NEET CHAPTER 1 SEXUAL REPRODUCTION IN FLOWERING PLANTS

NEET NCERT Class 12th 1st chapter’s(SEXUAL REPRODUCTION IN FLOWERING PLANTS) questions line by line. Every year in the NEET exam, 1-2 questions come from this chapter.

Introduction

Are we not lucky that plants reproduce sexually? The myriads of flowers that we enjoy gazing at, the scents and the perfumes that we swoon over, and the rich colors that attract us, are all there to aid sexual reproduction. Flowers exist not only for us to be used for our selfishness. All flowering plants show sexual reproduction. A look at the diversity of structures of the inflorescences, flowers, and floral parts, shows an amazing range of adaptations to ensure the formation of the end products of sexual reproduction, the fruits and seeds. In this chapter,
let us understand the morphology, structure, and processes of sexual reproduction in flowering plants (angiosperms).

Question 1:

What is the primary purpose of the myriads of flowers, scents, and colors in flowering plants?
a) To provide aesthetic pleasure to humans
b) For selfish reasons plants
c) As an aid to sexual reproduction
d) To attract insects for pollination

Question 2:

Which structures in flowering plants play a crucial role in ensuring the formation of fruits and seeds?
a) Leaves
b) Roots
c) Inflorescences
d) Flowers and floral parts

Question 3:

In the context of flowering plants, what is the primary significance of sexual reproduction?
a) Aesthetic diversity
b) Fruit production
c) Oxygen generation
d) Adaptation for survival

1.1 FLOWER – A FASCINATING ORGAN OF ANGIOSPERMS

Human beings have had an intimate relationship with flowers since time immemorial. Flowers are objects of aesthetic, ornamental, social, religious, and cultural value–they have always been used as symbols for conveying important human feelings such as love, affection, happiness, grief, mourning, etc. List at least five flowers of ornamental value commonly cultivated in homes and gardens. Find out the names of five more flowers used in social and cultural celebrations in your family.

Have you heard of floriculture – what does it refer to? To a biologist, flowers are morphological and embryological marvels and the sites of sexual reproduction. In earlier classes, you have read the various parts of a flower. Figure 1.1 will help you recall the parts of a typical flower. Can you name the two parts in a flower where the two most important units of sexual reproduction develop?

Question 1:

What is the primary significance of flowers to human beings?
a) Aesthetic and ornamental value
b) Medicinal purposes
c) Source of food
d) None of the above

Question 2:

List at least five flowers commonly cultivated at homes and in gardens for ornamental purposes.

a) Rose, Lily, Sunflower, Tulip, Orchid
b) Marigold, Jasmine, Daffodil, Poppy, Lotus
c) Carnation, Lavender, Pansy, Chrysanthemum, Daisy
d) Petunia, Begonia, Geranium, Zinnia, Aster

Question 3:

What role do flowers play in conveying human emotions?
a) They have no role in conveying emotions
b) Symbolic representation of feelings such as love, affection, and grief
c) Used for medicinal purposes
d) Representing cultural traditions

Question 4:

What does floriculture refer to?
a) Study of flower biology
b) The cultivation of flowers for ornamental and commercial purposes
c) Arranging flowers in a garden
d) The study of floral arrangements in various cultures

Question 5:

In a flower, which two parts are the primary sites of sexual reproduction?
a) Sepals and petals
b) Stamen and Carpel
c) Receptacle and pedicel
d) Anther and filament

Question 6:

Why are flowers considered morphological and embryological marvels by biologists?
a) Due to their role in photosynthesis
b) Because of their vibrant colors
c) Their significance in sexual reproduction
d) None of the above

1.2 PRE-FERTILISATION: STRUCTURES AND EVENTS

Much before the actual flower is seen on a plant, the decision that the plant is going to flower has taken place. Several hormonal and structural changes are initiated which lead to the differentiation and further development of the floral primordium. Inflorescences are formed which bear the floral buds and then the flowers. In the flower, the male and female reproductive structures, the androecium and the gynoecium differentiate and develop.
You would recollect that the androecium consists of a whorl of stamens representing the male reproductive organ and the gynoecium represents the female reproductive organ.

Question 1:

What initiates the decision of a plant to flower?
a) Presence of sunlight
b) Hormonal and structural changes
c) Water availability
d) Soil nutrients

Question 2:

What is the term for the structures that bear the floral buds and flowers?
a) Sepals
b) Petals
c) Inflorescences
d) Stamens

Question 3:

Differentiate between the androecium and the gynoecium in a flower.
a) Androecium is the male reproductive organ, and gynoecium is the female reproductive organ.
b) Androecium is the female reproductive organ, and gynoecium is the male reproductive organ.
c) Androecium represents both male and female reproductive organs.
d) Gynoecium represents both male and female reproductive organs.

1.2.1 Stamen, Microsporangium and Pollen Grain(A)

Figure 1.2a shows the two parts of a typical stamen – the long and slender stalk called the filament, and the terminal generally bilobed structure called the anther. The proximal end of the filament is attached to the thalamus or the petal of the flower. The number and length of stamens are variable in flowers of different species. If you were to collect a
stamen each from ten flowers (each from a different species) and arrange them on a slide, you would be able to appreciate the large variation in size seen in nature. Careful observation of each stamen under a dissecting microscope and making neat diagrams would elucidate the range in shape and attachment of anthers in different flowers.

A typical angiosperm anther is bi-lobed with each lobe having two theca, i.e., they are dithecous (Figure 1.2b). Often a longitudinal groove runs lengthwise separating the theca. Let us understand the various types of tissues and their organization in the transverse section of an anther (Figure 1.3a). The bilobed nature of an anther is very distinct in the transverse section of the anther. The anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe. The microsporangia develop further and become pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains.

Question 1:

What are the two parts of a typical stamen?
a) Sepal and petal
b) Filament and anther
c) Pistil and ovary
d) Style and stigma

Question 2:

What is the proximal end of the filament attached to?
a) Filament of another stamen
b) Ovary
c) Thalamus or petal of the flower
d) Anther

Question 3:

How is the anther of a typical angiosperm structured?
a) Unilobed with one theca
b) Trilobed with three theca
c) Bilobed with two theca
d) Multilobed with multiple theca

Question 4:

What is the characteristic feature of an angiosperm anther in the transverse section?
a) Bilobed structure
b) Tetragonal shape
c) Presence of sepals
d) Unilocular nature

Question 5:

What develops into pollen sacs in the anther?
a) Ovules
b) Microsporangia
c) Sepals
d) Petals

Question 6:

How are the microsporangia arranged in an anther?
a) Singular, occupying the center
b) Paired, one in each corner of the anther
c) Tetragonal, forming the sides of the anther
d) Circular, surrounding the center of the anther

1.2.1 Stamen, Microsporangium and Pollen Grain(B)

Structure of microsporangium: In a transverse section, a typical microsporangium appears near circular in outline. It is generally surrounded by four wall layers (Figure 1.3b)– the epidermis, endothecium, middle layers, and the tapetum. The outer three wall layers perform the function of protection and help in the dehiscence of another to release the pollen.

The innermost wall layer is the tapetum. It nourishes the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus. Can you think of how tapetal cells could become bi-nucleate? When the anther is young, a group of compactly arranged homogenous
cells called the sporogenous tissue to occupy the center of each microsporangium.
Microsporogenesis: As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads. What would be the ploidy of the cells of the tetrad?

Each cell of the sporogenous tissue is capable of giving rise to a
microspore tetrad. Each one is a potential pollen or microspore mother
cell. The process of formation of microspores from a pollen mother cell (PMC)
through meiosis is called microsporogenesis. The microspores, as they
are formed, are arranged in a cluster of four cells–the microspore tetrad
(Figure 1.3a). As the anthers mature and dehydrate, the microspores
dissociate from each other and develop into pollen grains (Figure 1.3 b).
Inside each microsporangium several thousands of microspores or pollen
grains are formed that are released with the dehiscence of anther (Figure 1.3c).

Question 1:

What are the four wall layers surrounding a typical microsporangium?
a) Epidermis, mesophyll, endodermis, pericycle
b) Epidermis, endothecium, middle layers, tapetum
c) Cuticle, cortex, vascular bundle, pith
d) Stomata, guard cells, parenchyma, sieve tube

Question 2:

Which wall layer of the microsporangium nourishes the developing pollen grains?
a) Epidermis
b) Endothecium
c) Middle layers
d) Tapetum

Question 3:

How do tapetal cells become bi-nucleate?
a) By mitotic division
b) By meiotic division
c) By binary fission
d) By conjugation

Question 4:

What is the ploidy of the cells of the microspore tetrad?
a) Diploid
b) Haploid
c) Triploid
d) Tetraploid

Question 5:

What is the process of formation of microspores from a pollen mother cell (PMC) through meiosis called?
a) Microsporogenesis
b) Pollination
c) Megaspore formation
d) Anther dehiscence

1.2.1 Stamen, Microsporangium and Pollen Grain(C)

Pollen grain: The pollen grains represent the male gametophytes. If you touch the opened anthers of Hibiscus or any other flower you would find deposition of yellowish powdery pollen grains on your fingers. Sprinkle these grains on a drop of water taken on a glass slide and observe under a microscope. You will be amazed at the variety of architecture –sizes, shapes, colors, and designs – seen on the pollen grains from different species (Figure 1.4).

Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has a prominent two-layered wall. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkalis. No enzyme that degrades sporopollenin is so far known. Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent. Pollen grains are well-preserved as fossils because of the presence of sporopollenin.

The exine exhibits a fascinating array of patterns and designs. Why do you think the exine should be hard? What is the function of germ pores? The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin. The cytoplasm of pollen grain is surrounded by a plasma membrane. When the pollen grain is mature it contains two cells, the vegetative cell and the generative cell (Figure 1.5b). The vegetative cell is bigger, has an abundant food reserve, and a large irregularly shaped nucleus.

The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus. In over 60 percent of angiosperms, pollen grains are shed at this 2-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).

Pollen grains of many species cause severe allergies and bronchial afflictions in some people often leading to chronic respiratory disorders– asthma, bronchitis, etc.

It may be mentioned that Parthenium or carrot grass that came into India as a contaminant with imported wheat,has become ubiquitous in occurrence and causes pollen allergy. Pollen grains are rich in nutrients. It has become a fashion in recent years to use pollen tablets as food supplements. In Western countries, a large number of pollen products in the form of tablets and syrups are available in the market. Pollen consumption has been claimed to increase the performance of athletes and racehorses (Figure 1.6). When once they are shed, pollen grains have to land on the stigma before they lose viability if they have to bring about fertilization.

How long do you think the pollen grains retain viability? The period for which pollen grains remain viable is highly variable and to some extent depends on the prevailing temperature and humidity. In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes of their release, and in some members of Rosaceae, Leguminoseae, and Solanaceae, they maintain viability for months. You may have heard of storing semen/sperms of many animals including humans for artificial insemination. It is possible to store pollen grains of a large number of species for years in liquid nitrogen (-1960C). Such stored pollen can be used as pollen banks,
similar to seed banks, in crop breeding programs.

Question 1:

What is the outer layer of the pollen grain called, and what is it made of?
a) Intine, cellulose, and pectin
b) Exine, sporopollenin
c) Plasma membrane, lipids
d) Germ pore, sporopollenin

Question 2:

What is the function of sporopollenin in the pollen grain?
a) It provides nutrients to the developing pollen grain
b) It makes the pollen grain hard and resistant
c) It acts as a protective layer against pathogens
d) It helps in the germination of pollen grains

Question 3:

What is the inner layer of the pollen grain called?
a) Exine
b) Intine
c) Plasma membrane
d) Germ pore

Question 4:

How many cells are present in a mature pollen grain?
a) One
b) Two
c) Three
d) Four

Question 5:

What is the name of the larger cell in a mature pollen grain?
a) Generative cell
b) Vegetative cell
c) Sporophyte cell
d) Gametophyte cell

Question 6:

In how many angiosperms do pollen grains reach the 2-celled stage before shedding?
a) 20%
b) 40%
c) 60%
d) 80%

Question 7:

What health issues can be caused by pollen grains in some people?
a) Skin rashes
b) Digestive problems
c) Respiratory disorders
d) Vision impairment

Question 8:

Which plant, introduced as a contaminant with wheat, causes pollen allergy in India?
a) Parthenium
b) Hibiscus
c) Sunflower
d) Rose

Question 9:

What is the primary reason for using pollen tablets as food supplements?
a) To cure allergies
b) To enhance athletic performance
c) To prevent respiratory disorders
d) To improve vision

Question 10:

In which countries are a large number of pollen products available in the market?
a) Asian countries
b) African countries
c) Western countries
d) South American countries

Question 11:

What has been claimed about the consumption of pollen products?
a) It causes allergies
b) It leads to weight gain
c) It increases performance in athletes
d) It induces sleep

Question 12:

How long do pollen grains of rice and wheat typically retain viability?
a) 5 minutes
b) 30 minutes
c) 1 hour
d) 24 hours

Question 13:

In some species, for how long can pollen grains maintain viability?
a) Weeks
b) Months
c) Years
d) Decades

Question 14:

What is the temperature of liquid nitrogen used for storing pollen grains?
a) -50°C
b) -80°C
c) -196°C
d) -273°C

Question 15:

What is the term used for storing pollen grains in liquid nitrogen for crop breeding?
a) Seed bank
b) Sperm bank
c) Pollen bank
d) Gamete bank

Question 16:

How do pollen banks compare to seed banks in crop breeding?
a) Pollen banks are more common
b) Seed banks are more effective
c) Both are equally important
d) Neither is relevant in crop breeding

1.2.2 The Pistil, Megasporangium (ovule) and Embryo sac(A)

The gynoecium represents the female reproductive part of the flower. The
gynoecium may consist of a single pistil (monocarpellary) or may have
more than one pistil (multicarpellary). When there are more than one,
the pistils may be fused together (syncarpous) (Figure 1.7b) or may be
free (apocarpous) (Figure 1.7c).

Each pistil has three parts (Figure 1.7a),the stigma, style, and ovary. The stigma serves as a landing platform for pollen grains. The style is the elongated slender part beneath the
stigma. The basal bulged part of the pistil is the ovary. Inside the ovary
is the ovarian cavity (locule). The placenta is located inside the ovarian
cavity. Recall the definition and types of placentation that you studied in class XI. Arising from the placenta are the megasporangia, commonly
called ovules.

The number of ovules in an ovary may be one (wheat,
paddy, mango) to many (papaya, watermelon, orchids).
The Megasporangium (Ovule): Let us familiarise ourselves with the
structure of a typical angiosperm ovule (Figure 1.7d).

The ovule is a small structure attached to the placenta by means of a stalk called funicle.
The body of the ovule fuses with a funicle in the region called the hilum. Thus,
the hilum represents the junction between the ovule and the funicle. Each ovule has
one or two protective envelopes called integuments. Integuments encircle
the nucellus except at the tip where a small opening called the micropyle
is organised. Opposite the micropylar end, is the chalaza, representing
the basal part of the ovule.

Question 1:

Which part of the flower represents the female reproductive organ?
a) Stamen
b) Gynoecium
c) Androecium
d) Sepal

Question 2:

What are the two main types of gynoecium based on the number of pistils?
a) Syncarpous and apocarpous
b) Monocarpellary and multicarpellary
c) Stigma and style
d) Ovary and ovule

Question 3:

Which part of the pistil serves as a landing platform for pollen grains?
a) Ovary
b) Stigma
c) Style
d) Ovule

Question 4:

What is the basal bulged part of the pistil called?
a) Stigma
b) Style
c) Ovary
d) Funicle

Question 5:

What is the ovarian cavity also known as?
a) Placenta
b) Nucellus
c) Locule
d) Micropyle

Question 6:

Where is the placenta located in the ovary?
a) On the integuments
b) Inside the ovarian cavity
c) At the micropylar end
d) At the chalaza

Question 7:

What is the structure attached to the placenta by the funicle?
a) Stigma
b) Style
c) Ovule
d) Nucellus

Question 8:

What is the region where the ovule fuses with the funicle?
a) Placenta
b) Hilum
c) Micropyle
d) Chalaza

1.2.2 The Pistil, Megasporangium (ovule) and Embryo sac(B)

Enclosed within the integuments is a mass of cells called the nucellus.
Cells of the nucellus have abundant reserve food materials. Located in the
nucleus is the embryo sac or female gametophyte. An ovule generally has
a single embryo sac formed from a megaspore.
Megasporogenesis: The process of formation of megaspores from the
megaspore mother cell is called megasporogenesis. Ovules generally
differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus. It is a large cell containing dense cytoplasm and a
prominent nucleus. The MMC undergoes meiotic division. What is the
importance of the MMC undergoing meiosis? Meiosis results in the
production of four megaspores (Figure 1.8a).

Female gametophyte: In a majority of flowering plants, one of the
megaspores is functional while the other three degenerate. Only the
functional megaspore develops into the female gametophyte (embryo
sac). This method of embryo sac formation from a single megaspore is termed
monosporic development. What will be the ploidy of the cells of the nucellus,
MMC, the functional megaspore and female gametophyte? Let us study about the formation of the embryo sac in detail.(Figure 1.8b).

The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. It is of interest to note that these mitotic divisions are strictly free nuclear, that is, nuclear divisions are not followed immediately
by cell wall formation. After the 8-nucleate stage, cell walls are laid down leading to the organization of the typical female gametophyte or embryo sac. Observe the distribution of cells inside the embryo sac (Figure 1.8b,c).

Six of the eight nuclei are surrounded by cell walls and organized into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell. There is a characteristic distribution of the cells within the embryo sac. Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell.

The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which plays an important role in guiding the pollen tubes into the synergid. Three cells are at the chalazal
end and are called the antipodals. The large central cell, as mentioned
earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at
maturity, though 8-nucleate is 7-celled.

Question 1:

What is the mass of cells enclosed within the integuments called?
a) Nucellus
b) Micropyle
c) Hilum
d) Chalaza

Question 2:

Where is the embryo sac or female gametophyte located?
a) Integuments
b) Nucellus
c) Micropyle
d) Hilum

Question 3:

What is the process of formation of megaspores from the megaspore mother cell called?
a) Megagametogenesis
b) Megasporogenesis
c) Microgametogenesis
d) Microsporogenesis

Question 4:

How many megaspores are produced as a result of meiotic division of the megaspore mother cell?
a) One
b) Two
c) Three
d) Four

Question 5:

What is the term for the formation of the embryo sac from a single megaspore?
a) Monogametogenesis
b) Monosporic development
c) Megasporification
d) Megagametogenesis

Question 6:

What is the ploidy of the cells of the nucellus, megaspore mother cell, functional megaspore, and female gametophyte?
a) Haploid
b) Diploid
c) Triploid
d) Tetraploid

Question 7:

How many nuclei are present in the embryo sac at the 2-nucleate stage?
a) One
b) Two
c) Four
d) Eight

Question 8:

What is the term for nuclear divisions that are not followed immediately by cell wall formation?
a) Free mitosis
b) Cytokinesis
c) Syncytium
d) Cleavage

Question 9:

What is the characteristic distribution of cells within the mature embryo sac?
a) 8-celled
b) 7-celled
c) 6-celled
d) 5-celled

Question 10:

Which cells constitute the egg apparatus in the embryo sac?
a) Synergids and polar nuclei
b) Egg cell and polar nuclei
c) Antipodals and synergids
d) Polar nuclei and antipodals

Question 11:

What are the special cellular thickenings at the micropylar tip of the synergids called?
a) Filiform apparatus
b) Micropylar structures
c) Synergid thickening
d) Stigmatic projections

Question 12:

How many cells are grouped together at the micropylar end of the embryo sac?
a) One
b) Two
c) Three
d) Four

Question 13:

What is the ploidy of the nuclei in the central cell of the mature embryo sac?
a) Haploid
b) Diploid
c) Triploid
d) Tetraploid

Question 14:

What is the term for the stage when cell walls are laid down in the embryo sac?
a) Embryogenesis
b) Cytokinesis
c) Organogenesis
d) Differentiation

Question 15:

What structures play an important role in guiding pollen tubes into the synergid?
a) Polar nuclei
b) Filiform apparatus
c) Egg cell
d) Antipodals

1.2.3 Pollination(A)

In the preceding sections, you have learned that the male and female gametes
in flowering plants are produced in the pollen grain and embryo sac,
respectively. As both types of gametes are non-motile, they have to be
brought together for fertilization to occur. How is this achieved?
Pollination is the mechanism to achieve this objective. Transfer
of pollen grains (shed from the anther) to the stigma of a pistil is
termed pollination. Flowering plants have evolved an amazing array
of adaptations to achieve pollination. They make use of external
agents to achieve pollination. Can you list the possible external
agents? Kinds of Pollination: Depending on the source of pollen, pollination
can be divided into three types.

(i) Autogamy: In this type, pollination is achieved within the same flower. Transfer of pollen grains from the anther to the stigma of the same flower (Figure 1.9a). In a normal flower that opens and exposes the anthers and the stigma, complete autogamy is rather
rare. Autogamy in such flowers requires synchrony in pollen release
and stigma receptivity and also, the anthers and the stigma should lie close to each other so that self-pollination can occur. Some plants such as Viola (common pansy), Oxalis, and Commelina produce two types of flowers –chasmogamous flowers which are similar to
flowers of other species with exposed anthers and stigma, and cleistogamous flowers which
do not open at all (Figure 1.9c).

In such flowers,the anthers and stigma lie close to each other.
When anthers dehisce in the flower buds,pollen grains come in contact with the stigma
to effect pollination. Thus, cleistogamous flowers are invariably autogamous as there is
no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce
assured seed-set even in the absence of pollinators. Do you think that cleistogamy is
advantageous or disadvantageous to the plant? Why?

(ii) Geitonogamy – Transfer of pollen grains from
the anther to the stigma of another flower of
the same plant. Although geitonogamy is
functionally cross-pollination involving a
pollinating agent, genetically it is similar to
autogamy since the pollen grains come from
the same plant.

(iii) Xenogamy – Transfer of pollen grains from
anther to the stigma of a different plant (Figure
1.9b). This is the only type of pollination which
during pollination brings genetically different
types of pollen grains to the stigma.

Question 1:

What is the purpose of pollination in flowering plants?
a) Seed dispersal
b) Gamete production
c) Fertilization
d) Pollen formation

Question 2:

What is the term for the transfer of pollen grains from the anther to the stigma?
a) Fertilization
b) Germination
c) Pollination
d) Gametogenesis

Question 3:

Which of the following is a non-motile gamete in flowering plants?
a) Pollen grain
b) Egg cell
c) Synergid
d) Antipodal cell

Question 4:

What external agents do flowering plants use to achieve pollination?
a) Wind, water, and animals
b) Insects, bacteria, and fungi
c) Sunlight, soil, and air
d) Rocks, minerals, and birds

Question 5:

Which type of pollination occurs within the same flower?
a) Geitonogamy
b) Xenogamy
c) Autogamy
d) Allogamy

Question 6:

In cleistogamous flowers, what is the primary advantage of being autogamous?
a) Increased genetic diversity
b) Enhanced seed dispersal
c) Assured seed-set
d) Attraction of pollinators

Question 7:

What is the genetic similarity between geitonogamy and autogamy?
a) No genetic similarity
b) Partial genetic similarity
c) Complete genetic similarity
d) Intermittent genetic similarity

Question 8:

Which type of pollination brings genetically different pollen grains to the stigma?
a) Autogamy
b) Geitonogamy
c) Xenogamy
d) Cleistogamy

Question 9:

What is the primary difference between geitonogamy and xenogamy?
a) Source of pollen
b) Involvement of pollinators
c) Genetic diversity
d) Number of flowers involved

Question 10:

In which type of pollination is there a chance of cross-pollen landing on the stigma?
a) Cleistogamy
b) Xenogamy
c) Autogamy
d) Geitonogamy

Question 11:

Which of the following types of pollination involves flowers that do not open at all?
a) Cleistogamy
b) Autogamy
c) Xenogamy
d) Geitonogamy

Question 12:

What is the role of pollinators in geitonogamy?
a) They transfer pollen from one flower to another on the same plant.
b) They transfer pollen from one plant to another plant of the same species.
c) They transfer pollen from one flower to another on different plants.
d) They do not play a role in geitonogamy.

Question 13:

What is the primary advantage of xenogamy to the plant?
a) Increased genetic diversity
b) Assured seed-set
c) Attraction of pollinators
d) Enhanced seed dispersal

Question 14:

Which type of pollination is functionally cross-pollination but genetically similar to autogamy?
a) Autogamy
b) Xenogamy
c) Geitonogamy
d) Cleistogamy

Question 15:

What is the only type of pollination that brings genetically different pollen grains to the stigma?
a) Cleistogamy
b) Xenogamy
c) Autogamy
d) Geitonogamy

1.2.3 Pollination(B)

(i) Autogamy: In this type, pollination is achieved within the same flower. Transfer of pollen grains from the anther to the stigma of the same flower (Figure 1.9a). In a normal flower which opens and exposes the anthers and the stigma, complete autogamy is rather
rare. Autogamy in such flowers requires synchrony in pollen release and stigma receptivity and also, the anthers and the stigma should be close to each other so that self-pollination
can occur. Some plants such as Viola (common pansy), Oxalis, and Commelina
produce two types of flowers –chasmogamous flowers which are similar to
flowers of other species with exposed anthers and stigma, and cleistogamous flowers which
do not open at all (Figure 1.9c).

In such flowers,the anthers and stigma lie close to each other.
When anthers dehisce in the flower buds,pollen grains come in contact with the stigma
to effect pollination. Thus, cleistogamous flowers are invariably autogamous as there is
no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce
assured seed set even in the absence of pollinators. Do you think that cleistogamy is
advantageous or disadvantageous to the plant? Why?

(ii) Geitonogamy – Transfer of pollen grains from
the anther to the stigma of another flower of
the same plant. Although geitonogamy is
functionally cross-pollination involving a
pollinating agent, genetically it is similar to
autogamy since the pollen grains come from
the same plant.

(iii) Xenogamy – Transfer of pollen grains from anther to the stigma of a different plant (Figure1.9b). This is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma.

Agents of Pollination: Plants use two abiotic (wind and water) and one biotic (animals) agents to achieve pollination. The majority of plants use biotic agents for pollination. Only a small proportion of plants use abiotic agents. Pollen grains coming in contact with the stigma is a chance factor in both wind and water pollination. To compensate for this uncertainties and the associated loss of pollen grains, the flowers produce an enormous amount of pollen when compared to the number of ovules available for pollination.

Autogamy (Question 1-4):

Question 1:

What is autogamy in pollination?
a) Transfer of pollen from one flower to another on the same plant
b) Transfer of pollen from anther to stigma of the same flower
c) Transfer of pollen from one plant to another plant of the same species
d) Transfer of pollen from anther to stigma of different plants

Question 2:

Why is complete autogamy rather rare in normal flowers?
a) Lack of pollen release
b) Lack of synchrony in pollen release and stigma receptivity
c) Excessive pollen production
d) Lack of floral parts

Question 3:

Which type of flowers do plants like Viola, Oxalis, and Commelina produce for autogamy?
a) Chasmogamous flowers
b) Cleistogamous flowers
c) Nectarless flowers
d) Hermaphroditic flowers

Question 4:

What is the primary advantage of cleistogamy to the plant?
a) Attraction of pollinators
b) Genetic diversity
c) Assured seed-set even in the absence of pollinators
d) Enhanced seed dispersal

Geitonogamy (Question 5-8):

Question 5:

What is geitonogamy in pollination?
a) Transfer of pollen from anther to stigma of the same flower
b) Transfer of pollen from one flower to another on the same plant
c) Transfer of pollen from one plant to another plant of the same species
d) Transfer of pollen from anther to stigma of different plants

Question 6:

What is the genetic similarity between geitonogamy and autogamy?
a) Complete genetic similarity
b) Partial genetic similarity
c) No genetic similarity
d) Intermittent genetic similarity

Question 7:

What is the primary difference between geitonogamy and autogamy?
a) Involvement of pollinators
b) Number of flowers involved
c) Source of pollen
d) Genetic diversity

Question 8:

In geitonogamy, where do the pollen grains come from?
a) The same plant
b) Different plants of the same species
c) Different species
d) The same flower

Xenogamy (Question 9-12):

Question 9:

What is xenogamy in pollination?
a) Transfer of pollen from anther to stigma of the same flower
b) Transfer of pollen from one flower to another on the same plant
c) Transfer of pollen from one plant to another plant of the same species
d) Transfer of pollen from anther to stigma of different plants

Question 10:

Which type of pollination brings genetically different pollen grains to the stigma?
a) Autogamy
b) Geitonogamy
c) Cleistogamy
d) Xenogamy

Question 11:

What is the only type of pollination that brings genetically different pollen grains to the stigma?
a) Cleistogamy
b) Xenogamy
c) Autogamy
d) Geitonogamy

Question 12:

What is the primary advantage of xenogamy to the plant?
a) Increased genetic diversity
b) Assured seed-set
c) Attraction of pollinators
d) Enhanced seed dispersal

Agents of Pollination (Question 13-15):

Question 13:

Which of the following is not an abiotic agent of pollination?
a) Wind
b) Water
c) Animals
d) Insects

Question 14:

What is the primary compensatory mechanism for uncertainties and loss of pollen grains in abiotic pollination?
a) Enormous pollen production
b) Limited ovule production
c) Reduced flower size
d) Increased flower fragrance

Question 15:

Why do flowers produce an enormous amount of pollen in wind and water pollination?
a) To attract pollinators
b) To ensure successful fertilization
c) To compensate for uncertainties and associated loss of pollen grains
d) To enhance seed dispersal

1.2.3 Pollination(C)

Pollination by wind is more common
amongst abiotic pollinations. Wind pollination
also requires that the pollen grains are light
and non-sticky so that they can be
transported in wind currents. They often
possess well-exposed stamens (so that the
pollens are easily dispersed into wind currents,
Figure 1.10) and large often-feathery stigma
to easily trap air-borne pollen grains. Wind-pollinated flowers often have a single ovule in
each ovary and numerous flowers packed into
an inflorescence; a familiar example is the corn
cob – the tassels you see are nothing but the
stigma and style which wave in the wind to
trap pollen grains. Wind-pollination is quite
common in grasses.

Pollination by water is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons. As against
this, you would recall that water is a regular mode of transport for the male gametes among
the lower plant groups such as algae, bryophytes, and pteridophytes.

It is believed,particularly for some bryophytes and pteridophytes, that their distribution is limited
because of the need for water for the transport of male gametes and fertilization. Some
examples of water-pollinated plants are Vallisneria and Hydrilla which grow in fresh water and several marine sea-grasses such as Zostera. Not all aquatic plants use water for pollination. In a majority of aquatic plants such as water hyacinth and water lily, the flowers emerge above the level
of water and are pollinated by insects or wind as in most of the land plants.

In Vallisneria, the female flower reaches the surface of water by the long stalk, and the male flowers or pollen grains are released onto the surface of water. They are carried passively by water currents (Figure1.11a); some of them eventually reach the female flowers and the stigma.In another group of water-pollinated plants such as seagrasses, female flowers remain submerged in water and the pollen grains are released inside the water. Pollen grains in many such species are long, ribbon-like
and they are carried passively inside the water; some of them reach the stigma and achieve pollination.

In most of the water-pollinated species, pollen grains are protected from wetting by a mucilaginous covering. Both wind and water-pollinated flowers are not very colorful and do
not produce nectar. What would be the reason for this? The majority of flowering plants use a range of animals as pollinating agents. Bees, butterflies, flies, beetles, wasps, ants, moths, birds(sunbirds and hummingbirds) and bats are the common pollinating agents. (Figure 1.11b).

Among the animals, insects, particularly bees are the dominant biotic pollinating agents. Even larger animals such as some primates (lemurs), arboreal (tree-dwelling) rodents, or even reptiles (gecko lizard and garden lizard) have also been reported as pollinators in some species. Often flowers of animal-pollinated plants are specifically adapted for a particular species of animal.

Majority of insect-pollinated flowers are large, colorful, fragrant, and rich in nectar. When the flowers are small, a number of flowers are clustered into an inflorescence to make them conspicuous. Animals are attracted to flowers by color and/or fragrance.

The flowers pollinated by flies and beetles secrete foul odors to attract these animals. To sustain animal visits,
the flowers have to provide rewards to the animals. Nectar and pollen grains are the usual floral rewards. For harvesting the reward(s) from the flower the animal visitor comes
in contact with the anthers and the stigma. The body of the animal gets a coating of pollen grains, which are generally sticky in animal-pollinated flowers. When the animal carrying pollen on its body comes in contact with the stigma, it brings about pollination.

In some species floral rewards are in providing safe places to lay eggs; an example is that of the tallest flower of Amorphophallus (the flower itself is about 6 feet in height). A similar relationship exists between a species of moth and the plant Yucca where both species – moth and the plant – cannot complete their life cycles without each other. The moth deposits its eggs in the locule of the ovary and the flower, in turn, gets
pollinated by the moth.

The larvae of the moth come out of the eggs as the seeds start developing.Why don’t you observe some flowers of the following plants (or any others available to you): Cucumber, Mango, Peepal, Coriander, Papaya,Onion, Lobia, Cotton, Tobacco, Rose, Lemon, Eucalyptus, Banana? Try to find out which animals visit them and whether they could be
pollinators.

You’ll have to patiently observe the flowers over a few days
and at different times of the day. You could also try to see whether there
is any correlation in the characteristics of a flower to the animal that
visits it. Carefully observe if any of the visitors come in contact with the
anthers and the stigma as only such visitors can bring about pollination.
Many insects may consume pollen or the nectar without bringing about
pollination. Such floral visitors are referred to as pollen/nectar robbers.
You may or may not be able to identify the pollinators, but you will surely
enjoy your efforts!

Wind Pollination (Question 1-4):

Question 1:

What is a characteristic feature of pollen grains in wind-pollinated plants?
a) Sticky
b) Heavy
c) Non-sticky
d) Fragrant

Question 2:

Why do wind-pollinated flowers possess well-exposed stamens?
a) To attract pollinators
b) To trap air-borne pollen grains
c) To store pollen grains
d) To enhance fragrance

Question 3:

What is a familiar example of wind-pollinated flowers with a single ovule in each ovary?
a) Rose
b) Hibiscus
c) Corn cob
d) Sunflower

Question 4:

Why do wind-pollinated flowers often have numerous flowers packed into an inflorescence?
a) To attract pollinators
b) To trap air-borne pollen grains
c) To enhance fragrance
d) To store nectar

Water Pollination (Question 5-8):

Question 5:

Why is pollination by water rare in flowering plants?
a) Lack of water
b) High risk of pollen loss
c) Limited availability of water
d) Preference for wind pollination

Question 6:

What is an example of a water-pollinated plant with female flowers remaining submerged?
a) Rose
b) Water lily
c) Water hyacinth
d) Sunflower

Question 7:

How are pollen grains carried in water-pollinated species like Vallisneria?
a) Wind currents
b) Insects
c) Water currents
d) Birds

Question 8:

What is a common characteristic of pollen grains in water-pollinated species?
a) Sticky covering
b) Feather-like structure
c) Hard exine
d) Mucilaginous covering

Animal Pollination (Question 9-10):

Question 9:

What is a common characteristic of insect-pollinated flowers to attract animals?
a) Foul odours
b) Large size
c) Lack of fragrance
d) Single flower per inflorescence

Question 10:

What is the primary floral reward for animal pollinators?
a) Nectar and pollen grains
b) Fragrance
c) Foul odours
d) Fertile soil

Observing Flowers (Question 11-12):

Question 11:

Why is it important to observe flowers at different times of the day for identifying pollinators?
a) To enjoy the beauty of flowers
b) To find the correlation between flower characteristics and visiting animals
c) To gather data on the number of flowers in an inflorescence
d) To determine the water requirements of flowers

Question 12:

What are floral visitors referred to if they consume pollen or nectar without bringing about pollination?
a) Pollen/nectar lovers
b) Pollinators
c) Nectar robbers
d) Pollen/nectar robbers

1.2.3 Pollination(D)

Outbreeding Devices: Majority of flowering plants produce hermaphrodite
flowers and pollen grains are likely to come in contact with the stigma of
the same flower. Continued self-pollination results in inbreeding depression.
Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination. In some species, pollen
release and stigma receptivity are not synchronized. Either the pollen is
released before the stigma becomes receptive or the stigma becomes receptive
much before the release of pollen. In some other species, the anther and
stigma are placed at different positions so that the pollen cannot come in
contact with the stigma of the same flower. Both these devices prevent
autogamy. The third device to prevent inbreeding is self-incompatibility.

This is a genetic mechanism and prevents self-pollen (from the same flower
or other flowers of the same plant) from fertilizing the ovules by inhibiting
pollen germination or pollen tube growth in the pistil. Another device to
prevent self-pollination is the production of unisexual flowers. If both male
and female flowers are present on the same plant such as castor and maize
(monoecious), it prevents autogamy but not geitonogamy. In several species
such as papaya, male and female flowers are present on different plants,
that is each plant is either male or female (dioecy). This condition prevents
both autogamy and geitonogamy.

Pollen-pistil Interaction: Pollination does not guarantee the transfer
of the right type of pollen (compatible pollen of the same species as the
stigma). Often, pollen of the wrong type, either from other species or from
the same plant (if it is self-incompatible), also lands on the stigma. The
pistil has the ability to recognize the pollen, whether it is of the right type
(compatible) or of the wrong type (incompatible).

If it is of the right type,the pistil accepts the pollen and promotes post-pollination events that leads to fertilisation. If the pollen is of the wrong type, the pistil rejects the pollen by preventing pollen germination on the stigma or the pollen tube growth in the style. The ability of the pistil to recognize the pollen followed by its acceptance or rejection is the result of a continuous dialogue
between the pollen grain and the pistil.

This dialogue is mediated by chemical components of the pollen interacting with those of the pistil. It is only in recent years that botanists have been able to identify some of the pollen
and pistil components and the interactions leading to the recognition,followed by acceptance or rejection.

As mentioned earlier, following compatible pollination, the pollen grain
germinates on the stigma to produce a pollen tube through one of the
germ pores (Figure 1.12a).

The contents of the pollen grain move into the pollen tube. Pollen tube grows through the tissues of the stigma and style and reaches the ovary (Figure 1.12b, c). You would recall that in some plants, pollen grains are shed at two-celled conditions (a vegetative
cell and a generative cell). In such plants, the generative cell divides and
forms the two male gametes during the growth of the pollen tube in the stigma.
In plants which shed pollen in the three-celled condition, pollen tubes
carry the two male gametes from the beginning. Pollen tube, after reaching
the ovary, enters the ovule through the micropyle and then enters one of
the synergids through the filiform apparatus (Figure 1.12d, e).

Many recent studies have shown that filiform apparatus present at the micropylar part
of the synergids guides the entry of pollen tube. All these events–from
pollen deposition on the stigma until pollen tubes enter the ovule–are
together referred to as pollen-pistil interaction. As pointed out earlier,
pollen-pistil interaction is a dynamic process involving pollen recognition
followed by promotion or inhibition of the pollen. The knowledge gained
in this area would help the plant breeder in manipulating pollen-pistil
interaction, even in incompatible pollinations, to get desired hybrids.
You can easily study pollen germination by dusting some pollen from
flowers such as pea, chickpea, Crotalaria, balsam and Vinca on a glass slide
containing a drop of sugar solution (about 10 percent).

After about 15–30 minutes, observe the slide under the low power lens of the microscope. You
are likely to see pollen tubes coming out of the pollen grains.
A breeder is interested in crossing different species and often genera
to combine desirable characters to produce commercially ‘superior’
varieties. Artificial hybridisation is one of the major approaches of crop
improvement programme. In such crossing experiments it is important
to make sure that only the desired pollen grains are used for pollination
and the stigma is protected from contamination (from unwanted pollen).

This is achieved by emasculation and bagging techniques.
If the female parent bears bisexual flowers, removal of anthers from
the flower bud before the anther dehisces using a pair of forceps is
necessary. This step is referred to as emasculation. Emasculated flowers
have to be covered with a bag of suitable size, generally made up of butter
paper, to prevent contamination of its stigma with unwanted pollen. This
process is called bagging. When the stigma of bagged flower attains
receptivity, mature pollen grains collected from anthers of the male parent
are dusted on the stigma, and the flowers are rebagged, and the fruits
allowed to develop.

If the female parent produces unisexual flowers, there is no need for
emasculation. The female flower buds are bagged before the flowers open.
When the stigma becomes receptive, pollination is carried out using the
desired pollen, and the flower rebagged.

Outbreeding Devices (Questions 1-5):

Question 1:

What is the main reason flowering plants develop outbreeding devices?
a) To encourage self-pollination
b) To prevent cross-pollination
c) To discourage self-pollination and encourage cross-pollination
d) To increase inbreeding depression

Question 2:

Which of the following is NOT an outbreeding device in plants?
a) Synchronised pollen release and stigma receptivity
b) Unisexual flowers
c) Self-pollination
d) Self-incompatibility

Question 3:

What is self-incompatibility in plants?
a) Preventing pollen germination
b) Inhibiting pollen tube growth
c) Encouraging self-pollination
d) Allowing cross-pollination

Question 4:

In which plant species do male and female flowers exist on different plants?
a) Monocarpellary plants
b) Dioecious plants
c) Cleistogamous plants
d) Autogamous plants

Question 5:

How does the production of unisexual flowers prevent autogamy?
a) By synchronising pollen release and stigma receptivity
b) By inhibiting pollen tube growth
c) By preventing self-pollen from fertilising ovules
d) By allowing geitonogamy

Pollen-Pistil Interaction (Question 6-9):

Question 6:

What determines whether the pistil accepts or rejects pollen?
a) Number of pollen grains
b) Colour of pollen grains
c) Chemical components in pollen and pistil
d) Size of pollen grains

Question 7:

What is the result of compatible pollination?
a) Pollen tube rejection
b) Fertilisation
c) Inhibition of pollen germination
d) Absence of pollen

Question 8:

Where does the pollen tube enter after reaching the ovary?
a) Petals
b) Sepals
c) Ovule through the micropyle
d) Filiform apparatus

Question 9:

What is the continuous dialogue between pollen grain and pistil mediated by?
a) Mechanical components
b) Physical force
c) Chemical components
d) Atmospheric pressure

Pollen Germination and Hybridisation (Question 10-13):

Question 10:

How can pollen germination be observed in plants like pea and chickpea?
a) Dusting pollen on a glass slide with water
b) Dusting pollen on a glass slide with sugar solution
c) Dusting pollen on a glass slide with oil
d) Dusting pollen on a glass slide with salt solution

Question 11:

Why is artificial hybridisation essential in crop improvement programs?
a) To encourage self-pollination
b) To prevent cross-pollination
c) To ensure controlled pollination for desired traits
d) To promote inbreeding depression

Question 12:

What is the process of removing anthers from the flower bud before dehiscence called?
a) Bagging
b) Emasculation
c) Germination
d) Hybridisation

Question 13:

What is the purpose of covering emasculated flowers with a bag?
a) To prevent contamination of stigma
b) To enhance fragrance
c) To encourage self-pollination
d) To protect anthers

1.3 DOUBLE FERTILISATION

After entering one of the synergids, the pollen tube releases the two male
gametes into the cytoplasm of the synergid. One of the male gametes moves
towards the egg cell and fuses with its nucleus thus completing the
syngamy. This results in the formation of a diploid cell, the zygote. 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 1.13a).

As this involves the fusion of three haploid nuclei it
is termed triple fusion. Since 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. The central cell after
triple fusion becomes the primary endosperm cell (PEC) and develops
into the endosperm while the zygote develops into an embryo.

Double Fertilisation (Question 1-5):

Question 1:

What is the outcome of the fusion between one of the male gametes and the egg cell in the embryo sac?
a) Diploid zygote
b) Haploid gamete
c) Triploid primary endosperm nucleus
d) Tetraploid nucleus

Question 2:

What is formed as a result of the fusion between one of the male gametes and the two polar nuclei in the embryo sac?
a) Diploid zygote
b) Triploid primary endosperm nucleus
c) Haploid gamete
d) Diploid primary endosperm cell

Question 3:

How many haploid nuclei are involved in triple fusion during double fertilisation?
a) One
b) Two
c) Three
d) Four

Question 4:

What is the term used to describe the fusion of one male gamete with the egg cell in the embryo sac?
a) Triple fusion
b) Syngamy
c) Quadruple fusion
d) Meiosis

Question 5:

What is the ultimate fate of the zygote in double fertilisation?
a) Develops into the primary endosperm cell
b) Develops into the embryo
c) Forms the pollen tube
d) Undergoes meiosis

1.4 POST-FERTILISATION: STRUCTURES AND EVENTS

Following double fertilization, events of endosperm and embryo
development, maturation of ovule(s) into seed(s) and ovary into fruit, are
collectively termed post-fertilisation events.

1.4.1 Endosperm
Endosperm development precedes embryo development. Why? The primary endosperm cell divides repeatedly and forms a triploidendosperm tissue. The cells of this tissue are filled with
reserve food materials and are used for the nutrition of
the developing embryo. In the most common type of
endosperm development, the PEN undergoes successive
nuclear divisions to give rise to free nuclei.

This stage of endosperm development is called free-nuclear endosperm. Subsequently cell wall formation occurs and the
endosperm becomes cellular. The number of free nuclei
formed before cellularisation varies greatly. The coconut
water from tender coconut that you are familiar with, is
nothing but free-nuclear endosperm (made up of
thousands of nuclei) and the surrounding white kernel is
the cellular endosperm.

Endosperm may either be completely consumed by the
developing embryo (e.g., pea, groundnut, beans) before seed
maturation or it may persist in the mature seed (e.g. castor
and coconut) and be used up during seed germination. Split
open some seeds of castor, peas, beans, groundnut, fruit of
coconut and look for the endosperm in each case. Find out
whether the endosperm is persistent in cereals – wheat, rice
and maize.

Endosperm Development (Questions 1-6):

Question 1:

What is the purpose of endosperm development in flowering plants?
a) Provides mechanical support to the ovule
b) Facilitates pollination
c) Nutrition for the developing embryo
d) Aids in seed dispersal

Question 2:

Why does endosperm development precede embryo development in flowering plants?
a) To protect the ovule
b) To facilitate seed germination
c) To provide nutrition to the developing embryo
d) To prevent self-pollination

Question 3:

What is the term used to describe the stage of endosperm development where successive nuclear divisions occur, leading to the formation of free nuclei?
a) Cellular endosperm
b) Persistent endosperm
c) Free-nuclear endosperm
d) Germinating endosperm

Question 4:

What is the coconut water composed of in terms of endosperm development?
a) Cellular endosperm
b) Persistent endosperm
c) Free-nuclear endosperm
d) Germinating endosperm

Question 5:

In some seeds, endosperm is completely consumed by the developing embryo before seed maturation. Which of the following is an example of such a seed?
a) Castor
b) Pea
c) Coconut
d) Wheat

Question 6:

Where does the endosperm persist in some mature seeds, serving as a nutritional source during seed germination?
a) In the cotyledons
b) In the radicle
c) In the embryo sac
d) In the seed coat

1.4.2 Embryo

Embryo develops at the micropylar end of the embryo sac where
the zygote is situated. Most zygotes divide only after certain
amount of endosperm is formed. This is an adaptation to
provide assured nutrition to the developing embryo. Though
the seeds differ greatly, the early stages of embryo development
(embryogeny) are similar in both monocotyledons and
dicotyledons.

Figure 1.13 depicts the stages of embryogeny in
a dicotyledonous embryo. The zygote gives rise to the
proembryo and subsequently to the globular, heart-shaped
and mature embryo.A typical dicotyledonous embryo (Figure 1.14a), consists
of an embryonal axis and two cotyledons. The portion of
embryonal axis above the level of cotyledons is the epicotyl,
which terminates with the plumule or stem tip. The cylindrical
portion below the level of cotyledons is hypocotyl that
terminates at its lower end in the radicle or root tip. The root
tip is covered with a root cap.

Embryos of monocotyledons (Figure 1.14 b) 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 theradical and root cap enclosed in an undifferentiated 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.
Soak a few seeds in water (say of wheat, maize, peas, chickpeas,
ground nut) overnight. Then split the seeds and observe the various
parts of the embryo and the seed.

Embryo Development (Questions 1-5):

Question 1:

Where does the embryo develop in the embryo sac?
a) Chalazal end
b) Synergidal end
c) Micropylar end
d) Antipodal end

Question 2:

What adaptation ensures that most zygotes divide only after a certain amount of endosperm is formed?
a) Micropylar elongation
b) Cotyledon expansion
c) Suspended embryonic growth
d) Radicle development

Question 3:

What is the portion of the embryonal axis above the cotyledons called in a typical dicotyledonous embryo?
a) Coleoptile
b) Epicotyl
c) Radicle
d) Scutellum

Question 4:

What is the undifferentiated sheath covering the radical and root cap in monocotyledonous embryos?
a) Coleoptile
b) Coleorrhiza
c) Hypocotyl
d) Plumule

Question 5:

In the grass family, what is the name of the cotyledon in monocotyledonous embryos?
a) Plumule
b) Scutellum
c) Epicotyl
d) Coleoptile

1.4.3 Seed

In angiosperms, the seed is the final product of sexual reproduction. It is
often described as a fertilised ovule. Seeds are formed inside fruits. A
seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.
The cotyledons (Figure 1.15a) of the embryo are simple structures,
generally thick and swollen due to storage of food reserves (as in legumes).
Mature seeds may be non-albuminous or ex-albuminous. Nonalbuminous seeds have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut).

Albuminous seeds retain a part of endosperm as it is not completely used
up during embryo development (e.g., wheat, maize, barley, castor).
Occasionally, in some seeds such as black pepper and beet, remnants of
nucellus are also persistent. This residual, persistent nucellus is the
perisperm.

Integuments of ovules harden as tough protective seed coats
(Figure 1.15a). The micropyle remains as a small pore in the seed coat.
This facilitates entry of oxygen and water into the seed during germination.
As the seed matures, its water content is reduced and seeds become
relatively dry (10-15 per cent moisture by mass). The general metabolic
activity of the embryo slows down. The embryo may enter a state of
inactivity called dormancy, or if favourable conditions are available
(adequate moisture, oxygen and suitable temperature), they germinate.
As ovules mature into seeds, the ovary develops into a fruit, i.e., the
transformation of ovules into seeds and ovary into fruit proceeds
simultaneously.

The wall of the ovary develops into the wall of fruit called
pericarp. The fruits may be fleshy as in guava, orange, mango, etc., or
may be dry, as in groundnut, and mustard, etc. Many fruits have evolved
mechanisms for dispersal of seeds. Recall the classification of fruits and
their dispersal mechanisms that you have studied in an earlier class. Is
there any relationship between number of ovules in an ovary and the
number of seeds present in a fruit?

In most plants, by the time the fruit develops from the ovary, other
floral parts degenerate and fall off. However, in a few species such as apple,
strawberry, cashew, etc., the thalamus also contributes to fruit formation.
Such fruits are called false fruits (Figure 1.15b). Most fruits however
develop only from the ovary and are called true fruits. Although in most
of the species, fruits are the results of fertilisation, there are a few speciesin which fruits develop without fertilisation. Such fruits are called
parthenocarpic fruits. Banana is one such example. Parthenocarpy can
be induced through the application of growth hormones and such fruits
are seedless.

Seeds offer several advantages to angiosperms. Firstly, since
reproductive processes such as pollination and fertilisation are
independent of water, seed formation is more dependable. Also seeds have
better adaptive strategies for dispersal to new habitats and help the species
to colonise in other areas. As they have sufficient food reserves, young
seedlings are nourished until they are capable of photosynthesis on their
own. The hard seed coat provides protection to the young embryo. Being
products of sexual reproduction, they generate new genetic combinations
leading to variations.

Seed is the basis of our agriculture. Dehydration and dormancy of
mature seeds are crucial for storage of seeds which can be used as food
throughout the year and also to raise crop in the next season. Can you
imagine agriculture in the absence of seeds, or in the presence of seeds
which germinate straight away soon after formation and cannot be stored?
How long do the seeds remain alive after they are dispersed?

This period again varies greatly. In a few species the seeds lose viability within
a few months. Seeds of a large number of species live for several years.
Some seeds can remain alive for hundreds of years. There are several
records of very old yet viable seeds. The oldest is that of a lupine, Lupinus
arcticus excavated from Arctic Tundra. The seed germinated and flowered
after an estimated record of 10,000 years of dormancy. A recent record of
2000 years old viable seed is of the date palm, Phoenix dactylifera
discovered during the archeological excavation at King Herod’s palace
near the Dead Sea.

After completing a brief account of sexual reproduction of flowering
plants it would be worth attempting to comprehend the enormous
reproductive capacity of some flowering plants by asking the following
questions: How many eggs are present in an embryo sac? How many
embryo sacs are present in an ovule? How many ovules are present in
an ovary? How many ovaries are present in a typical flower? How many
flowers are present on a tree? And so on…

Can you think of some plants in which fruits contain very large
number of seeds. Orchid fruits are one such category and each fruit
contain thousands of tiny seeds. Similar is the case in fruits of some
parasitic species such as Orobanche and Striga. Have you seen a tiny
seed of Ficus? How large is the tree of Ficus developed from that tiny
seed. How many billions of seeds does each Ficus tree produce? Can
you imagine any other example in which such a tiny structure can
produce such a large biomass over the years? 15 questions on above patterns

Seed and Fruit Development (Questions 1-5):

Question 1:

What are the three main components of a typical seed?
a) Seed coat, epicotyl, cotyledon
b) Seed coat, cotyledon, embryo axis
c) Pericarp, cotyledon, embryo axis
d) Pericarp, epicotyl, cotyledon

Question 2:

Which type of seeds have no residual endosperm and are completely consumed during embryo development?
a) Non-albuminous
b) Albuminous
c) Parthenocarpic
d) Epigeal

Question 3:

What is the role of the micropyle in seeds?
a) Entry of water and oxygen during germination
b) Protection of the embryo
c) Formation of seed coat
d) Dispersal of seeds

Question 4:

In some species, what is the term for the residual, persistent nucellus in the seed?
a) Cotyledon
b) Epicotyl
c) Perisperm
d) Plumule

Question 5:

What are fruits called that develop without fertilization?
a) False fruits
b) True fruits
c) Parthenocarpic fruits
d) Accessory fruits

Question 6:

What is the primary purpose of the seed coat?
a) Protection of the embryo
b) Storage of food reserves
c) Initiation of germination
d) Pollination facilitation

Question 7:

Which part of the seed is often described as a fertilized ovule?
a) Cotyledon
b) Embryo axis
c) Seed coat
d) Pericarp

Question 8:

What does the term “parthenocarpic” refer to in the context of fruit development?
a) Formation of seeds without fertilization
b) Presence of only one seed in a fruit
c) Dispersal of seeds by wind
d) Ripening of fruits

Question 9:

In which part of the seed does food reserve storage mainly occur?
a) Cotyledon
b) Embryo axis
c) Seed coat
d) Perisperm

Question 10:

What is the significance of seeds having dormancy?
a) Prevents seed germination
b) Facilitates storage of seeds
c) Promotes rapid seed germination
d) Aids in pollination

Question 11:

What is the function of the micropyle in a seed?
a) Initiate germination
b) Facilitate seed dispersal
c) Allow entry of oxygen and water
d) Protect the embryo

Question 12:

What is the role of the perisperm in seeds?
a) Protection of the embryo
b) Storage of food reserves
c) Facilitation of seed dispersal
d) Initiation of germination

Question 13:

Which type of seed retains a part of the endosperm after embryo development?
a) Albuminous
b) Non-albuminous
c) Parthenocarpic
d) Dormant

Question 14:

Why is dehydration crucial for mature seeds?
a) Facilitates dormancy
b) Aids in pollination
c) Allows seed dispersal
d) Enables storage of seeds

Question 15:

What is the primary advantage of seeds in angiosperms?
a) Dependence on water for reproduction
b) Requirement for external pollination
c) Colonization of new habitats
d) Reliance on vegetative propagation

Question 16:

Which type of fruit is formed without fertilization?
a) True Fruit
b) Parthenocarpic fruit
c) Accessory fruit
d) Aggregate fruit

1.5 APOMIXIS AND POLYEMBRYONY

Although seeds, in general are the products of fertilisation, a few flowering
plants such as some species of Asteraceae and grasses, have evolved a
special mechanism, to produce seeds without fertilisation, called apomixis.
What is fruit production without fertilisation called? Thus, apomixis is a
form of asexual reproduction that mimics sexual reproduction. There are
several ways of development of apomictic seeds. In some species, the
diploid egg cell is formed without reduction division and develops into
the embryo without fertilisation.

More often, as in many Citrus and Mangovarieties some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos. In
such species each ovule contains many embryos. Occurrence of more
than one embryo in a seed is referred to as polyembryony. Take out
some seeds of orange and squeeze them. Observe the many embryos of
different sizes and shapes from each seed. Count the number of embryos
in each seed. What would be the genetic nature of apomictic embryos?
Can they be called clones?

Hybrid varieties of several of our food and vegetable crops are being
extensively cultivated. Cultivation of hybrids has tremendously increased
productivity. One of the problems of hybrids is that hybrid seeds have
to be produced every year. If the seeds collected from hybrids are sown,
the plants in the progeny will segregate and do not maintain hybrid
characters. Production of hybrid seeds is costly and hence the cost of
hybrid seeds become too expensive for the farmers.

If these hybrids are made into apomicts, there is no segregation of characters in the hybrid
progeny. Then the farmers can keep on using the hybrid seeds to raise
new crop year after year and he does not have to buy hybrid seeds every
year. Because of the importance of apomixis in hybrid seed industry,
active research is going on in many laboratories around the world to
understand the genetics of apomixis and to transfer apomictic genes
into hybrid varieties.

Apomixis and Hybrid Seed Production (Questions 17-26):

Question 17:

What is the term for the production of seeds without fertilization in flowering plants?
a) Fertilization
b) Apomixis
c) Parthenogenesis
d) Pollination

Question 18:

How do some species, like Citrus and Mango, exhibit apomixis?
a) Formation of diploid egg cells without reduction division
b) Self-fertilization of ovules
c) Pollination by wind
d) Cross-fertilization with related species

Question 19:

What is polyembryony?
a) Formation of multiple embryos in a seed
b) Production of seeds through fertilization
c) Asexual reproduction in plants
d) Development of embryos in ovules

Question 20:

When nucellar cells surrounding the embryo sac develop into embryos, what is the result?
a) Hybrid seeds
b) Parthenocarpy
c) Polyembryony
d) Endosperm development

Question 21:

What is the genetic nature of apomictic embryos?
a) Genetically identical to the parent plant
b) Hybrid
c) Haploid
d) Diploid

Question 22:

What can the embryos produced through apomixis be called?
a) Gametes
b) Clones
c) Seeds
d) Zygotes

Question 23:

Why is apomixis important in the context of hybrid seed production?
a) Reduces productivity
b) Leads to segregation of characters
c) Facilitates seed dispersal
d) Maintains hybrid characters without segregation

Question 24:

What is one problem associated with hybrid seeds that apomixis addresses?
a) Reduced cost of hybrid seeds
b) Improved seed dispersal
c) Maintenance of hybrid characters
d) Increased productivity

Question 25:

Why is active research conducted on apomixis in laboratories worldwide?
a) To study pollination mechanisms
b) To understand the genetics of hybrid seeds
c) To enhance seed dispersal
d) To promote self-fertilization

Question 26:

What is the potential benefit for farmers if hybrids are made into apomicts?
a) Reduced need for hybrid seeds every year
b) Increased cost of hybrid seeds
c) Segregation of characters in the progeny
d) Dependence on traditional seed varieties

SUMMARY

Flowers are the seat of sexual reproduction in angiosperms. In the flower,
androecium consisting of stamens represents the male reproductive
organs and gynoecium consisting of pistils represents the female
reproductive organs.

A typical anther is bilobed, dithecous and tetrasporangiate. Pollen
grains develop inside the microsporangia. Four wall layers, the
epidermis, endothecium, middle layers and the tapetum surround the
microsporangium. Cells of the sporogenous tissue lying in the centre of
the microsporangium, undergo meiosis (microsporogenesis) to form
tetrads of microspores.

Individual microspores mature into pollen grains.
Pollen grains represents the male gametophytic generation. The
pollen grains have a two-layered wall, the outer exine and inner intine.
The exine is made up of sporopollenin and has germ pores. Pollen grains
may have two cells (a vegetative cell and generative cell) or three cells (a
vegetative cell and two male gametes) at the time of shedding.
The pistil has three parts – the stigma, style and the ovary. Ovules
are present in the ovary.

The ovules have a stalk called funicle, protective
integument(s), and an opening called micropyle. The central tissue is
the nucellus in which the archesporium differentiates. A cell of the
archesporium, the megaspore mother cell divides meiotically and one of
the megaspores forms the embryo sac (the female gametophyte). The
mature embryo sac is 7-celled and 8-nucleate. At the micropylar end is
the egg apparatus consisting of two synergids and an egg cell. At the
chalazal end are three antipodals. At the centre is a large central cell
with two polar nuclei.

Pollination is the mechanism to transfer pollen grains from the
anther to the stigma. Pollinating agents are either abiotic (wind and
water) or biotic (animals).Pollen-pistil interaction involves all events from the landing of pollen grains on the stigma until the pollen tube enters the embryo sac (when
the pollen is compatible) or pollen inhibition (when the pollen is
incompatible).

Following compatible pollination, pollen grain germinates
on the stigma and the resulting pollen tube grow through the style,
enter the ovules and finally discharges two male gametes in one of the
synergids. Angiosperms exhibit double fertilisation because two fusion
events occur in each embryo sac, namely syngamy and triple fusion.
The products of these fusions are the diploid zygote and the triploid
primary endosperm nucleus (in the primary endosperm cell). Zygote
develops into the embryo and the primary endosperm cell forms the
endosperm tissue. Formation of endosperm always precedes
development of the embryo.

The developing embryo passes through different stages such as
the proembryo, globular and heart-shaped stages before maturation.
Mature dicotyledonous embryo has two cotyledons and an embryonal
axis with epicotyl and hypocotyl. Embryos of monocotyledons have a
single cotyledon. After fertilisation, ovary develops into fruit and ovules
develop into seeds.
A phenomenon called apomixis is found in some angiosperms,
particularly in grasses. It results in the formation of seeds without
fertilisation. Apomicts have several advantages in horticulture and
agriculture.
Some angiosperms produce more than one embryo in their seed.
This phenomenon is called polyembryony.

Summary Questions:

Question 1:

What are the two main reproductive organs in the flower?
a) Sepals and petals
b) Stamens and pistils
c) Anthers and ovaries
d) Filaments and styles

Question 2:

What is the term for the male reproductive organ in the flower?
a) Pistil
b) Stamen
c) Style
d) Ovary

Question 3:

What is the typical structure of an anther?
a) Unilobed, monothecous, bisporangiate
b) Bilobed, dithecous, tetrasporangiate
c) Trilobed, polythecous, monosporangiate
d) Bilobed, trithecous, pentasporangiate

Question 4:

What surrounds the microsporangium in the anther?
a) Only the epidermis
b) Epidermis, endothecium, middle layers, and tapetum
c) Endothecium and tapetum
d) Middle layers and tapetum

Question 5:

What is the result of microsporogenesis?
a) Formation of microspores
b) Formation of megaspores
c) Maturation of pollen grains
d) Development of ovules

Question 6:

What is the outermost layer of the pollen grain wall?
a) Intine
b) Exine
c) Epidermis
d) Tapetum

Question 7:

How many cells do pollen grains have at the time of shedding?
a) One cell
b) Two cells
c) Three cells
d) Four cells

Question 8:

What are the three parts of the pistil?
a) Stigma, ovule, style
b) Stigma, style, ovary
c) Ovary, style, stigma
d) Style, ovary, stigma

Question 9:

What is the female gametophyte in angiosperms?
a) Microspore
b) Pollen grain
c) Megaspore
d) Embryo sac

Question 10:

What is the mechanism to transfer pollen grains from the anther to the stigma?
a) Syngamy
b) Fertilization
c) Pollination
d) Double fertilization

Question 11:

What are the products of double fertilization?
a) Zygote and megaspore
b) Zygote and endosperm
c) Zygote and microspore
d) Zygote and pollen grain

Question 12:

What phenomenon results in the formation of seeds without fertilization?
a) Apomixis
b) Polyembryony
c) Parthenocarpy
d) Pollination