produced in larger numbers than the female gametes. These male gametes are the carrier of pollen grains while female gametes are the carrier of eggs or ovules. In pollination, the pollen grains are carried through wind, water or insects to the stigma of another flower.

The male gametes of angiosperms consist of two sperm cells within a pollen grain or a pollen tube. They are derived from a single generative cell, which is formed as the smaller cell by unequal cell division in the microspore after meiosis. The gametes formed will be non-motile as there is no flagellated stage in the life cycle of angiosperms.

Sporopollenin is major component of the tough outer walls called as exine of plant spores and pollen grains. It is chemically very stable and is usually well preserved in soils and sediments. The exine layer is often intricately sculptured in species-specific patterns, allowing material recovered from lake sediments to provide useful information to palynologists about plant and fungal populations in the past. The chemical composition of sporopollenin is not exactly known, due to its unusual chemical stability and resistance to degradation by enzymes and strong chemical reagents.

Ubisch bodies have been largely observed in plants, after they were first recorded around the aggregates of mimosoid pollen by Rosanoff. The tapetum may be secretory (glandular) or amoeboid (periplasmodial). The cells of tapetum form pro-ubisch bodies which get surrounded by sporopollenin and now called ubisch bodies. They participate in the formation of pollen exine. 

Gymnosperms are having a haploid endosperm. But, angiosperms have a triploid endosperm due to the double fertilization. The pollen grain is the haploid in nature. Therefore, in gymnosperms, the pollen grains and the megaspores are the haploids. The functional megaspores from the embryo sac. The endosperm is formed prior to the fertilization and hence it is haploid.

Germ pore is the pore in the outer wall of a pollen grain through which the germ tube or pollen tube makes its exit on germination. When this fusion happens, the zygote is produced. The germ pore also releases the extra nucleus which results in the formation of triploid endosperm.

Intine - cellulose
Pollen - microspore mother cell
Pin-eyed - Long styled
Thrum-eyed - Short styled
The pollen grain has an inner and outer layer. The inner layer is called as intine and the outer one is called as exine. Intine is composed of cellulose.
A diploid cell in plants that divides by  meiosis to give rise to four haploid microspores. In flowering plants microspore mother cells are formed within the pollen sacs of the anthers by mitosis; the microspores they produce develop into pollen grains.
Pin-eyed : Having the stigma in the mouth of the corolla, on one end of a long style with the stamens lower in the tube
Thrum-eyed: Having on a short style below the anthers, which like in the mouth of the corolla on big stamens.

Sporopollenin is a major component of the tough outer walls called as exine walls of plant spores and pollen grains. It is chemically very stable and usually well preserved in soils and sediments. The chemical composition of sporopollenin is not exactly known due to its unusual chemical stability and resistance to degradation by enzymes and string chemical reagents.

Pollen grains are haploid microspores produced in higher plants like gymnosperms and angiosperms. In higher plants the pollen grains produce male gametes by the process of mitosis. The study of pollen grains is called palynology. It is useful in forensics, evolutionary studies etc. The important pollen characters that are studied are number, position and character of aperture, encrustations of exine, viability, longevity etc.

Pollen sacs are located in the anther. It is the structure where pollen are produced. Angiosperms have 4 pollen sac in anther. 

Sporopollenin is a major component of the tough outer (exine) walls of plant spores and pollen grains. It is chemically very stable and is usually well preserved in soils and sediments. The exine layer is often intricately sculptured in species-specific patterns, allowing material recovered from (for example) lake sediments to provide useful information to palynologists about plant and fungal populations in the past. The chemical composition of sporopollenin is not exactly known, due to its unusual chemical stability and resistance to degradation by enzymes and strong chemical reagents. Analyses have revealed a mixture of biopolymers, containing mainly long chain fatty acids, phenylpropanoids, phenolics and traces of carotenoids. Tracer experiments have shown that phenylalanine is a major precursor, but other carbon sources also contribute. It is likely that sporopollenin derives from several precursors that are chemically cross-linked to form a rigid structure.

Sporopollenin is the major component of exine of spores and pollen. It is extremely resistant to chemicals and enzymes. It is chemically stable and remains unchanged in pollen grains even that are 500 million years old. Hence it is the most resistant biological material known. Thus the correct answer is option C.

Stamen is the male reproductive organ that consists of long slender filament and broad knob like anther; each anther lobes of anther contains two long and cylindrical microsporangia. Since, stamen bear microsporangia in anther lobes, they are also called as microsporophylls. Option A is incorrect. Gynoecium represents the megasporophyll as it carries megasporangia, i.e., the ovule, at basal part of pistil. Option B is incorrect. Megaspore represents the first cell of embryo sac/female gametophyte; depending upon the number of megaspores involved in development of female gametophyte, single/two/three successive mitotic division form eight nucleate-seven celled female gametophyte. Option C is incorrect. Pollen grain is the first cell of male gametophyte that divides mitotically to form small generative cell and large tube/vegetative cell. Pollen with small generative and large vegetative or with one vegetative cell and two male gametes is known as mature male gametophyte.

The pollen grain is the immature male gametophyte that precociously develops into mature male gametophyte through micro gametogenesis. Meiosis in microspore mother cell forms four haploid microspores which are initially covered with a common wall of callose ($\beta$ 1,3-glucan).  Microspore matures into pollen grain which is a uni-nucleate cell with the two-layered cell wall. The exine is the outer covering of pollen grain which is highly sculptured with sporopollenin; a chemically inert polymer of carotenoid ester which is highly resistant to degradation by any enzyme, as known so far.  It can withstand extremes of temperature and desiccation. Cutin is an insoluble heterogenous lipid-polymer of long chain substituted aliphatic acids which are main constituent of cuticles and serves as the waterproof substance to reduce water loss; it does not withstand extremes of temperatures and is not present in exine. Suberin is a waterproofing waxy substance which restricts the water movement to reduce the risk of embolism caused by wear and tear damage to leaf/petiole. It is not a constituent of exine. Thus, the correct answer is option C. 

Pollen grain is a uninucleate cell with two layered cell wall. Exine is the outer covering of pollen grain which is highly sculptured with sporopollenin; a chemically inert polymer of carotenoid ester which is highly resistant to degradation by any enzyme, as known so far. Option C is correct. 

Pollen grain is an immature male gametophyte as it has two cells out of which one develops into two male gametes after pollination. The generative cell of pollen undergoes mitosis to form two male gametes. So, pollen is a immature or partially developed male gametophyte. Correct answer is D.

Micropyle is a small opening in the surface of an ovule.

In the Pinus, winged pollen grains exist. It is an outgrowth of exine on the two lateral sides to make the wings of pollen. It is the characteristic feature, only in Pinus. In Pinus, some surface tissues of the ovuliferous scale are still attached to the seed. Thus forming a membranous wing.

Thus the correct answer is Exine.

The correct option is B.

The exile of the pollen grains consists most resistant biological material known as Sporopollenin. It is resistant to both microbial and chemical decomposition by which it preserves pollen grains during fossilization.

Sporopollenin is one of the most chemically inert biological polymers. It is a major component of the tough outer (exine) walls of plant spores and pollen grains. It is chemically very stable and is usually well preserved in soils and sediments.

The exine of the pollen grains consist most resistant biological material known as sporopollenin. It is resistant to both microbial and chemical decomposition by which it preserves pollen grains during fossilaization.

Germ pore is the area where pollen tube will grow and the exine is absent in that particular area.

Plastids are found in almost all cells of the plant body either in the form of colorless plastids or colored plastids or proplastids.

Wheat root cells are diploid (2n = 42). Pollen grains are haploid (n) products of meiosis. Therefore, the number of chromosomes in a pollen grain cell is 42 / 2 = 21.

The exine of the pollen grains consists most resistant biological material known as Sporopollenin. It is resistant to both microbial and chemical decomposition by which it preserves pollen grains during fossilization. Exine consists of sporopollenin.

Statement I is wrong because the exine, not the intine, is made of sporopollenin. Statement III is wrong because sporopollenin is highly resistant to enzymatic degradation, not easily degraded by them.

(a) Tapetum provides nutrition to developing pollen. (b) Hilum is the junction between ovule and funicle. (d) Primary endosperm nucleus is triploid (3n) due to double fertilization. (c) is incorrect because Water Hyacinth and Water Lily are pollinated by insects or wind, not water.

Tapetum is the innermost layer and completely surrounds the sporogenous tissue. The cells of Tapetum have dense cytoplasm and generally possess more than one nuclei. Tapetum nourishes the developing pollen grains.

Germ pores are specific areas on the pollen grain wall where the exine is thin or absent, meaning they lack the resistant sporopollenin, allowing the pollen tube to emerge.

Grass is wind-pollinated (anemophilous). Statement P is false because grass flowers are not colorful or fragrant. Statement Q is true because pollen must be light and dry for wind transport. R is true and explains why Q is necessary.

The smallest pollen grain is found in the plant Myosotis. It is about 0.006 mm in diameter.

Pollen tubes are the structure that helps in the entry of the male nuclei into the ovule and then the fertilization takes place. Pollen tube does not form spores. It is just a medium that carries the male gametes.

In a fully developed male gametophyte, three nuclei are present i.e., one vegetative nucleus and two male nuclei.

A typical carpel consists of ovary which can have many ovules. Each ovule has an embryo sac, which in turn has a single egg cell. Similarly, in majority of angiosperms each stamen consists of a bilobed anther, which in turn has two pollen sacs in each lobe, consisting of pollen grains.

A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. They act as conduits to transport the male gamete cells from the pollen grain—either from the stigma (in flowering plants) to the ovules at the base of the pistil or directly through ovule tissue in some gymnosperms. Pollen tube formation is important for sexual reproduction to occur in seed plants. Pollen tubes are unique to plants and their structure had evolved over plant history.

Polyspermy refers to a situation where more than two sperms are released in an embryo sac. This may result because of the formation of more than two sperms in a pollen tube or due to the penetration of an embryo sac by more than one pollen tube. In angiosperms, the pollen tube is normally an unbranched structure which collapses soon after fertilization, the pollen tube may persist for as long as three weeks. Intine is the inner, more or less uniform, pectocellulosic layer of pollen grain wall, which is destroyed during acetolysis. Morphologically, the exine is divisible into two distinct layers: (i) an outer sculptured layer called sexine, and (ii) an inner non-sculptured layer, nexine. The sexine further comprises an internal layer of upright rod-like elements, the columellae(bacula) covered over by a roof-like layer, the tectum. The exine is, thus, typically33-layered consisting of tectum, columella, and nexine. The tectum may be smooth or have various types of processes.

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, paleontology, archaeology, and forensics.

Each pollen grain is a minute body, of varying shape and structure, formed in the male structures of seed-bearing plants and transported by various means (wind, water, insects, etc.) to the female structures, where fertilization occurs. In angiosperms, pollen is produced by the anthers of the stamens in flowers.

Ubisch bodies found in tapetal cells help in pollen formation by participating in the external thickening of exine. They occur during spore wall formation and migrate to the cell cavity, where they are coated with sporopollenin and are released into the anther cavity. 

Staminode is the term used for the sterile stamen that does not make the pollen grain and appears like petals, as seen in Canna. 

The pollen exine of the pollen grains consists most resistant biological material known as Sporopollenin. It is resistant to both microbial and chemical decomposition by which it preserves pollen grains during fossilization.

An organic substance which can withstand environmental extremes and which cannot be degraded by an enzyme is sporopollenin. It is the substance present in the exine layer of pollen grains. Due to its presence, fossils of pollen grains can be found.