Stamen, Microsporangium and Pollen Grain Questions in English

(B) The outer layer of the pollen grain is known as the exine $(A)$. It is composed of a highly resistant organic material called sporopollenin $(B)$. This layer is discontinuous and is absent at specific regions known as germ pores $(C)$,which allow for the emergence of the pollen tube during germination. Therefore,the correct sequence is $A$-exine,$B$-sporopollenin,$C$-germ pore.

(A) In insect-pollinated plants,a sticky material is deposited on the surface of pollen grains,which is known as 'pollen kit material'.
This material is secreted by the cells of the tapetum,which is the innermost layer of the microsporangium wall.

(C) The wall of a microsporangium consists of four layers: the epidermis,endothecium,middle layers,and tapetum.
The outermost three layers—the epidermis,endothecium,and middle layers—perform the functions of protection and aid in the dehiscence of the anther to release the pollen grains.
The innermost layer,the tapetum,provides nourishment to the developing pollen grains.

(D) The correct identification is: $A-$Connective tissues,$B-$Epidermis,$C-$Endothecium,$D-$Sporogenous tissue,$E-$Tapetum,$F-$Middle layer.
The microsporangium is surrounded by four wall layers: epidermis,endothecium,middle layers,and tapetum.
$(i)$ The outer three wall layers (epidermis,endothecium,and middle layers) perform the function of protection and aid in the dehiscence of the anther to release the pollen.
$(ii)$ The innermost wall layer is the tapetum,which nourishes the developing pollen grains.

(B) The process of development of a male gametophyte from a pollen mother cell $(PMC)$ involves the following steps:
$1$. The diploid $(2n)$ pollen mother cell undergoes meiosis to form a haploid $(n)$ microspore tetrad.
$2$. Each microspore then undergoes a single mitotic division to form a two-celled stage,consisting of a vegetative cell and a generative cell.
$3$. This two-celled stage is considered the partially developed or mature male gametophyte at the time of pollen release.
Therefore,the formation involves one meiotic division followed by one mitotic division.

(D) In the process of microsporogenesis,each microspore mother cell $(MMC)$ undergoes meiosis to produce a tetrad of four haploid microspores.
Since each microspore develops into a pollen grain,one $MMC$ produces $4$ pollen grains.
Therefore,if there are $4$ microspore mother cells,the total number of pollen grains produced will be $4 \times 4 = 16$.

(D) The tapetum is the innermost layer of the anther wall.
It serves as the primary nourishing layer for developing pollen grains.
During development,the cells of the tapetum often become multinucleate and polyploid due to processes known as endomitosis (replication of chromosomes without nuclear division) and endopolyploidy (increase in the number of sets of chromosomes within a nucleus).

(C) Exine is primarily composed of sporopollenin.
Sporopollenin is one of the most resistant organic materials known; it can withstand high temperatures,strong acids,and alkali.
However,the exine is discontinuous at certain points where sporopollenin is absent. These apertures are known as germ pores.
Therefore,the statement that germ pores contain sporopollenin is false.

(C) The given diagram shows the stages of microsporogenesis and the maturation of a pollen grain.
$A$ represents the pollen tetrad,which is a group of four microspores formed after meiosis.
$B$ represents the vacuoles that appear in the developing microspore.
$C$ represents the nucleus of the microspore.
$D$ represents the asymmetric spindle formed during the first mitotic division of the microspore nucleus.
$E$ represents the vegetative cell,which is larger,contains abundant food reserves,and has a large irregularly shaped nucleus.
$F$ represents the generative cell,which is small,spindle-shaped with dense cytoplasm,and floats in the cytoplasm of the vegetative cell.

(D) $(i)$ $A$ typical angiospermic anther is a bilobed structure having two theca in each lobe.
$(ii)$ It is tetragonal in shape with $4$ microsporangia located at each lobe in its corners.
$(iii)$ Microsporangia develop into pollen sacs,which produce pollen grains.
$(iv)$ Based on the diagram: $A$ represents the Filament,$B$ represents the Line of dehiscence,$C$ represents the Pollen sac,and $D$ represents the Pollen grains.

(A) typical angiosperm anther is bilobed,meaning it consists of two lobes. Each lobe contains two microsporangia,making it tetrasporangiate in total (two microsporangia per lobe $\times$ two lobes = four microsporangia).

(A) The innermost wall layer of the anther is called the $Tapetum$.
Its primary function is to provide nutrition to the developing pollen grains.
It also secretes enzymes,hormones,and components of the pollen wall (like sporopollenin).

(B) The process of formation of microspores from a pollen mother cell $(PMC)$ or microspore mother cell $(MMC)$ through meiosis is known as microsporogenesis.
During this process,a diploid $(2n)$ $MMC$ undergoes meiosis to produce four haploid $(n)$ microspores,which are arranged in a cluster called a microspore tetrad.

(B) The pollen grain is the immature male gametophyte in seed plants. It contains the vegetative cell and the generative cell,which eventually produces the male gametes. Therefore,it represents the male gametophyte stage of the plant life cycle.

(C) The outermost layer of the pollen grain wall is called the exine.
It is composed of a highly resistant organic material known as Sporopollenin.
Sporopollenin is a carotenoid derivative that is resistant to high temperatures,strong acids,and alkali.
No enzyme is known that can degrade Sporopollenin,which allows pollen grains to be preserved as fossils for long periods.

(A) The germ pore is a small aperture in the exine of the pollen grain where the sporopollenin is absent.
During the process of germination,the pollen tube emerges through one of these germ pores.
Therefore,the primary function of the germ pore is to allow the growth of the pollen tube during the germination of the pollen grain.

(B) The pollen grain wall consists of two layers: the outer $Exine$ and the inner $Intine$.
$Exine$ is thick,hard,and discontinuous due to the presence of germ pores.
$Intine$ is a thin,continuous layer composed of cellulose and pectin.

(A) In over $60\%$ of angiosperms, pollen grains are shed at the two-celled stage.
These two cells are the larger vegetative cell (or tube cell) and the smaller generative cell.
The vegetative cell is rich in food reserves and has an irregularly shaped nucleus.
The generative cell is small and floats in the cytoplasm of the vegetative cell.
Therefore, the correct option is $A$.

(B) In flowering plants (angiosperms),the pollen grains are shed at either a two-celled or a three-celled stage.
In over $60\%$ of angiosperms,pollen grains are shed at the two-celled stage (one vegetative cell and one generative cell).
In the remaining $40\%$ of angiosperms,the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed,resulting in a three-celled stage.

(A) Pollen grains are generally spherical in outline.
They typically measure about $25-50 \mu m$ in diameter.
Therefore,the correct option is $A$.

(B) In the development of a pollen grain,the microspore undergoes an asymmetrical mitotic division.
This division results in two unequal cells: a large vegetative cell and a small generative cell.
The vegetative cell is larger,contains abundant food reserves,and possesses a large,irregularly shaped nucleus.

(C) The statement in option $C$ is incorrect. While the cells of the tapetum often show an increase in $DNA$ content due to endomitosis or polyploidy to support the high metabolic demand of pollen development,the cells of the endothecium do not typically exhibit this phenomenon. The endothecium functions primarily in the dehiscence of the anther due to its hygroscopic nature.

(B) Statement $I$ is correct: Each cell of the sporogenous tissue acts as a microspore mother cell and can give rise to a microspore tetrad.
Statement $II$ is correct: Pollen grains represent the male gametophyte in angiosperms.
Statement $III$ is incorrect: Pollen grains are generally spherical and measure about $25-50 \mu m$ in diameter,not triangular and $10-15 \mu m$.
Statement $IV$ is incorrect: Sporopollenin is one of the most resistant organic materials known; it cannot be degraded by any known enzyme,strong acids,or strong alkalis.

(C) The figure represents a cross-section of a young anther.
- Part $a$ points to the Endothecium, which is the layer beneath the epidermis.
- Part $b$ points to the Middle layers, which are $2-3$ layers of cells between the endothecium and the tapetum.
- Part $c$ points to the Tapetum, the innermost nutritive layer.
- Part $d$ points to the Microspore mother cells (sporogenous tissue) located in the center of the microsporangium.
Therefore, the correct identification is $a$: Endothecium, $b$: Middle layer, $c$: Tapetum, $d$: Microspore mother cell.

(B) Microsporogenesis is the process of formation of microspores from a pollen mother cell $(PMC)$ through meiosis. It involves only one meiotic division to produce four haploid microspores. It does not involve mitosis. Therefore,the statement that it involves one meiosis and two mitosis is incorrect.

(C) The exine is the hard outer layer of the pollen grain,which is composed of sporopollenin,a highly resistant organic material.
Germ pores are the specific regions on the exine where sporopollenin is absent,allowing the pollen tube to emerge during germination.
Therefore,the statement that 'Exine has apertures where sporopollenin is present' is incorrect because apertures (germ pores) are sites where sporopollenin is absent.

(A) The sporogenous tissue consists of microspore mother cells $(MMC)$. Each $MMC$ undergoes meiosis to form a microspore tetrad. Thus,the Assertion is correct.
In dicots,cytokinesis is simultaneous,leading to tetrahedral tetrads,which are the most common type. In monocots,cytokinesis is successive,leading to isobilateral tetrads. Since simultaneous cytokinesis is the most common process in angiosperms,the Reason is also correct and explains why the potential for forming tetrads is widespread in sporogenous tissue.

(A) The sporoderm (pollen wall) consists of two main layers: the inner intine and the outer exine.
The intine is composed of pectin and cellulose (pectocellulosic).
The exine is composed of sporopollenin,which is one of the most resistant organic materials known.
Thus,the assertion is correct as the sporoderm layers are protected by the highly resistant sporopollenin.
The exine is further differentiated into an outer layer called the ektexine and an inner layer called the endexine.
Therefore,both the assertion and the reason are correct,and the reason provides a structural description of the exine,which is part of the sporoderm.

(A) In the development of the male gametophyte in angiosperms,the pollen grain undergoes mitosis to form a large vegetative (tube) cell and a small generative cell.
Initially,the generative cell is surrounded by a callose wall. As the pollen grain matures,this wall dissolves,allowing the generative cell to detach and float freely within the cytoplasm of the vegetative cell.
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for why the generative cell lies freely in the cytoplasm.

(C) In flowering plants,the flower consists of four whorls.
$1$. The Calyx $(A)$ and Corolla $(B)$ are accessory whorls.
$2$. The Androecium $(C)$ is the male reproductive whorl,which consists of stamens. Each stamen produces pollen grains containing male gametes.
$3$. The Gynoecium $(D)$ is the female reproductive whorl.
Therefore,the Androecium is the whorl responsible for producing male gametes.

(C) The stamen consists of two parts: the long and slender stalk called the filament,and the terminal,generally bilobed structure called the anther. The term 'stigma' refers to the receptive surface of the carpel (pistil),not the terminal part of the stamen. Therefore,the statement in option $C$ is incorrect.

(A) typical angiosperm anther is bilobed,meaning it consists of two lobes. Each lobe contains two thecae,making it dithecous. Since each theca develops into a microsporangium,the anther is tetrasporangiate (containing four microsporangia). Therefore,the anther is bilobed and dithecous,which results in four microsporangia.

(A) In the given diagram of a mature anther:
$P$ points to the pollen grains,which are the male gametophytes produced within the anther.
$Q$ points to the pollen sacs (also known as microsporangia),which are the structures where pollen grains develop.
According to standard $NCERT$ terminology for this specific diagram,$P$ represents pollen grains and $Q$ represents the pollen sacs (microsporangia). Therefore,the correct identification is $P = \text{Pollen grains}$ and $Q = \text{Pollen sacs}$.

(D) typical angiosperm anther is dithecous,meaning it consists of two lobes. Each lobe contains two microsporangia,one at each corner. Therefore,a typical bilobed anther contains a total of $4$ microsporangia. These microsporangia develop further and become pollen sacs.

(C) In flowering plants,the stamen consists of a filament and an anther. The anther is a bilobed structure,and each lobe typically has two thecae,making it dithecous. The microsporangia develop at the four corners of the anther,two in each lobe. These microsporangia further develop into pollen sacs,which contain pollen grains. Therefore,the development of the microsporangium occurs within the anther.

(B) The wall of a microsporangium consists of four layers: the epidermis,endothecium,middle layers,and tapetum.
These layers are arranged from the outside to the inside as: Epidermis $\rightarrow$ Endothecium $\rightarrow$ Middle layers $\rightarrow$ Tapetum.
Therefore,when identifying the layers from the inside to the outside,the sequence is: Tapetum $\rightarrow$ Middle layers $\rightarrow$ Endothecium $\rightarrow$ Epidermis.

(B) The wall of the microsporangium consists of four layers: the epidermis, endothecium, middle layers, and tapetum.
$1$. The $Epidermis$ is the outermost protective layer.
$2$. The $Endothecium$ develops fibrous thickenings of $\alpha$-cellulose at maturity, which helps in the dehiscence of anthers to release pollen grains. Due to these fibrous thickenings, it is often referred to as the fibrous layer.
$3$. The $Middle$ $layers$ are ephemeral and provide nourishment.
$4$. The $Tapetum$ is the innermost layer that provides nutrition to the developing pollen grains.
Therefore, the correct answer is $Endothecium$.

(A) The tapetum is the innermost layer of the microsporangium.
Tapetal cells typically possess dense cytoplasm and are often multinucleate due to endomitosis.
Its primary function is to provide nourishment to the developing pollen grains.
Therefore,option $A$ is incorrect because tapetal cells are frequently multinucleate,not uninucleate.

(D) The anther wall consists of four layers: Epidermis,Endothecium,Middle layers,and Tapetum.
$1$. The Epidermis provides protection.
$2$. The Endothecium helps in the dehiscence of the anther to release pollen grains.
$3$. The Middle layers provide protection and also assist in dehiscence.
$4$. The Tapetum is the innermost layer that provides nourishment to the developing pollen grains. It does not play a role in protection or dehiscence.

(B) The sporogenous tissue is a group of compactly arranged homogeneous cells that occupy the center of each microsporangium when the anther is young.
As the anther develops,the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads.
Therefore,in a mature anther,the sporogenous tissue is no longer present as such,because its cells have differentiated into microspore mother cells and subsequently into pollen grains.
Thus,the statement that it is present in a mature anther is incorrect.

(D) microspore mother cell $(MMC)$ undergoes meiosis,which is a reductional division.
During meiosis,a single diploid $(2n)$ $MMC$ divides to form four haploid $(n)$ microspores.
These four microspores are arranged in a cluster known as a microspore tetrad.
Therefore,a single microspore mother cell produces $4$ microspores.

(C) The microsporangium is surrounded by four wall layers: the epidermis,endothecium,middle layers,and tapetum.
In the given diagram,$P$ points to the second layer from the outside,which is the endothecium.
$Q$ points to the central cells within the microsporangium,which are the microspore mother cells (MMCs) that undergo meiosis to form pollen grains.
Therefore,$P$ is the endothecium and $Q$ is the microspore mother cell.

(A) microspore mother cell $(MMC)$ undergoes meiosis to produce a cluster of four haploid cells,which is known as a microspore tetrad.
Therefore,one microspore mother cell gives rise to exactly $1$ microspore tetrad containing $4$ microspores.

(D) In flowering plants, the $Pollen$ $grain$ represents the partially developed male gametophyte.
It contains two cells: the vegetative cell (tube cell) and the generative cell.
The vegetative cell is larger and contains abundant food reserves, while the generative cell is small and floats in the cytoplasm of the vegetative cell.
Therefore, the $Pollen$ $grain$ is the structure that represents the male gametophyte stage.

(D) The image shows a group of four haploid microspores formed after the meiotic division of a microspore mother cell. This cluster of four cells is known as a pollen tetrad or microspore tetrad. As the anthers mature and dehydrate,the microspores dissociate from each other and develop into pollen grains.

(D) Pollen grains are generally spherical in shape.
They measure approximately $25 - 50 \, \mu m$ in diameter.
This information is provided in the $NCERT$ textbook under the topic of 'Pollen Grain' in the chapter 'Sexual Reproduction in Flowering Plants'.

(A) Sporopollenin is a highly resistant organic material that forms the exine (outer layer) of the pollen grain,not the intine (inner layer).
Therefore,the statement that it is the hard inner layer is incorrect.
It is chemically stable,resistant to high temperatures,strong acids,and strong alkalis.
No known enzyme can degrade sporopollenin,which allows pollen grains to be well-preserved as fossils.

(C) In most angiosperms,the pollen grain is shed at the two-celled stage.
These two cells are:
$1$. The Vegetative cell: It is bigger,has abundant food reserve,and a large irregularly shaped nucleus.
$2$. The Generative cell: It is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.
Therefore,the correct option is $C$.

(B) In angiosperms,the pollen grain represents the male gametophyte.
During the development of the pollen grain,the microspore nucleus undergoes mitosis to form a vegetative cell and a generative cell.
In about $60\%$ of angiosperms,pollen grains are shed at this $2$-celled stage.
In the remaining species (less than $40\%$),the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed,resulting in a $3$-celled stage.

(C) In a mature pollen grain,there are two types of cells:
$1$. The $P$ (Vegetative cell) is bigger,has abundant food reserve,and a large irregularly shaped nucleus.
$2$. The $Q$ (Generative cell) is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.
Therefore,the correct sequence is $P$ = Vegetative cell and $Q$ = Generative cell.