Reproduction in Lower and Higher Plants [ Exercise,Solutions,Notes ]
![Reproduction in Lower and Higher Plants [ Exercise,Solutions,Notes ]](https://blogger.googleusercontent.com/img/a/AVvXsEiFAwIXcyMuv_S_BklOEfq7_TAQCKeOED_9IBjDGY6bSnsm04gIKApLZW-gtQoAEWl0w-2KW_cgvRRIYpc8YKE6OunhejAR6a5P0Uw1lSUPmXS3L0YGFZXFA_gbT8TylK6Mp2tgyBKrYECAdKOy1B6dg-WZ1YF26wKKpjEEgXH2GFuSsC6c5QuX-XuC=w640-h640-rw "Reproduction in Lower and Higher Plants [ Exercise,Solutions,Notes ]")
Multiple choice question.
Exercise | Q 1.1 | Page 16
Insect pollinated flowers usually posses ______
OPTIONS
- Sticky pollens with rough surface
- Large quantities of pollens
- Dry pollens with smooth surface
- Light coloured pollens
SOLUTION
Exercise | Q 1.2 | Page 16
In ovule, meiosis occurs in ______
OPTIONS
- integument
- nucellus
- megaspore
- megaspore mother cell
SOLUTION
In ovule, meiosis occurs in megaspore mother cell.
Exercise | Q 1.3 | Page 16
The ploidy level is not the same in ______.
OPTIONS
- Integuments and nucellus
- Root tip and shoot tip
- Secondary nucleus and endosperm
- Antipodals and synergids
SOLUTION
The ploidy level is not the same in secondary nucleus and endosperm.
Exercise | Q 1.4 | Page 16
Which of the following types require pollinator but the result is genetically similar to autogamy?
OPTIONS
- Geitonogamy
- Xenogamy
- Apogamy
- Cleistogamy
SOLUTION
Geitonogamy
Exercise | Q 1.5 | Page 16
If diploid chromosome number in a flowering plant is 12, then which one of the following will have 6 chromosomes?
OPTIONS
- Endosperm
- Leaf cells
- Cotyledons
- Synergids
SOLUTION
Synergids
Exercise | Q 1.6 | Page 16
In angiosperms, endosperm is formed by/ due to ______
OPTIONS
- free nuclear divisions of megaspore
- polar nuclei
- polar nuclei and male gamete
- synergids and male gamete
SOLUTION
In angiosperms, endosperm is formed by/ due to polar nuclei and male gamete.
Exercise | Q 1.7 | Page 16
Point out the odd one.
OPTIONS
- Nucellus
- Embryo sac
- Micropyle
- Pollen grain
SOLUTION
Pollen grain.
Very short answer type question:
Exercise | Q 2.01 | Page 16
Name the part of gynoecium that determines the compatible nature of pollen grain.
SOLUTION
The stigma of the pistil determines the compatibility of the pollen grains, by allowing only compatible pollen grains to germinate.
Pistil or carpel is the individual member of the female reproductive whorl, the Gynoecium.
Exercise | Q 2.02 | Page 16
How many haploid cells are present in a mature embryo sac?
SOLUTION
Total 6 haploid cells are present in a mature embryo sac. They are antipodal cells (3), synergids (2), and egg cell (1).
Exercise | Q 2.03 | Page 16
Even though each pollen grain has 2 male gametes, why at least 20 pollen grains are required to fertilize 20 ovules in a particular carpel?
SOLUTION
During double fertilization, one of the male gamete of pollen grain fuses with egg cell, while other male gamete fuses with secondary nucleus. Thus to fertilize 20 ovules in a particular carpel, 20 pollen grains are required.
Exercise | Q 2.04 | Page 16
Define megasporogenesis.
SOLUTION
It is the process of formation of haploid megaspores from diploid megaspore mother cell (MMC) by meiotic division.
Exercise | Q 2.05 | Page 16
What is hydrophily?
Pollination carried out by water is called hydrophily.
Exercise | Q 2.06 | Page 16
Name the layer which supplies nourishment to the developing pollen grains.
Tapetum supplies nourishment to the developing pollen grains.
Exercise | Q 2.07 | Page 16
Define Parthenocarpy.
SOLUTION
It is the condition in which fruit is developed without the process of fertilization is called parthenocarpy.
Exercise | Q 2.08 | Page 16
Are pollination and fertilization necessary in apomixis?
SOLUTION
In apomixis, the embryo is formed without the formation of gametes and fertilization.
Thus, pollination and fertilization are not necessary for apomixis.
Exercise | Q 2.09 | Page 16
Name the parts of pistil which develop into fruits and seeds.
SOLUTION
After fertilization, the ovary of the pistil develops into fruit and ovules into seeds.
Exercise | Q 2.1 | Page 16
What is the function of filiform apparatus?
SOLUTION
Filiform apparatus guide the entry of pollen tube towards the egg.
Exercise | Q 3.1 | Page 16
How polyembryony can be commercially exploited?
SOLUTION
Polyembryony increases the chances of survival of the new plants.
Genetically uniform parental type seedlings are obtained from nucellar embryos, thus nucellar adventive polyembryony is of great significance in horticulture.
Plantlets obtained from these embryos are disease-free.
These embryos can be isolated and grown on embryo culture to produce clones.
Exercise | Q 3.2 | Page 16
Pollination and seeds formation is very crucial for fruit formation. Justify the statement.
SOLUTION
Pollination is a very important part of the life cycle of a flowering plant.
The flowers must be pollinated in order to bring about the process of fertilization.
Pollination brings male and female gametes of a flower together during fertilization.
As a result of fertilization, ovary develops into fruits and ovules into seeds.
Seeds on germination give rise to a new plant that further grows and develops fruits and seeds. Thus pollination and seed formation are required to create offsprings for the next generation.
Exercise | Q 3.3 | Page 16
Incompatibility is a natural barrier in the fusion of gametes. How will you explain this statement?
SOLUTION
- Incompatibility refers to inability of certain gametes even from genetically similar plant species to fuse with each other.
- It is considered as the most prevalent and effective device to avoid inbreeding and outbreeding.
- Pollen pistil interaction is a dynamic process that involves pollen recognition followed by promotion or inhibition of the pollen.
- Chemical substances released by the style act as a barrier.
- Typically the pollen belonging to the correct mating type germinates on stigma, develops a pollen tube, and brings about fertilization.
- The pollens belonging to the other mating type are discarded.
- Thus, incompatibility is a natural barrier in the fusion of gametes.
Exercise | Q 3.4 | Page 16
Describe three devices by which cross-pollination is encouraged in angiosperms by avoiding self- pollination.
SOLUTION
Genetic diversity is an essential factor for evolution by natural selection. Continued self-pollination results in inbreeding depression. Thus, plants have developed many devices to encourage cross-pollination. Examples of outbreeding devices are as follows:
Unisexuality:
In this, the plant bears either male or female flowers. It is also called as dioecism.
As flowers are unisexual, self-pollination is not possible. Plants may be monoecious,
e.g. Maize or dioecious, e.g. Mulberry, Papaya.
Dichogamy:
In this, anthers and stigmas mature at different times in a bisexual flower due to which self-pollination is prevented. It can be further divided into two types:
a. Protandry:
In this type, anthers mature first, but the stigma of the same flower is not receptive at that time.
e.g. in the disc florets of sunflower.
b. Protogyny:
In this type, stigma of carpel matures earlier than anthers of the same flower. e.g. Gloriosa.
Prepotency:
In this, pollen grains of other flowers germinate rapidly over the stigma than the pollen grains from the same flower, e.g. Apple.
Heterostyly (heteromorphy):
Plants like Primula (Primrose) produce two or three types of flowers in which stigmas and anthers are placed at different levels (heterostyly and heteroanthy).
This prevents the pollens from reaching the stigma and pollinating it. In heteromorphic flowers, pollen grains produced from anther pollinate stigmas produced at the same level.
Thus self-pollination is not possible in such cases.
Herkogamy:
It is a mechanical device to prevent self-pollination in a bisexual flower. In plants, a natural physical barrier is present between two sex organs and avoid contact of pollen with the stigma of the same flower, in e.g. Calotropis, pentangular stigma is positioned above the level of anthers (pollinia).
Self-incompatibility (self-sterility):
This is a genetic mechanism due to which the germination of pollen on the stigma of the same flower is inhibited, e.g. Tobacco, Thea.
Exercise | Q 4.1 | Page 16
Describe the process of double fertilization.
SOLUTION
Double fertilization:
The fusion of one male gamete with an egg and that of another male gamete with a secondary nucleus is called as double fertilization.
It is the characteristic feature of angiosperms.
It was discovered by Nawaschin in the liliaceous plants like Lilium and Fritillaria.
When pollen grain reaches the surface of the stigma, it germinates and forms a pollen tube.
Pollen tube penetrates the stigma, style, ovary chamber and then enters the ovule.
The growth of the pollen tube is guided by the chemicals secreted by the synergids.
Usually, when a pollen tube enters the ovule through the micropyle, it is termed as porogamy.
But in some cases, it enters through chalaza which is known as chalazogamy. In some plants, it enters by piercing the integuments which are called mesogamy.
A pollen tube penetrates the embryo sac of ovule through its micropylar end.
The pollen tube carrying male gametes penetrates in one of the synergids.
Watery contents of synergid are absorbed by the pollen tube, due to which it ruptures and releases the contents, including the two non-motile male gametes.
As non-motile male gametes are carried through a hollow pollen tube, it is known as siphonogamy that ensures fertilization to take place.
Fertilization mainly involves two processes: Syngamy and Triple fusion.
a. Syngamy:
It is the fusion of haploid male gamete with a haploid female gamete (egg). It results in the formation of a diploid zygote which develops to form an embryo. Syngamy is a type of generative fertilization.
b. Triple fusion:
It is the fusion of second haploid male gamete with diploid secondary nucleus. It results in the formation of Primary Endosperm Nucleus (PEN) which develops into triploid endosperm. Triple fusion is a type of vegetative fertilization.
In this process, both the male gametes participate, due to which fertilization occurs twice in the same embryo sac, hence it is described as double fertilization.
Exercise | Q 4.2 | Page 16
Explain the stages involved in the maturation of microspore into a male gametophyte.
SOLUTION
Development of male gametophyte
Pollen grain/microspore marks the beginning of male gametophyte, thus it is the first cell of the male gametophyte.
It undergoes the first mitotic division to produce bigger, naked vegetative cells and small, thin-walled generative cells.
The vegetative cell is rich in food and having an irregularly shaped nucleus.
The generative cell floats in the cytoplasm of the vegetative cells.
The second mitotic division is concerned with generative cells only and gives rise to two non-motile male gametes.
The mitotic division of the generative cells takes place either in the pollen grain or in the pollen tube.
The pollen grains are shed from the anther, at this two-celled stage in most of the angiosperms.
Exercise | Q 4.3 | Page 17
Explain the development of dicot embryo.
SOLUTION
A: Oospore.
B: Two celled proembryo.
e: embryonal initial;
t: suspensor initial;
m: Embryo sac membrane.
B1 : 4-celled I-shaped proembryo;
e1, e2: embryonal initial; s1, s2 : suspensor initial.
C: Further development of embryo.
S: Suspensor, h: Hypophysis; E: Embryonal mass
D: L. S. of ovule
Endo: Endosperm in free nuclear stage.
Anti: Antipodal tissue.
Embryo: Developing embryo
E: Embryo showing further development of embryonic octants and hypophysis.
F: L. S. of ovule. Endosperm becoming cellular.
G: Embryo; Cot: Cotyledons; Hypo: Hypocotyl; Rad: Radicle; R.c.: Rootcap;
H: Mature seed; Pl: Plumule. Endosperm has been consumed almost completely
Development of dicot embryo:
The zygote divides to form two-celled proembryo.
The larger cell towards the micropyle is called basal or suspensor initial cell and smaller cell towards chalaza is called terminal or embryonal initial cell.
The suspensor cell divides transversely in one plane to produce filamentous suspensor of 6-10 cells.
The first cell of the suspensor towards the micropylar end becomes swollen and functions as a haustorium.
The lowermost cell of the suspensor is known as hypophysis.
The suspensor helps in pushing the embryo in the endosperm.
The embryonal initial undergoes three successive mitotic divisions to form octant.
The planes of divisions are at right angles to each other.
The lower tier of four cells of octant gives rise to hypocotyl and radicle whereas four cells of the upper-tier form the plumule and the one or two cotyledons.
The hypophysis by further division gives rise to the part of radicle and root cap.
Subsequently, the cells in the upper tier of the octant divide into several planes so as to become heart-shaped which then forms two lateral cotyledons and a terminal plumule.
Further enlargement of hypocotyl and cotyledons result in a curvature of the embryo and it appears horseshoe-shaped.
Exercise | Q 4.4 | Page 17
Draw a labeled diagram of the L.S. of anatropous ovule and list the components of the embryo sac and mention their fate after fertilization.
SOLUTION
1. Structure of anatropous ovule:
| Components of the embryo sac | Fate after fertilization |
| Ovule | Seed |
| Egg | Embryo |
| Nucellus | Perisperm |
| Secondary nucleus | Endosperm |
| Outer integument | Testa (outer seed coat) |
| Inner integument | Tegmen (inner seed coat) |
| Micropyle | An opening in the seed (i.e. micropyle) |
| Synergids | Degenerate |
| Antipodals | Degenerate |
Fill in the blank:
Exercise | Q 5.01 | Page 17
The _________ collect the pollen grains.
SOLUTION
The stigma collect the pollen grains.
Exercise | Q 5.02 | Page 17
The male whorl, called the ________ produces ________.
SOLUTION
The male whorl, called the androecium produces pollen grains.
Exercise | Q 5.03 | Page 17
The pollen grains represent the ________.
SOLUTION
The pollen grains represent the male gametophyte.
Exercise | Q 5.04 | Page 17
The ________contains the egg or ovum.
SOLUTION
The embryo sac contains the egg or ovum.
Exercise | Q 5.05 | Page 17
________takes place when one male gamete and the egg fuse together. The fertilized egg grows into a seed from which the new plants can grow.
SOLUTION
Syngamy (fertilization) takes place when one male gamete and the egg fuse together. The fertilized egg grows into a seed from which the new plants can grow.
Exercise | Q 5.06 | Page 17
The ______ is the base of the flower to which other floral parts are attached.
SOLUTION
The thalamus is the base of the flower to which other floral parts are attached.
Exercise | Q 5.07 | Page 17
________is the transfer of pollen grains from the anther of the flower to the stigma of the same or a different flower
SOLUTION
Pollination is the transfer of pollen grains from the anther of the flower to the stigma of the same or a different flower.
Exercise | Q 5.08 | Page 17
Once the pollen reaches the stigma, the pollen tube traverses down the ________to the ovary where fertilization occurs.
SOLUTION
Once the pollen reaches the stigma, the pollen tube traverses down the style to the ovary where fertilization occurs.
Exercise | Q 5.09 | Page 17
The ______ are coloured to attract the insects that carry the pollen. Some flowers also produce ______ or ______ that attracts insects.
SOLUTION
The petals are coloured to attract the insects that carry the pollen. Some flowers also produce sweet odour or nectar that attracts insects.
Exercise | Q 5.1 | Page 17
The whorl ________is green that protects the flower until it opens.
SOLUTION
The whorl calyx is green that protects the flower until it opens.
Exercise | Q 6 | Page 17
Label the parts of seed. - Biology
![Reproduction in Lower and Higher Plants [ Exercise,Solutions,Notes ]](https://blogger.googleusercontent.com/img/a/AVvXsEh5vyIBr2EkCZqYPF68ipISA1GMg0GITqEcW97u84a9r0osdvR0py03nnXh6Ttb1Zt7rF59uK5bY1TKAkjeYguRUkCMOXIsQDkubMdDJUCvkkh0UCBoYJO2-rUOtiMU_emxusWj5qCXfvpIG0YUtka8RbVo6N-a-JlawwLaCkVnwLpUQJBb8O4xnt5k=w640-h489-rw "Reproduction in Lower and Higher Plants [ Exercise,Solutions,Notes ]")
Exercise | Q 7 | Page 17
Match the column.
Match the column.
| Column - I (Structure before seed formation) | Column - II (Structure after seed formation) | ||
| A. | Funiculus | I. | Hilum |
| B. | Scar of ovule | II. | Tegmen |
| C. | Zygote | III. | Testa |
| D. | Inner integument | IV. | Stalk of seed |
| V. | Embryo | ||
OPTIONS
A - V, B - I, C - II, D - IV
A - III, B - IV, C - I, D - V
A - IV, B - I, C - V, D - II
A - IV, B - V, C - III, D - II
SOLUTION
A - IV, B - I, C - V, D – II
Reproduction in Lower and Higher Plants [ Exercise,Solutions,Notes ]
1.1 Asexual Reproduction :
Asexual reproduction does not involve
fusion of two compatible gametes or sex cells.
It is the process resulting in the production of
genetically identical progeny from a single
organism and inherits the genes of the parent.
Such morphologically and genetically identical individuals are called clones. Organisms
choose to reproduce asexually by different
modes or ways:
i.
Fragmentation : Multicellular organisms
can break into fragments due to one or
the other reasons. e.g. Spirogyra. These
fragments grow into new individuals
ii. Budding : It is the most common method
of asexual reproduction in unicellular
Yeast. Usually it takes place during
favourable conditions by producing one
or more outgrowths (buds). These buds on
seperation develop into new individual
iii. Spore formation : In Chlamydomonas
asexual reproduction occurs by flagellated,
motile zoospores which can grow
independently into new individuals.
Other methods of asexual reproduction
include - Binary fission which occurs in
Amoeba, Paramoecium; Conidia formation
in Penicillium and Gemmules formation in
Sponges.
Vegetative Reproduction :
Plants reproduce asexually through their
vegetative parts. Hence, the new plants formed
are genetically identical to their parents.
There are also few methods which would
not occur naturally in the plants. Agriculture
and horticulture exploit vegetative reproduction
in order to multiply fresh stocks of plants.
Artificial methods are used to propagate desired
varieties according to human requirements.
The various methods are as follows :
a. Cutting :
The small piece of any vegetative part
of a plant having one or more buds is used
for propagation viz. Stem cutting - e.g. Rose,
Bougainvillea; leaf cutting - e.g. Sansvieria;
root cutting e.g. Blackberry
b. Grafting :
Here parts of two plants are joined in
such a way that they grow as one plant. In
this method, part of the stem containing more
than one bud (Scion) is joined onto a rooted
plant called stock, is called grafting. Whereas
budding is also called bud grafting in which
only one bud is joined on the stock, e.g. Apple,
Pear, Rose, etc. carefully grown to give many plant lets.
Micropropagation method is also used now
a days.
1.2 Sexual Reproduction :
It involves fusion of two compatible
gametes or sex cells. All organisms reach to the
maturity in their life before they can reproduce
sexually. In plants, the end of juvenile or
vegetative phase marks the begining of the
reproductive phase and can be seen easily in
the higher plants at the time of flowering.
The flower is specialized reproductive
structure of a plant in which sexual reproduction
takes place. The function of flower is to produce
haploid gametes and to ensure that fertilization
will take place. Typical flower consists of four
different whorls viz. calyx, corolla androecium
and gynoecium.
Sexual reproduction involves two major
events viz. meiosis and fusion of gametes
to form diploid zygote and the production of
genetically dissimilar offsprings.
Variations
are useful from the point of view of the survival
and the evolution of species, over the time.
Sexual reproduction is characterised
by fusion of the male and female gametes
(fertilization), the formation of zygote and
embryogenesis. Sequential events that occur
in sexual reproduction are grouped into three
distinct stages viz, Pre-fertilization, Fertilization
and the Post-fertilization.
The male reproductive whorl of flower
is called androecium. Individual member of
androecium, is called stamen. Stamen consists
of filament, connective and anther.
Structure of Anther :
An immature stage of anther is represented
by group of parenchymatous tissue surrounded
by single layered epidermis. Anther is
generally dithecous (having two lobes) and
tetrasporongiate. Each monothecous anther
contains two pollen sacs. In dithecous anther
four pollen sacs are present. Therefore, it
is tetrasporongiate. The heterogenesity
(differenciation) arises when some hypodermal
cells get transformed into archesporial cells.
T. S. of Anther :
The archesporial cell divides into an inner
sporogenous cell and outer primary parietal
cell. Sporogenous cell forms sporogenous
tissue. Each cell of sporogenous tissue is
capable of giving rise to a microspore tetrad.
Parietal cell undergoes divisions to form anther
wall layers. The wall of mature anther consists
of four layers. Epidermis is the outermost
protective layer made up of tabular (flattened)
cells. Endothecium is sub-epidermal layer
made up of radially elongated cells with
fibrous thickenings. Inner to endothecium is
middle layer made up of thin walled cells (1-2
layered), which may disintegrate in mature
anther. Tapetum is the inner most nutritive
layer of anther wall. It immediately encloses
the sporogenous tissue (microspore mother
cells).
1.3 Microsporogenesis :
Each microspore mother cell divides
meiotically to form tetrad of haploid
microspores (pollen grains).
Structure of microspore :
Typical pollen grain is a non-motile,
haploid, unicellular body with single nucleus.
It is surrounded by a two layered wall called
sporoderm. The outer layer exine is thick
and made up of complex, non-biodegradable,
substance called sporopollenin. It may
be smooth or with a sculptured pattern
(characteristic of the species). It is resistant to
chemicals. At some places exine is very thin
showing thin areas known as germ-pores.
These are meant for the growth of emerging
pollen tube during germination of pollen grain.
The inner wall layer, intine consists of cellulose
and pectin.
Development of male gametophyte :
Pollen grain marks the begining of
male gametophyte. It undergoes first mitotic
division to produce bigger, naked vegetative
cell and small, thin walled generative cell.
The vegetative cell is rich in food and having
irregular shaped nucleus. The generative cell
floats in the cytoplasm of vegetative cell.
Female reproductive whorl of flower is
gynoecium (Pistil). Individual member of
gynoecium is called carpel (megasporophyll).
A flower with many, free carpels is called
apocarpous (e.g. Michelia). A syncarpous
flower is one that has many carpels fused
together (e.g. Brinjal). Typical carpel has three
parts viz, ovary, style and stigma. The number
of ovules in the ovary varies e.g. paddy, wheat
and mango are uniovulate whereas tomato
and lady’s finger are multiovulate.
The second mitotic division is concerned
with generative cell only and gives rise to two
non-motile male gametes. The mitotic division
of generative cell takes place either in pollen
grain or in the pollen tube. The pollen grains
are shed from the anther, at this two- celled
stage in most of the angiosperms.
1.4 Structure of Anatropous ovule :
Each ovule develops inside the ovary and
is attached to the placenta by a small stalk
called funiculus. The place of attachment of
funiculus with the main body of ovule, is called
hilum. In angiosperms, the most common type
of ovule is anatropous in which micropyle is
directed downwards and is present adjacent to
the funiculus (funicle). The ovule consists of
central parenchymatous tissue, the nucellus
which is surrounded usually by two protective
coverings called integuments viz. Outer and
an inner integument.
A narrow opening at the apex of the ovule
is called micropyle. Chalaza is the base of
ovule directly opposite to micropyle. Embryo
sac (female gametophyte) is oval multicellular
structure embedded in the nucellus
1.5 Megasporogenesis :
It is the process of formation of haploid
megaspores from diploid megaspore mother
cell (MMC). Megaspore mother cell becomes
distinguished in the nucellus, more or less in
the centre but towards micropylar end of ovule.
Development of female gametophyte :
Megaspore mother cell undergoes
meiosis to form linear tetrad of haploid cells
i.e. megaspore. Upper three megaspores abort
and lowest one towards centre of nucellus
remains functional. It acts as the first cell of
female gametophyte. Generally one megaspore
towards centre is functional megaspore. It is
infact the first cell of female gametophyte.
It undergoes three successive, free nuclear
mitotic divisions. Thus total eight nuclei are
formed, four of which are located at each pole.
One nucleus from each pole migrates towards
the centre and are called polar nuclei. Three
nuclei towards micropylar end constitute egg
apparatus. It consists of large central, haploid
egg cell and two supporting haploid synergid
cells. Synergid shows hair like projections
called filiform apparatus, which guide the
pollen tube towards the egg.
Antipodal cells are group of three cells
present at the chalazal end. The two haploid
polar nuclei of large central cell fuse to form
diploid secondary nucleus or definitive
nucleus, just prior to fertilization. This sevencelled and eight nucleated structure is called
an embryo sac. This method of embryo
sac development from a single megaspore
is described as monosporic development.
In angiosperms, the development of female
gametophyte is endosporous i.e. within the
megaspore. Female gametophyte is colourless,
endosporic and is concealed in the ovule
enclosed by ovary
1.6 Pollination :
Pollen grains being non motile, angiosperms
have evolved the strategy to use abiotic agents
(wind, water) and biotic agents (birds, insects,
snails) to their flowers, feeding the visitors
and exploiting their mobility for pollination
and also seed dispersal. Pollen grains are
non-motile and they are usually carried from
flower to flower by means of external agents.
Pollination is the transfer of pollen grains from
anther to the stigma of the flower. It is the prerequisite for fertilization because both the male
and female gametes are non-motile. Moreover
gametes are produced at two different sites.
Self pollination is a type of pollination
which occurs in a single flower or two flowers
on a single plant. It results in inbreeding or
selfing. In contrast cross pollination is the
transfer of pollen grains from the anther of
one flower to the stigma of another flower of
different plants of same species. Pollination
can be further divided into three types on the
basis of source of pollination.
a. Autogamy (self pollination) :
It is a type of pollination in which bisexual
flower is pollinated by its own pollen grains.
Offsprings are genetically identical to their
parents e.g. pea.
c. Xenogamy (cross polination/ out
breeding) :
It is a type of cross pollination when pollen
grain of one flower is deposited on the stigma
of a flower of different plant belonging to same
species, with the help of pollinating agency. It
generates genetically varied offsprings.
Majority of flowering plants depend on
the transfer of pollen grains. Virtually all seed
plants need to be pollinated. Most of the food
and fibre crops grown throughout the world,
depend upon pollinators for reproduction.
The agents responsible for pollination have
been grouped into two main categories :
A. Abiotic agents
B. Biotic agents
A. Abiotic Agents : These are non-living
agents which include wind and water.
1. Pollination by wind (Anemophily) :
Most of the important crop plants are
wind pollinated. These includes wheat, rice,
corn, rye, barley and oats. Palms are also wind
pollinated
Adaptations in anemophilous flowers :
• The flowers are small, inconspicuous,
colourless, without nectar and fragrance
(odour).
• The pollen grains are light in weight, dry
and produced in large numbers to increase
chances of pollination considering wastage
of pollengrains.
• Stigma is feathery to trap pollens carried
by wind currents.
• Stamens are exserted with long filaments
and versatile anthers.
• Stamens and stigmas are exposed to air
currents.
2. Pollination by water (Hydrophily) :
Found only in some 30 genera of
aquatic monocots. E.g. Vallisneria, Zostera,
Ceratophyllum etc.
Adaptations in hydrophilous flowers :
• Flowers are small and inconspicuous.
• Perianth and other floral parts are
unwettable.
• Pollen grains are long and unwettable due
to presence of mucilage.
• Nectar and fragrance are lacking in
flowers.
Hydrophily is of two types -
Hypohydrophily : Pollination occurs below
the surface of water. Here the pollen grains are
heavier than water, sink down and caught by
stigmas of female flowers, e.g. In Zostera (sea grass) the pollen grains are long, ribbon like
and without exine.
Epihydrophily : The pollen grains float on
the water surface and reach the stigma of
female flower. e.g. Vallisneria is a submerged
dioecious, fresh water aquatic plant in which
female flowers reach the water surface
temporarily to ensure pollination and male
flowers float on the surface of water.
• Specific gravity of pollen grain is equal
to that of water. That is why they float on
surface of water.
• Some aquatic plants are anemophilous e.g.
Potamogeton, Halogaris, etc.
• Some aquatic plants are entomophilous e.g.
Lotus, water hyacinth, waterlily, etc.
B. Biotic Agents : It includes living agents.
About 80% of plants require the help of other
living, moving creatures such as insects, birds,
bats, snails to transfer their pollens from one
flower to another. These also sustain our
ecosystems and produce natural resources by
helping plants to reproduce.
1. Pollination by insects (Entomophily) :
It occurs in Rose, Jasmine, Cestrum, etc.
Adaptations in entomophilous flowers :
• They are large, showy and often brightly
coloured.
• The flowers produce sweet odour (smell)
and have nectar glands.
The stigma is rough due to presence of hair
or is sticky due to mucilaginous secretion.
• The pollen grains are spiny and surrounded
by a yellow sticky substance called pollenkit.
• Some plants have special adaptations
for the insect visitor to help in cross
pollination, e.g. lever mechanism or turnpipe mechanism in Salvia
Pollination by birds (Ornithophily) :
Only a few types of birds are specialised
for pollination. They usually have small size
and long beaks e.g. Sun birds and humming
birds. Some ornithophilous plants are Bombax,
Callistemon (Bottle Brush), Butea, etc.
Adaptations in ornithophilous flowers :
• Flowers are usually brightly coloured,
large and showy.
• They secrete profuse, dilute nectar.
• Pollen grains are sticky and spiny.
• Flowers are generally without fragrance,
as birds have poor sense of smell.
3. Pollination by Bats (Chiropteryphily) :
Bats can transport pollens over long
distance, some times several kilometers.
Adaptations in Chiropterphilous flowers :
• Flowers are dull coloured with strong
fragrance.
• They secrete abundant nectar.
• Flowers produce large amount of edible
pollen grains, e.g. Anthocephalous
(kadamb tree), Adansonia (Baobab tree),
Kigelia (Sausage tree)
Outbreeding devices (contrivances):
Many plants have mechanisms that
discourage or prevent self pollination. To
promote cross pollination and increase genetic
diversity, plants have evolved a wide variety
of sexual strategies.
Genetic diversity is
an essential factor for evolution by natural
selection. Continued self pollination results in
the inbreeding depression.
Thus plants have developed many devices
to encourage cross pollination. The examples
of outbreeding devices are as follows:
Unisexuality :
In this case, the plant bears either male or
female flowers. It is also called as dioecism.
As flowers are unisexual, self pollination is
not possible. Plants may be monoecious, e.g.
Maize or dioecious, e.g. Mulberry, Papaya.
Dichogamy :
In this device, anthers and stigmas mature
at different times in a bisexual flower so as
to prevent self pollination. It can be further
divided into two types:
1. Protandry : In this type, androecium
matures earlier than the gynoecium, e.g. in
the disc florets of sunflower.
2. Protogyny : In this type, gynoecium matures
earlier than the androecium, e.g. Gloriosa
Prepotency :
Pollen grains of other flowers germinate
rapidly over the stigma than the pollen grains
from the same flower, e.g. Apple.
Heterostyly (heteromorphy):
In some plants like Primula (Primrose,
there are two or three forms/ types of flowers
in which stigmas and anthers are placed at
different levels (heterostyly and heteroanthy).
This prevents the pollens from reaching the
stigma and pollinating it. In heteromorphic
flowers, pollen grains produced from anther
pollinate stigmas produced at the same level.
Herkogamy :
It is a mechanical device to prevent self
pollination in a bisexual flower. In plants,
natural physical barrier is present between
two sex organs and avoid contact of pollen
with stigma of same flower, e.g. Calotropispentangular stigma is positioned above the
level of anthers (pollinia).
Self incompatibility (self sterility):
This is a genetic mechanism due to which
the germination of pollen on stigma of the same
flower is inhibited, e.g. Tobacco, Thea.
Pollen - Pistil Interaction :
It is the interaction of pollen grains with
sporophytic tissue (stigma). It begins with
pollination and ends with fertilization. All
the events from the deposition of pollen grain
on stigma to the entry of pollen tube in the
ovule (synergid) are referred as pollen - pistil
interaction. Pollination does not guarantee the
transfer of right type of pollen, often wrong type
also land on stigma. The pistil has the ability
to recognise and accept the right or compatible
pollen of the same species. Thus wrong type
of pollen is discarded by pistil. Compatibility
and incompatibility of the pollen-pistil is
determined by special proteins.
This process
involves pollen recognition followed by
promotion or inhibition of pollen.
The stigmatic surface of flower refuse
other wrong type or incompatible pollen
grains. A physiologial mechanism operates to
ensure that only intraspecific pollen germinate
successfully. The compatible pollen absorbs
water and nutrients from the surface of stigma,
germinates and produces pollen tube. Its growth
through the style is determined by specific
chemicals.
The stigmatic surface provides
the essential prerequisites for a successful
germination, which are absent in the pollen.
The pollen tube is finally pushed through the
ovule and reaches the embryo sac. The tip of
the pollen tube enters in one of the synergids
and then ruptures to release the contents. Due
to pollen pistil interaction, intense competition
develops even in the compatible pollen grains
(gametes).
It also plays important role in sexual
reproduction and seed formation.
Pollen grain
can also be induced to germinate in a synthetic
medium. Sucrose induces pollen germination
and tube growth in vitro. Addition of boric acid
facilitates and accelarates pollen germination.
Artificial hybridization :
It is one of the major approaches used
in the crop improvement. Only the desired
Double Fertilization :
Double fertilization is a complex
fertilization mechanism in flowering
(angiospermic) plants. It was discovered by
Nawaschin in the liliaceous plants like Lilium
and Fritillaria.
After a pollen grain has reached the surface
of the stigma, it germinates and forms a pollen
tube, which penetrates the stigma, style, ovary
chamber and then enters ovule. The growth of
pollen tube is guided by the chemicals secreted
by the synergids. It usually enters ovule through
the micropyle. It is termed as porogamy. But in
some cases, it is found to enter through chalaza,
known as chalazogamy and in some plants by
piercing the integuments, called mesogamy.
Finally, it penetrates embryo sac of ovule
through its micropylar end.
The pollen tube carrying male gametes
penetrates in one of the synergids. Watery
contents of synergid are absorbed by pollen
tube which then ruptures and release the
contents, including the two non-motile male
gametes. As non motile male gametes are
carried through hollow pollen tube, it is known
as siphonogamy that ensures fertilization to
take place. Syngamy and triple fusion are two
events of sexual reproduction in angiospermic
flowering plants. Syngamy is the fusion of
haploid male gamete with haploid female
gamete (egg) to produce a diploid zygote,
whereas in triple fusion, second haploid male
gamete fuses with diploid secondary nucleus
producing primary endosperm nucleus (PEN)
that developes into triploid endosperm. The
zygote develops into an embryo. Syngamy is
a type of generative fertilization whereas triple
fusion is a type of vegetative fertilization.
Here, both the male gametes participate
and therefore, it is described as or called double
fertilization
Significance of Double Fertilization :
• It is a unique feature of angiosperms. It
ensures that the parent plant invests a
seed with a food store, only if the egg is
fertilized.
• The diploid zygote develops into an embryo
which conseq
Polyembryony :
It is the development of more than one
embryos, inside the seed and the condition
is described as polyembryony. It was first
noticed by Leeuwenhoek (1719) in the seeds
of Citrus genus. It is the occurrence of more
than one embryo in a seed which consequently
results in the emergence of multiple seedlings.
The additional embryos result from the
differentiation and development of various
maternal and zygotic tissues associated with
the ovule of seed.
Polyembryony may be true
or false depending upon whether many embryos
arise in the same embryo sac or in different
embryo sacs in the same ovule. In adventive
polyembryony, an embryo develop directly
from the diploid cell of nucellus and integuments
as in Citrus. In cleavage polyembryony, zygote
proembryo sometimes divides (cleaves) into
many parts or units. Each unit then developes
into an embryo. Polyembryony increases
the chances of survival of the new plants.
Nucellar adventive polyembryony is of great
significance in horticulture.