BIOLOGY 2 Marks Questions

Self-incompatibility is a genetic mechanism to prevent self pollen from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
Self-pollination does not lead to seed formation in self-incompatible species because of interaction between the chemicals of the pollen and those of the stigma.

Parthenocarpy is the process of developing fruits without involving the process of fertilization or seed formation. Therefore, the seedless varieties of economically important fruits such as orange, lemon, water melon etc. are produced using this technique. This technique involves inducing fruit formation by the application of plant growth hormones such as auxins.

Sporogenous tissue → pollen mother cell → microspore tetrad → Pollen grain → male gamete.

Continued self-pollination decreases the vigour and vitality of a particular race. Thus, flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination.
Dichogamy and self-sterility are two most common devices that ensure cross-pollination.
Dichogamy: Maturation of anther and stigma at different times in a bisexual flower prevent self-pollination.
Self-sterility (or self-incompatibility): Due to the presence of self-sterile gene in some flowers, pollen grains do not germinate on the stigma of that flowers. e.g.,- tobacco, potato.

The male gametophyte or the pollen grain develops inside the pollen chamber of the anther, whereas the female gametophyte (also known as the embryo sac) develops inside the nucellus of the ovule from the functional megaspore.

Botanically ripened ovary is called a true fruit. The fruits in which thalamus and other floral parts develop along with the ovary are called false fruits. For example – apple, strawberry, cashew etc. In apple the main edible portion of the fruit is the fleshy thalamus. Ovary forms the fruit after fertilization or without fertilization in parthenocarpic fruits.

In many flowering plants, only one out of the four megaspores enlarges and develops into female gametophyte or embryo sac. The other three megaspores degenerate. This type of embryo sac formation is called as monosporic type of development.

The zygote is dormant for sometime in fertilized ovule the embryo develops only after the formation of the endosperm as endosperm is the source of nutrition for the developing embryo. Thus, the zygote waits for endosperm formation.

Pea - flowers of pea plants are bisexual, monoecious/self-pollinated (to produce pods with viable seeds).
Papaya - Dioecious plant/unisexual plant bearing male and female flowers on separate plants, unable to produce viable seeds as there is no cross pollination/it could be a male plant which is unable to produce fruit and seeds.

An unusual form of reproduction see in flowering plants, in which one sperm nucleus fases with an egg to form a Zygote there both are haploid (homogenous cells), and after fusion they make a diploid cell, like zygote. Simultaneous fusion of the second male gamete with the polar nuclei, resulting in a primary endosperm nucleus, which is often triploid, 3n’, a unique characteristic all angiosperms.

Advantage- Self-pollination assured/seed production assured.
Disadvantage- Least variations/leading to inbreeding depression.

Some of the nucellar cells, surrounding embryo sac divide, protrude into the embryo sac, develop into the embroys thus each seed contains many embryos of different sizes.

Any one diagram one label from seed, endocarp & mesocarp.
Thalamus also contributes to fruit formation.

Since there is no fertilisation in apomixis it resemble asexual reproduction, and development of embryo/seed/fruit formation is mimicing sexual reproduction.
In Citrus/ Mango some of the nucellar cells surrounding the embryo sac, act as diploid egg cell, which are formed without reduction division, and develop into embryo, without fertilisation.

Synergids = haploid, egg = haploid, polar nuclei = haploid, antipodals = haploid, all types of cell of female gametophyte are haploid.

A microspore mother cell/PMC on meiosis forms 4 functional pollen grains/male gametophyte.
A megaspore mother cell/MMC on meiosis also forms four megaspores but out of it only one is functional and other three degenerate.

Exine- Sporopollenin.
Intine- Cellulose and pectin.
Role- Most resistant organic material/can withstand high temperature, acid and alkali.

Geitonogamy is a kind of pollination in which the pollen grains are transferred from the anther of a flower to the stigma of another flower on the same plant.Geitonogamous flowering plants are genetically autogamous as the gametes come from the same parent plant but since the pollen grains are being transferred to a different flower which needs a pollinating agent, it is functionally an example of cross-pollination.

Present in exine, of pollen/pollen grain.
Sporopollenin is the most resistant organic material hence protects the pollen/gamete/gametophyte, provides protection to pollen from unfavourable conditions or chemicals (acids, enzymes, high temperature etc.)

Albuminous- (with residual) endosperm is not completely used up during embryonic development eg. wheat/maize/castor/sunflower.
Non-albuminous- (without residual) endosperm is completely consumed during embryonic development eg. pea/groundnut.

Zygote formed by the fusion of male gamete (n) and egg cell (n), hence diploid (2n),
Primary endosperm nucleus formed by the fusion of two polar nuclei/secondary nucleus (n + n) and male gamete (n), hence triploid (3n).

Potato plant bears bisexual flowers. This is the reason that the process of emasculation, that is, removal of stamen will be performed in potato and not in papaya that bears unisexual flowers. However, bagging is performed in both the flowers because the stigma needs to be protected from getting pollinated with undesired pollen.

Pollination is transfer of pollen grains from anthers to the stigma of a flower. Pollination is prerequisite for fertilisation, events after fertilisation like endosperm development, seed setting and fruit formation. Thus, pollination plays an important role in the growth and development of angiosperms.

Endosperm forms the kernel of coconut.
The coconut water is free-nuclear endosperm whereas kernel is cellular endosperm.

Pollen grains or microspores are generally spherical in outline. The wall of pollen grain is called sporoderm, which is made up of two layers-An outer thick exine and an inner thin intine.
The exine is made up of sporopollenin which is biologically the most resistant substance and can withstand high temperature and strong acids and alkali. At one or more places, the exine is very thin or absent. These regions are called germ pores.
Intine is continuous layer made up of cellulose and pectin. A mature pollen grain has two cells-a vegetative cell and a generative cell. The vegetative cell is bigger and has food reserve and a large irregular nucleus. The generative cell is small, spindle shaped and floats in the cytoplasm of vegetative cell. In dicots, pollen grains are with three germ pores, while in monocots, they are with single germinal furrow.

In both cases pollen grains come from the same plant. So they are genetically similar.

Since banana fruit is formed from the ovary, it is true fruit. It is parthenocarpic because the ovary develops into fruit without fertilization

The integuments of ovule get transformed into seed coats, outer testa and inner tegmen. Anatropous ovule.

At the time of shedding wheat pollen consists of one vegetative and two male gametes, which is the 3celled stage. While pea pollen consists of one vegetative and one generative cell which is the 2-celled stage. Germ pores are present on the exine where sporpollenin is absent.

Pollen analysis has become a standard tool: Government agencies analyze the pollen content of fake Viagra and other prescription drugs to determine where they came from. Museums use pollen to authenticate paintings by master artists. Oil companies study fossil pollen to locate hydrocarbon deposits. Archaeologists study pollen to learn how ancient human communities used plants, and even the seasons at which they occupied a particular site. And paleobotanists study pollen evidence to reconstruct former environments, thousands or even millions of years into the past.

Geitonogamous flowers are genetically autogamous because both male and female flowers are borne on the same flower. They are functionally cross-pollinated because the pollen from one flower is transferred to the stigma of a different flower.

A bisexual flower is emasculated to prevent self-pollination in the flower and it is bagged after emasculation to prevent contact of unwanted pollen grain with the stigma of the flower.

1.  

S. No.	Hypocotyl	Epicotyl
1.	The portion of the embryonal axis which lies below the cotyledon in a dicot embryo is known as the hypocotyl.	The portion of the embryonal axis which lies above the cotyledon in a dicot embryo is known as the epicotyl.
2.	It terminates with the radicle.	It terminates with the plumule.

2.  

S. No.	protandry	protogyny
1.	protandry is (botany) the condition of flowers whose male parts mature before the female ones.	protogyny is (botany) the condition of flowers whose female parts mature before the male ones.
2.	the condition in which an organism begins life as a male and then changes into a female.	the condition in which an organism begins life as a female and then changes into a male.

These are the stigma and style which wave in the wind to trap pollen grains.

Primary Endosperm Nucleus (PEN) shows triploid condition. When one of the male gametes fuses with the polar nuclei, it results in the formation of triploid PEN.

• Emasculation is necessary to prevent the self-pollen grains falling on the stigma in case of bisexual flowers.
• Emasculation is not needed when the female parent chosen bears unisexual, i.e. female flowers.

• Apple is categorised as false fruit, because the thalamus, a part other than the ovary, also contributes to fruit formation.
• When any part of the flower other than the ovary becomes a part of the fruit, the fruit is said to be a false fruit.
• The other examples are strawberry and cashew.

Nucellus:

• The food reserve in the nucellus is used by the developing megaspore as well as the embryo.
• The residual persistant nucellus in the mature seed, is called perisperm.

Endosperm:

• The food reserve in the endosperm is used by the embryo during seed development.
• The food reserve in the residual endosperm (in endospermic seeds like castor) is used by the embryo during seed germination.

Endosperm Development:

• Endosperm develops first followed by an embryo.
• Endosperm develops from PEN.
• The PEN undergoes successive nuclear divisions to give rise to free nuclei and this stage of endosperm development is called free nuclear endosperm.
• Subsequently, cell wall is formed on the periphery and endosperm becomes cellular.
• This division is followed by cytokinesis and thus endosperm becomes cellular. It is now called cellular endosperm.

Functions:

• The cells of endosperm tissue are triploid and filled with reserve food material to nourish the developing embryo.
• The water of tender coconut in the centre is free-nuclear endosperm and white kernel in the outer part is the multicellular endosperm.
• The endosperm may be completely consumed by the developing embryo, e.g., pea, beans, or it may persist in mature seed, e.g., coconut.

Yes, if the embryo develops from the cells of nucellus or integument it will be diploid.

(b) Selection of parents → (e) Emasculation → (c) Bagging → (f) Collection of pollen from male parent → (d) Dusting the pollen on stigma → (a) Re-bagging.

X – Meiosis (Reduction division)
A – Mitosis
B – Mitosis
C – Mitosis
D – Cells reorganised as polar nuclei, antipodals and egg apparatus.

Megaspore mother cell is diploid. In all the plants undergoing sexual reproduction their gamets must be haploid.
MMC undergoes meosis to produce haploid egg. This egg fuses with a sperm from pollen grain to produce seed.

Self-incompatibility imposes restriction to antogamy. The device to prevent inbreeding is self - incompatibility or self - sterlity. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil. Self incompatiblity is overcome by mixed pollination.

Exine-It can withstand high temperature/ strong acids. Intine-It is thin layer made up of cellulose and pectin.

The inability of seeds to germinate even if external conditions are favourable is called seed dormancy. Seed dormancy may be due to one or more of the following reasons:

1. Hard and impermeable seed coats.
2. Presence of certain inhibitory substances such as abscisic acid (ABA), coumarin etc.

Significance:

1. It helps the seeds to tide over the unfavourable period.
2. It enables the seeds to be disseminated safely and help them to germinate, when the environmental conditions are favourable.

1. Selection of parents: Only those plants should be selected which have desired traits.
2. Crossing over: Pollen grains from selected male plant is collected and transferred to the female plant after which it is bagged.

Parthenocarpic fruits are formed when ovary develops into fruit without fertilisation. Apomictic seeds are formed when formation of seeds take place without fertilisation.

1. Plumule: forms the shoot.
2. Cotyledons: Supply the food material.
3. Radicle: Grows into the root.

A pollen tube carrying two male gametes, enter a ovule and release two male gametes which fuse with two different structures in a embryo sac (the egg and two polar nuclei). Hence for fertilisation of 10 ovules, 10 pollen grains are required and not 5 pollen grains.

One of the male gametes fuses with the female gamete and forms the embryo. The embryo subsequently develops into a new plant. Another male gamete fuses with polar nuclei and eventually forms endosperm. Endosperm supplies food to the developing embryo.

The pollen grains can be best preserved as fossils. This is possible due to the effect of sporopollenin. It is the most resistant biological substance known and can withstand high temperature as well as action of alkali or acids.

A typical angiospermic embryo sac is 8-nucleated and 7-celled.

Lupinus arcticus germinated and flowered after about 10,000 years of dormancy. This happened in Arctic tundra.

The phenomenon of asexual reproduction that mimics sexual reproduction as it forms seed without fertilisation is called agamospermy.
Parthenogenesis refers to the development of unfertilised ovule into an adult individual. On the other hand, parthenocarpy is the phenomenon of formation of fruits without fertilisation of an ovary.

The triploid tissue in the ovule is the endosperm. Its triploid condition is attained due to the fusion of two polar nuclei and one nucleus of male gamete (also referred to as triple fusion).

In geitonogamy pollen grains are transferred from the anther to the stigma of another flower, of the same plant whereas in xenogamy pollen grains are transferred from the anther, to the stigma of a different flower.

The development of endosperm (preceding the embryo) takes place from primary endosperm nucleus (PEN) in both, castor and groundnut.
The developing embryo derives nutrition from endosperm.
PEN undergoes repeated division to give free nuclei. Subsequently cell wall is formed and endosperm becomes cellular. At this stage endosperm is retained in castor or is not fully consumed but in groundnut endosperm is consumed by growing embryo.

1. In Vallisneria, the male flowers are released on the surface of water and female flowers reach the surface by the long stalk.
2. Pollen grains are protected from wetting by a mucilaginous covering.

A bisexual flower is emasculated at unopened stage to prevent self-pollination in the flower and it is bagged after emasculation to prevent contact of unwanted pollen grain with the stigma of the flower. Artificial pollination is then performed when the stigma is ready and the flower is rebagged.

The wall of the ovary that develops into wall of the fruit is called pericarp.
Functions:

1. Protects the seed till its maturity.
2. Helps in dispersal.

• The plant Yucca and the moth, both of them cannot complete their life cycles without each other.
• The moth deposits its eggs in the locule of the ovary; during this, the flower gets pollinated by the moth.
• The larvae of the moth come out of the eggs, when the seeds start developing; some of the seeds are eaten by the larvae as food.

Depending on the source of pollen grain, pollination can be classified into:

1. Autogamy: It is the transfer of pollen grain from anther to the stigma of the same flower.
2. Geitonogamy: lt is the transfer of pollen grains from anther of one flower to the stigma of another flower on the same plant. Geitonogamy is functionally cross-pollination involving pollinating agent, but genetically it is equivalent to autogamy since, the pollen grains come from the same plant.
3. Xenogamy: It is the transfer of Pollen grains from anther to the stigma of different plants of same species. It brings genetically different types of pollen grains to the stigma.

Figure ‘a’ shows multicarpellary syncarpous condition and figure ‘b’ shows multicarpellary apocarpous condition.

1. Microspore mother cell = 16 (Diploid)
2. Endosperm cell = 24 (Triploid)

The zygote needs nourishment during its development. As the mature, fertilised embryo sac offers very little nourishment to the zygote, the primary endosperm cell (PEC) divides and generates the endosperm tissue which nourishes the zygote. Hence, the zygote always divides after division of PEC.

A zygote is formed by the fusion of haploid male gamete with the haploid egg to form a diploid cell; whereas, primary endosperm nucleus (PEN) is formed by the fusion of haploid male gamete with two haploid polar nuclei, forming a triploid nucleus.

• Yes, it is because, both are fleshy and juicy and are eaten raw.
• In apple, a false fruit, the thalamus becomes edible while in mango, a true fruit, the mesocarp is edible.

Difference between pericarp and perisperm are:

S. No.	Pericarp	Perisperm
1.	It is the covering of fruits that develops from ovary wall.	It is unused nucellus in the seed.
2.	It is a part of the fruit.	It is a part of the seed.
3.	It is usually fleshy or dry.	It is usually dry.
4.	It is found in all fruits.	It is present in only a few seeds.

It is a mode of cross pollination in which the pollen grains are transferred to the stigma through the insects like bees, butterflies, etc.
Features of Entomophilous flowers:

• The flowers are generally large, colourful, fragrant and rich in nectar.
• Pollen grains are often surrounded by a sticky substance, pollenkitt.

1. e.g. Michelia
2. e.g. Papaver

Germ pores allow germination of pollen grain and formation of pollen tubes.

a = Endothecium,
b = Middle layers,
c = Tapetum.

The synergids have special cellular thickenings at the micropylar tip called filiform apparatus. They guide the entry of pollen tubes into the synergids.

In about 60% of angiosperms, pollen grains ale shed at the 2-celled stage. However, in about 40% flowering plants, the generative cell may further divide mitotically to give rise to two male gametes and pollen grains are shed at this 3-celled stage.
The placement of cells within the pollen grain when shed at 2-celled stage can be visualised as,

Advantage: Self-pollination is assured, thus ensuring seed formation.
Disadvantage: Least variations observed and it leads to inbreeding depression.

An unfertilised embryo sac of angiosperm is composed of 8-nuclei and 7 cells. Among 8 nuclei, 6 are enclosed by cell walls and organised into cell, while the remaining 2 nuclei (called polar nuclei) are situated above the egg apparatus. Out of 6 cells, 3 are grouped at micropylar end and constitute the egg apparatus, made up of 2 synergids and 1 egg cell. The other 3 are located at chalazal end and are called antipodals. All the cells of the embryo sac are haptoid except the central cell which is first binucleate and then becomes diploid due to fusion of polar nuclei.

Male gametophytes are formed by meiosis of single microspore mother cell whereas female gametophytes are formed by meiosis of single megaspores mother cell to produce 4 megaspores, out of which 3 degenerate and only one survives. The surviving megaspore undergoes mitotic division to form the female gametophyte.

In flowering plants, microspore mother cells undergo meiosis and form microspore tetrad. After attaining maturity, these microspores separate from each other and each microspore develops into a male gametophyte. On the other hand, megaspore mother cell divides by meiotic division to produce four megaspores. From these, three degenerate while, the one undergoes further development and mitotic divisions to produce female gametophyte.

One male gamete fuses with egg cell while second male gamete fuses with central cell during the process of double fertilisation.

X- Meiosis (reduction division).
A- Mitosis.
B- Mitosis.
C- Mitosis.
D- Cells reorganised as polar nuclei, antipodals and egg apparatus.

• It is a case of polyembryony, the phenomenon of occurrence of more than one embryo in a seed.
• Some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into embroys.

1. Sporogenous tissue: It forms pollen grains.
2. Tapetum: It provides nourishment to the developing pollen grains.

Banana develops from an ovary without fertilisation having non-viable seeds so it is called parthenocarpic fruit. An orange contain seeds with more than one embryo thus, it shows polyembryony.

Tapetum provides nourishment to the developing pollen grains. The tapetal cells also secrete Ubisch granules that provide sporopollenin and other materials for exine formation.

Many insects may consume pollen or the nectar without bringing about pollination. These floral visitors are referred to as pollen/ nectar robbers.

Geitonogamy is functionally a cross-pollination, involving a pollinating agent. But genetically, it is similar to autogamy since, the pollen grains come from the same plant. So, it is also referred to as genetical autogamy.

S.No.	Albuminous seed		Non-albuminous seed
1.	In some seeds, the endosperm persists as food storage tissue.		If the endosperm is completely consumed by the developing embryo before seed maturation, the seeds are called non-albuminous.
2.	Example: Castor		Example: Pea

In apple only the thalamus (along with ovary) portion contributes to fruit. Therefore, it is a false fruit. Mango develops only from the ovary, therefore it is a true fruit.
Banana develops from ovary but without fertilisation. The method is known as parthenocarpy. Since there is no fertilisation, no seeds are formed in banana.

When the flower chosen is unisexual (female), there is no need for emasculation. Yet bagging is necessary to prevent contamination of the stig pollen grains.

Pollen tube carries two male gametes.
Pollen tube, after reaching the ovary, enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus which guides the entry of pollen tube into egg cell.

In orange, the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into a number of embryos of different sizes.

Structure of Pollen Grain: Pollen grains are generally spherical and are about 25-50 micrometres in diameter. It has has an outer layer exine and inner layer intine. Mature Pollen grain contains a bigger vegetative cell and a smaller generative cell.
Germination of Pollen Grain: The process of development of pollen tube is called germination. Hydration of stigma results in elongation of tube cells into the pollen tube. Pollen tube then grows towards the ovule for further process.

'A' is chasmogamous flower while 'B' is cleistogamous flower. A bisexual flower which normally open is called chasmogamous flower. Cleistogamous flowers do not open at all.
Cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma.
In a normal flower which opens and exposes the anthers and stigma complete autogamy is rather rare. Chasmogamous flower may show autogamy, geitonogamy or xenogamy.

Ways by which apomictic seeds develop are:

1. A diploid egg is formed without reduction division which develops into embryo without fertilisation.
2. Some cells of the nucellus, which are diploid in nature, start dividing and without fertilisation develop into embryo.

1. Guava, tomato, mango.
2. Groundnut, mustard, bean.

1. Parthenocarpic fruits are generally seedless or havenon-viable seeds.
2. In oranges, each seed has more than one embryo, i.e. polyembryony and seeds are viable.

The synergid cells are located in the female gametophyte and are essential for angiosperm reproduction. During the fertilization process, a pollen tube grows into one of the synergid cells, ceases growth, ruptures, and releases its two sperm cells into this cell.

The function of filiform apparatus is to guide entry of pollen tube and release of sperm cells.
Filiform apparatus: The synergid cell wall forming an extremely thick structure at the micropylar end is called the filiform apparatus. It bears several finger-like projections into the structure of synergid cytoplasm.

Coleorhiza protects the radical of (monocot) embryo.

• The fruits that are formed without fertilisation of the ovary, are called parthenocarpic fruits.
• They are generally seedless or seeds are not viable.

1. Pollens are produced in large numbers.
2. The sporopollenin in exine protects the pollen from harsh conditions.

No it cannot be assumed so because fertilisation does takes place but the endosperm is consumed during development.

The zygote needs nourishment during its development. As the mature, fertilised embryo sac offers very little nourishment to the zygote, the PEC divides and generates the endosperm tissue which nourishes the zygote. Hence, the zygote always divides after division of PEC.

Viola (common pansy) and Commelina can produce both chasmogamous and cleistogamous flowers on the same plant. These flowers produce seeds by outcrossing and selfing, respectively. This mixed breeding system is considered a successful reploductive strategy.

Pollens are shed at either 2-celled or 3-celled stage. Cells are formed mitotically from haploid cells.

All hydrophytes are not pollinated by water. Those hydrophytes whose flowers emerge above the surface of water are pollinated by insects or wind, e.g. water hyacinth and water lily are pollinated by insects.

1. In a pollen grain, germ pores are present in the exine, where sporopollenin is absent.
2. In the two-celled pollen grain, the generative cell floats in the cytoplasm of the vegetative cell.

S.No	Vegetative cell	Generative cell
1.	It is bigger in size	It is smaller and floats in the cytoplasm of vegetative cell.
2.	It has food reserves.	It gives rise to two male gametes.

Seeds that are produced without fertilisation are referred to as apomictic.
Advantage: Desired characters are retained in offspring (progeny) as there is no segregation of characters in offspring (progeny). Seed production is assured even in absence of pollinators. Apomictic seeds are economical as they can be used to grow crops year after year.
Disadvantage: Cannot control accumulation of deleterious genetic mutation. These are usually restricted to narrow ecological niches and lack ability to adapt to changing environment.

S. No	Cells of tapetum	Cells of other wall layers
(i)	They have a dense cytoplasm.	They have a less dense cytoplasm.
(ii)	They often have more than one nucleus.	They have a single nucleus.
(iii)	They do not develop thickenings on their walls.	They may develop wall thickenings.
(iv)	Their function is to nourish the developing pollen grains.	They help in the dehiscence of the anther and liberation of pollen grains.

1. Male and female flowers are present on different plants.
2. The stamens and stigma of a bisexual flower mature at different times.
   1. Anthers mature earlier than the stigma and release pollens.
   2. The stigma matures earlier than the anther.
3. Flowers are self-sterile or self-incompatible.
4. Chasmogamous flowers are present with exposed stamens and stigma which facilitate cross-pollination.

The petals of the Ophrys resemble the female of a bee species in size, colour and odour. Male bee mistakes the Ophrys for female bee and tries to copulate. Few pollen grains adhered to the body of the male bee fall over stigma of the flower thereby leading to pollination showing sexual deceit.

In Vallisnaria, the female flower reaches the surface of water by a long stalk and the pollen grains are released on the surface of water. The pollen grains are carried passively by water currents and reach the stigma eventually.

S. No.	Dicot embryo	Monocot embryo
(i)	It has two cotyledons.	It has one cotyledon.
(ii)	Radicle and plumule are not covered with sheath.	Radicle is covered with coleorhiza and plumule is covered by coleoptile.

The diploid megaspore mother cell divides meiotically and forms four megaspores, which are usually arranged in a longitudinal row (linear tetrad). Out of four megaspores formed, three degenerate. The remaining one acts as functional megaspore (chalazal).
The nucleus of functional megaspore undergoes three successive mitotic divisions forming eight haploid nuclei, which get organised into three groups-three nuclei migrate to the micropylar end and form the egg apparatus (one egg cell, two synergids).

• Three migrate towards chalazal end and form antipodal cells.
• Two polar nuclei fuse to form a diploid secondary nucleus in the centre.

• Tapetum is the innermost wall layer of the microsporangium.
• It nourishes the developing pollen grains.

Characteristics of tapetal cells:

1. They possess dense cytoplasm.
2. They have more than one nucleus.

In 2-celled stage the mature pollen grain contains one generative and vegetative cells, whereas in 3-celled stage one vegetative cell and two male gamete cells are present.
The generative cell being small floats in the cytoplasm of the vegetative cell. The pollen grains are shed at this 2-celled stage.

The device to prevent inbreeding is self-incompatibility or self-sterlity. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

Albuminous seeds are those which retain a part of endosperm as it is not completely used up during embryo development. For example, in wheat and maize. In some seeds remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm. For example, in black pepper and beet.

1. Certain nucellar cells around the embryo sac divide, protrude into embryo sac and develop into embryos, e.g. Citrus, mango, etc.
2. The diploid egg cell of the embryo sac, formed without meiosis in the megaspore mother cell develops into embryo without fertilisation.

To maintain hybrid characters (year after year in a desired plant) and to avoid buying hybrid seeds every year (expensive seeds) farmers prefer using apomictic seeds.

Apomixis is the mechanism:

• It refers to the form of asexual reproduction that mimics sexual reproduction and seeds are formed without fertilisation.
• Since there will be no segregation of the hybrid characters in the progeny plants, and they will be maintained for a number of generations.

Double fertilization is the joining of a female gametophyte with two male gametes. One sperm fertilizes the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the embryo sac. This helps in providing enough nutrition to the embryo during seed germination.

Provides nourishment to the developing pollen grains.

When a pollen grain is shed at the three celled stage, it contains a vegetative cell and two male gametes. The generative cell divides into two male gametes.

In normal conditions, the fruit develops from the ovary. However, in apple the thalamus also contributes to fruit formation. That is why it is called a false fruit.
Banana is called a parthenocarpic fruit because it develops without fertilisation and is thus seedless.

In a 3-celled pollen, the generative cell has already divided and formed 2 male gametes. Hence, it will not divide again in the pollen tube. Since in a 2-celled pollen, the generative cell has not divided, it divides in the pollen tube.