Sexual Reproduction In Flowering Plants

There are 4 floral whorls namely Calyx, corolla, androecium and gynoecium.

Calyx – is the outermost part whorl of flower parts, consisting of green leaf-like structures called sepals, which covers and protects the petals.

Corolla – is the second whorl of flower parts, which is composed of the flower’s petals.

Androecium - is the male reproductive part of the flower which is an essential whorl of the flower, it is made up of stamens.

Gynoecium - is the female reproductive part of the flower which is also an essential whorl of the flower, it is made up of pistils.

So, in the given list (i) and (iii) are correct names of the floral whorl. The other two are incorrect.

Therefore, the correct answer is (c).

Female gametophyte is also called as embryo sac. Embryo sac is developed inside ovule.

Male gametophyte is called Pollen grains. Pollen grains are produced by organs called anthers.

So, embryo sac is to ovule as pollen grains are to an anther.

Stamen is male reproductive organ of a flower. It produces the pollen.

Filament is a slender stalk and supports the anther, which is where pollen develops.

Androecium is the male reproductive part of the flower which is an essential whorl of the flower; it is made up of stamens.

Therefore, the correct answer is (c).

In a typical complete, bisexual and hypogynous flower the arrangement of floral whorls on the thalamus from the outermost to the innermost is as follows:

Calyx – is the outermost part whorl of flower parts, consisting green leaf-like structures called sepals, which covers and protects the petals.

Corolla – is the second whorl of flower parts, which is composed of the flower’s petals.

Androecium - is the male reproductive part of the flower which is an essential whorl of the flower, it is made up of stamens.

Gynoecium - is the female reproductive part of the flower which is also an essential whorl of the flower, it is made up of pistils.

Therefore, the correct answer is (a).

Dicotyledonous plant or dioecious plant means the male and female reproductive systems occur on separate plants. And both plants produce flowers.

- The unisexual male flower is staminate, which means it bear stamens only. In this fertilization is not possible at all. Hence in this case fruits and seeds are not produced.

- The unisexual female flower is pistillate, which means it bear pistils only. In this fertilization is possible with the help of pollinators and fruits and seeds can be produced.

Monoecious plant has both male and female flowers and produces fruits and seeds.

So, a dicotyledonous plant which bears flowers but never produces fruits and seeds. This situation is possible only when the plant is dioecious and bears only staminate flowers.

Therefore, the correct answer is (d).

In typical microsporangium, an anther is generally surrounded by four wall layers - 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.

The innermost wall layer is the tapetum. It nourishes the developing pollen grains.

Therefore, the correct answer is (d).

During Microsporogenesis, meiosis occurs in microspore mother cells.

As anther develops the cells of Sporongenous tissues undergo meiotic divisions to form microspore tetrads.

The microspore tetrad after dehydration is separated into pollen grains.

Endothecium is the lining of the cavity of an anther.

Therefore, the correct answer is option (b).

Gynoecium is the female reproductive part of the flower which is also an essential whorl of the flower. It is made up of pistils.

Each pistil has three parts – the stigma, style and ovary.

The ovary consists of ovarian cavity (locule), which in turn has placenta in it.

Megasporangia also called ovules come out of placenta.

Embryo sac is female gametophyte. And an ovule generally has single embryo sac.

Tapetum is one of the wall layers of typical microsporangium in an anther.

Stamen is present in Androecium, the male reproductive part of the flower.

And thalamus forms the base on which all the floral whorls lay.

So the terms associated with Gynoecium are pistil, stigma, style, ovary, ovules, ovarian cavity (locule), placenta, embryo sac.

Therefore, only option (a) matches this, so correct answer is option (a).

The parts of ovule in correct order are:

Egg, embryo sac, nucellus and interguments

Therefore, the correct answer is option (b).

A typical female gametophyte of a flowering plant also known as embryo sac at maturity is 8-nucleate and 7-celled.

The mitotic divisions in the formation of embryo sc are strictly free nuclear, that is, nuclear divisions are not followed immediately by cell wall formation.

Embryo sac is located in the nucellus within the integuments and not outside the nucellus. So option (iii) is wrong.

The cells in the embryo sac are distributed such that the three cells are grouped together at the micropylar end and constitute the egg apparatus. And the three cells are at the chalazal end and are called the antipodals. So the egg apparatus is situated micropylar end. So option (iv) is wrong.

Therefore, correct answer is option (c).

Autogamy where pollination is achieved within the same flower, i.e., it is type of self pollination. In this the pollens are transferred from anther to stigma of same flower.

Chasmogamous flower are flowers with exposed anthers and stigma.

So, for autogamy to occur in chasmogamous flower both pollen and ovules should mature simultaneously. As pollen release and stigma reception should be synchronised for autogamy to take place.

In chasmogamous flower length of anther and stigma plays a secondary role in autogamy.

Therefore, correct answer is option (c).

Chasmogamous flower have exposed anthers and stigma. Chasmogamous flower can exhibit autogamy as well as allogamy i.e., cross-pollination. And cross-pollination is of two types geitonogamy and xenogamy. So, Chasmogamous flower can exhibit autogamy as well as geitonogamy. Hence option (b) and (d) are incorrect.

Cleistogamous flowers are closed flowers i.e., they never open and the anther and stigma lie together inside. As this flower is always closed so, there is no chance of cross pollination or geitonogamy. So option (c) is incorrect.

Therefore, the correct answer is option (a).

Plants use two abiotic (wind and water) and one biotic (animals) agents to achieve pollination.

Wind pollination requires pollen grains that are light and non-sticky so that they can be easy transported in wind currents.

Water pollination is also known as hydrophily. In this if pollen grains are light and non-sticky, they will fly away instead of reaching the target flower.

Insects’ pollination is also known as Entomophilly. This type of pollination is through insects as the agents.

Animals’ pollination is also known as Zoophilly. This type of pollination is through animals as the agents.

Therefore, the correct answer is option (c).

In Autogamy, pollination is achieved within the same flower, i.e., it is type of self pollination. In this the pollens are transferred from anther to stigma of same flower.

Geitonogamy is pollination where pollination is achieved by transfer of pollen grains from anther to stigma of another flower of same plant.

So, Dioecious plant having only male or female flowers both autogamy and geitonogamy is not possible. As either stigma will be absent or anther will be absent.

Hence, Dioecious plant bearing only male or female flowers prevents both autogamy and geitonogamy.

Therefore the correct answer is option (b).

Synergids, antipodal, polar nuclei are haploid in nature.

Zygote is diploid cell.

Primary endosperm nucleus is triploid in nature.

Antipodal are haploid in nature

So in a fertilised embryo sac, the haploid, diploid and triploid structures are Synergid, zygote and primary endosperm nucleus respectively.

Therefore the correct answer is option (a).

The typical female gametophyte forms four cell types - synergid cell, egg cell, central cell and antipodal cell.

In fertilised embryo sac, antipodals and synergids gradually degenerate after the formation of zygote. Before that antipodals are present at chalazal end and synergids are present at micropylar end in embryo sac.

Therefore the correct answer is option (b).

Artificial hybridisation programme is an approache of crop improvement. In this it is important to make sure that only the desired pollen grains are used for pollination and the stigma is protected from contamination (that is stigma is protected from unwanted pollen). This is usually done by emasculation and bagging techniques.

Emasculation is removal of anther from the flower bud. Now the left out stigma is covered with butter paper bag to prevent contamination.

Now we know dioecious plants have unisexual flowers, hence there is no need of emasculation. Instead the female flowers are directly bagged to prevent contamination. And the rest steps are carried normally.

Therefore, the correct answer is option (c).

The embryos of typical dicot, consists of an embryonal axis and two cotyledons.

Embryos of monocotyledons consists only one cotyledon. And this cotyledon is called scutellum in grass family.

Coleorhiza is a sheath protecting the root of a germinating grass or cereal grain.

Coleoptile is a sheath protecting a young shoot tip in a grass or cereal.

The cylindrical portion below the level of cotyledons is hypocotyl that terminates at its lower end in the radical or root tip.

So the correct answer is option (c).

Parthenocarpy is development of a fruit without prior fertilization. Hence the fruits are seedless.

Apomixis is development of a seeds without prior fertilization. It is observed in few flowering. Apomixis is a form of asexual reproduction that mimics sexual reproduction. Apomixis avoids the processes of meiosis and fertilization, leading to embryo development.

Vegetative propagation is type of asexual reproduction in a plant where new plant is formed with involvement of just one parent. The new plant is genetically identical to the parent.

Sexual reproduction – here new offspring is formed with involvement of two parents.

Therefore, the correct answer is option (b).

In a flower, if the megaspore mother cell forms megaspores without undergoing meiosis and if one of the megaspores develops into an embryo sac, its nuclei would be diploid in nature. The embryo sac is formed by mitotic divisions.

Haploid cells are formed during sexual reproduction when cell will undergo meiosis.

Therefore, the correct answer is option (b).

Parthenocarpy is development of a fruit without prior fertilization. Hence the fruits are seedless. So in this the ovary develops into a fruit without fertilisation.

Apomixis is development of a seeds without prior fertilization. It is observed in few flowering. Apomixis is a form of asexual reproduction that mimics sexual reproduction. Apomixis avoids the processes of meiosis and fertilization, leading to embryo development.

Vegetative propagation is type of asexual reproduction in a plant where new plant is formed with involvement of just one parent. The new plant is genetically identical to the parent.

Sexual reproduction – here new offspring is formed with involvement of two parents.

Therefore, the correct answer is option (a).

Synergids, egg cells and the filliform apparatus

Explanation: The cells in the embryo sac are distributed such that the three cells are grouped together at the micropylar end and constitute the egg apparatus. Hence the component cells of the ‘egg apparatus’ in an embryo sac are two synergids (supporting cells), one egg cell and the filliform apparatus.

Pistil

Explanation: Gynoecium - is the female reproductive part of the flower which is also an essential whorl of the flower, it is made up of pistils.

This pistil has the ability to determine the compatible nature of pollen grain.

The pistil can recognise whether the pollen is compatible (right) or incompatible (wrong).

The pistil accepts the pollen only if the pollen is compatible and then the pistil promotes post-pollination processes leading to fertilisation.

If the pollen is incompatible the pistil rejects the pollen.

Due to continuous communication between pollen and pistil via chemical components of pollens helped pistil to recognise the compatible pollen.

the common functions that cotyledons and nucellus perform are:

i. They both have abundant reserve food materials, so they both store reserve food materials.

ii. They both nourish different parts by providing food. Cotyledon nourishes embryo whereas nucellus nourishes embryo sac.

Pollen mother cell → Pollen tetrad → Pollen grain

Explanation: The completed flow chart is as follows:

Pollen mother cell → Pollen tetrad → Pollen grain

Pollen mother cells are converted to pollen tetrad by meiosis division through the process of Microsporogenesis. The pollen tetrad are actually microspores tetrads.

Now as anther matures and dehydrates the pollen tetrads dissociate and form pollen grains.

When pollen grain is mature it contains two cells namely Vegetative cell and generative cell.

Megaspores – Meiosis, Embryo sac – Mitosis, Egg - Mitosis

Explanation: Megaspores are produced from megaspore mother cell through the process of Megasporogenesis by meiosis division.

One of the functional megaspore gets developed into embryo sac by the process called Monosporic development by mitotic division.

Embryo sac at maturity is 8-nucleate and 7-celled. And also the cells in the embryo sac are distributed such that the three cells are grouped together at the micropylar end and constitute the egg apparatus. And the three cells are at the chalazal end and are called the antipodals. So the egg apparatus is situated micropylar end. So conversion of embryo sac to egg is a mitotic division.

Pollen tube, Antipodal, Polar Nuclei, Egg cell, Synergids

Explanation: the path of a pollen tube from the pollen on the stigma into the embryo sac is as listed below (as shown in figure):

i. After the pistil selects the compatible pollen grain pollination takes place by pollen grain getting germinated on the stigma to produce a pollen tube. The pollen tube is generated from any of of the germ pores.

ii. The contents of the pollen grain now move into the pollen tube.

iii. Now the pollen tube grows through the tissue of the style and reaches the ovary.

iv. Now it enters the ovule through micropyle and enters the embryo sac through one of the two synergids. And the synergid is degenerated.

stigma, style and ovary

Explanation:

The parts of pistil are stigma, style and ovary.

- The stigma serves as a landing platform for pollen grains.

- The style is the elongated slender part beneath the stigma.

- Ovary has ovarian cavity in it, which in turn has placenta. From placenta ovules (or megasporangia) arise.

Ovary develops into fruits and ovules mature into seeds.

Embryos from Synergids – Haploid, Enbryos from nucellus - diploid

Explanation: Presence of more than one embryo in a seed is known as polyembryony.

The embryos developed from synergids are haploid in nature.

The embryos developed from nucellus are diploid in nature.

Yes, an unfertilised, apomictic embryo sac can give rise to a diploid embryo.

When megaspore develops into embryo sac without metiotic division, the egg developed will be diploid.

This diploid egg develops into embryo by mitotic division.

one vegetative cell and two male gametes

Explanation: In over 60 % of angiosperms, pollen grains are shed at this 2-celled stage namely vegetative cell and generative cell.

In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed. This is 3-celled stage i.e., one vegetative cell and two male gametes.

Self-incompatibility is a genetic mechanism that prevents self-pollination (i.e., pollination from same flower or other flowers of same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

Xenogamy

Explanation: In Self-incompatible plants self-pollination (i.e., pollination from same flower or other flowers of same plant) doesn’t occur. So in such case cross-pollination particularly xenogamy takes place.

That is transfer of pollen grains from anther to the stigma of a different plant.

The antipodal is 3-celled with 3 nucleus.

Polar nuclei and Central cell constitute 2 nucleus and it is 1-celled.

Egg apparatus is 3-celled with 3 nucleus.

So in total there are 7-celled and 8-nucleate.

The triploid tissue in a fertilised ovule is endosperm which is developed from Primary Endosperm nucleus.

This triploid condition is achieved by fusion of three haploid nuclei called triple fusion. The three haploid nuclei are one male gamete and two haploid polar nuclei.

Apomixis is a form of asexual reproduction that mimics sexual reproduction.

i. Apomixis is development of a seeds without prior fertilization. Apomixis avoids the processes of meiosis and fertilization, leading to embryo development.

ii. The diploid egg cell in many species is formed without reduction division and develops into the embryo without fertilization.

Hence pollination and fertilisation are not necessary in apomixes.

Carpel can be of two types Monocarpellary (single pistil) or Multicarpellary (more than one pistil).

In multipcarpellary the pistils may be fused called Syncarpous or pistils may be free called Apocarpous.

So the diagram (a) is Syncarpous multicarpellary (i.e., have more than one pistil fused together).

The diagram (b) is Apocarpous multicarpellary (i.e., have more than one pistil but free).

Pollination carried in water plants are in following ways:

i. Pollination above the surface of water: In few species, female flower reach the surface of water by the long stalk and pollen grains from male flowers are released on to the surface of water. These pollen grains are carried by water currents. And out of these some of them reach the female flowers and the stigma. For example in Vallisneria

ii. Pollination below the surface of water: In few species, female flowers remain submerged in water and the pollen grains are released inside the water. Pollen grains in such species are long, ribbon like and they are carried passively inside the water; some of them reach the stigma and achieve pollination. Example Seagrasses.

The two male gametes produced by each pollen grain in angiosperms helps in double fertilisation.

i. One of the male gametes moves towards the egg cell and fuses with its nucleus called Syngamy. This results in the formation of a diploid cell, the zygote.

ii. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) (Figure 2.13a).

As this involves the fusion of three haploid nuclei it is termed triple fusion.

Two types of fusions, syngamy and triple fusion, take place in an embryo sac the phenomenon is termed double fertilisation, an event unique to flowering plants.

Autogamy is pollination where pollination is achieved within the same flower, i.e., it is type of self pollination. In this the pollens are transferred from anther to stigma of same flower.

Chasmogamous flower are flowers with exposed anthers and stigma.

The strategies that a bisexual chasmogamous flower can evolve to prevent self-pollination are:

i. As pollen release and stigma reception should be synchronised for autogamy to take place. Avoiding synchronisation of pollen release and stigma reception can prevent autogamy that is avoiding the simultaneous maturing of anther and stigma. Such type of pollination is known as Dichogamy where stigma and anther matures at different time. Examples of such type of flowers are Sunflower, Datura, Solamium, etc.

ii. Self-incompatibility or self-sterility is a genetic mechanism that prevents self-pollination (i.e., pollination from same flower or other flowers of same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

iii. In chasmogamous flower length of anther and stigma plays a secondary role in autogamy. Therefore by placing anther and stigma at different position, in such a way that the pollen of a flower doesn’t come in contact with stigma of the same flower can prevent autogamy.

The correct sequential order in which the steps should be followed in the artificial hybridisation programme are:

(a) Selection of parents is done first.

(b) Emasculation – after parents are selected, in bisexual female flower anthers are removed from flowed bud before the anther dehisces, this is called Emasculation. In case of female parent producing unisexual female flowers Emasculation step is not needed.

(c) Bagging – Now the stigma is bagged with butter paper bag to prevent its contamination with unwanted pollen. This is called Bagging.

(d) Collection of pollen from male parent – Now mature pollens are collected from other male parent is done.

(e) Dusting the pollen on stigma – When the stigma of bagged flower attains receptivity, the collected mature pollen grains are dusted on the stigma.

(f) Re-bagging – Now again the flower is re-bagged till the fruits are developed.

i. Vivipary is defined, where the seed grows first to break through the seed coat then out of the fruit wall while still attached to the parent plant. But not many seeds can grow on single parent plant, hence they will fall to ground around the parent plant. Here in a struggle to get enough oxygen from the soil (mostly marshy lands), many seedlings die. Out of the left out seedlings as they grow they struggle to get enough sunlight, and many others don’t survive. So the numbers of offsprings’ are limited.

ii. Vivipary in aniamals – here embryo develops inside the body of female. A litter is the offspring at one birth of animals from the same mother. A litter usually are 3-8 offsprings together. The vivipary automatically limits the number of offsprings in a litter due to the reason that limited number of eggs are produced and fertilized during female reproductive cycle.

Yes, Self-incompatibility imposes restrictions on autogamy.

Self-incompatibility is a genetic mechanism that prevents self-pollination (i.e., pollination from same flower or other flowers of same plant) from fertilizing the ovules. This is done by inhibiting pollen germination or pollen tube growth in the pistil.

The restrictions are imposed as continued self-pollination leads to inbreeding depression. And to prevent such type of depression self-incompatibility is used.

In Self-incompatible plants self-pollination (i.e., pollination from same flower or other flowers of same plant) doesn’t occur. So in such case cross-pollination particularly xenogamy takes place.

That is transfer of pollen grains from anther to the stigma of a different plant.

This is longitudinal section of embryo of grass which is monocot in nature.

Embryos of monocotyledons possess only one cotyledon. In the grass family the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in a sheath called Coleorrhiza. The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.

Presence of more than one embryo in a seed is known as polyembryony.

It can be commercially exploitd in many ways, out of which few are listed below:

i. It helps in plant breeding

ii. It helps in horticulture

iii. Plantlets obtained from these embryos are virus free and more vigorous, resulting in high productivity.

iv. All these embryos can be isolated and grown on embryo culture to produce clones.

Yes parthenocarpy and apomixis different phenomena.

The zygote begins to divide only after the division of Primary endosperm cell (PEC) because of the following reasons:

i. Zygote needs nourishment during its development.

ii. The primary endosperm cell (PEC) forms the endosperm where food materials are stored for the developing embryo.

iii. A mature, fertilised embryo sac offers very little nourishment to the zygote.

iii. PEC divides and generates the endosperm tissue which nourishes the zygote.

So the zygote begins to divide only after the division of Primary endosperm cell (PEC) is an adaptation to provide assured nutrition to the developing embryo.

The generative cell in pollen grain divides mitotically to give rise to the two male gametes before pollen grains are shed. This is 3-celled stage i.e., one vegetative cell and two male gametes. And as the pollen grains are already divided this will not divide again in the pollen tube. So the pollen tube carries two male gametes from the beginning.

Pollen grains in 2-celled stage just have 2 cells namely vegetative cell and generative cell. As here division of generative cell did not take place, hence the generative cell of two-celled pollen divides in the pollen tube.

The required labelled diagram is as shown below with necessary labels.

The different stages of embryo development in a dicot are as shown below:

Chasmogamous flower are flowers with exposed anthers and stigma.

All the following types of pollinations are possible in chasmogamous flowers are:

i. Autogamy or self-pollination - for autogamy to occur in chasmogamous flower both pollen and ovules should mature simultaneously. As pollen release and stigma reception should be synchronised for autogamy to take place.

ii. Dichogamy - For this type of pollination synchronisation of pollen release and stigma reception should be avoided that is by avoiding the simultaneous maturing of anther and stigma. In this type of pollination stigma and anther matures at different time. Examples of such type of flowers are Sunflower, Datura, Solamium, etc.

iii. Self-incompatibility or self-sterility is a genetic mechanism that prevents self-pollination (i.e., pollination from same flower or other flowers of same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

iv. In chasmogamous flower length of anther and stigma plays a secondary role in autogamy. Therefore by placing anther and stigma at different position, in such a way that the pollen of a flower doesn’t come in contact with stigma of the same flower can prevent autogamy.

v. Geitonogamy – here pollen and pistil mature at different times but are self – compatible.

vi. Xenogamy – here pollen grains from the flower of one plant to the stigma of another plant. This pollination brings genetically different types of pollen grains.

The below is the labelled diagram of mature angiosperm embryo sac.

Parts of mature angiosperm embryo sac:

i. Antipodal: They are 3-celled at the chalazal end of the ovule. The large central cell is formed by the fusion of 2-polar nuclei. They are also referred as vegetative cells of the embryo sac.

ii. Polar Nuclei: One nucleus from each end of the chalazal end and micropylar end are called Polar Nuclei.

iii. Central cell: It is the largest cell, formed by the fusion of two polar nuclei. It is also known as secondary nucleus. It shows central vacuole and 2 haploid polar nuclei.

iv. Egg Apparatus: It includes one large egg cell, 2 synergids and filiform apparatus. Three cells present towards the micropylar end grouped together constitute egg apparatus.

The antipodal is 3-celled with 3 nucleus. Polar nuclei and Central cell constitute 2 nucleus and it is 1-celled. Egg apparatus is 3-celled with 3 nucleus. So a mature angiosperm embryo sac is 7-celled and 8-nucleate.

Role of synergids:

i. Synergids are essential for angiosperm reproduction.

ii. Synergids helps in growth of pollen tubes towards egg.

iii. Synergids helps in the nutrition of embryo sac by absorption and transport of food from nucleus through their filiform apparatus.

iv. The filiform apparatus (special cellular thickening of synergids) helps in guiding the pollen tube to enter one of the synergids.

The diagram of a microsporangium and labelling of its wall layers is as follows:

Role of endothecium:

i. Endothecium helps in dehiscence of anther.

ii. It is fibrous layer of cells lying below the epidermis of the wall of the anther.

iii. At maturity these cells loose water, contract and help in dehiscence of pollen sac.

- Apomixis is development of a seeds without prior fertilization. Example: replacement of the flower by bulbils and replacement of seed by a plant.

- It is observed in few flowering.

- Apomixis is a form of asexual reproduction that mimics sexual reproduction.

- Apomixis avoids the processes of meiosis and fertilization, leading to embryo development.

- The offsprings produced in this case are genetically identical to the parent plant.

- Embryo sacs of some apomictic species appear normal but contain diploid cells due to absence of meiotic division at the megaspore mother cell stage.

- As the megaspore mother cell has a diploid nucleus, if it undergoes mitosis instead of meiotic, all the resulting nuclei and cells will be diploid in nature.

- Apomixis is common in plants like Asteraceae, Poaceae.