This chapter includes flowers, their detailed structure and developmental processess which took place durin sexual reproduction. Helpful for Board and NEET students.
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2. flower
•A flower, sometimes known as
a bloom or blossom, is
the reproductive structure found
in flowering plants.
•The flower may be defined as a
condensed shoot which has become
specialized as reproductive organ.
• It is a condensed modified shoot or
branch in which the leaves are extremely
altered to form its parts.
• Flowers develop from a stem branch
either in a terminal or axillary position.
3. Parts of a typical flower
Flowers present a great diversity
in their structure.
A typical flower consists of four
distinct whorls of floral leaves and
These may be divided in to two main
groups.
1) Non essential Parts or Outer whorl
Calyx and Corolla
Protection during reproduction
2) Essential Parts or Inner whorl
androecium and gynoecium
They take part in reproduction
4. Sepals :- The outermost green leaf like parts that cover the unopened flower or bud are
sepals.Taken collectively they form the calyx.
Petals :- The inner leaf like structures are brightly coloured.Taken collectively they are called
corolla.
NOTE:- The sepals and petals, when not differentiated into calyx and corolla, are called
perianth.
Stamen:- It is the third inner part that produces pollen in which sperm or male cell
originate. Collectively the stamens from androecium. Each stamen consist of a slender
stalk or filament, supporting a knob like sac called the anther. It produces yellow or
reddish powdery grains called pollen grains which play an important part in
reproduction.
Pistil:- It is the innermost organ that produces ovules within which female gamete
originate. It has following parts:-
a) Stigma :- Sticky structure for receiving the pollen.
b) Style :- slender tube that supports stigma.
c) Ovary :- The basal bulged part of the pistil is ovary. Inside ovary lies the ovarian cavity
(locule).The placenta is located inside the locule.
d) Ovule :-From the placenta megasporangia arises, commonly called ovules.
5. Male reproductive organ
Stamen:- Stamens are composed of
anther and filament.The anther lobe
may be of two type:
Monothecus(onelobe)
Ex- China rose, cotton.
Dithecus(twolobe)
Ex- Datura.
On the basis of attachment of
anther lobe with filament
stamens are of four types :
Basifixed
Adnate
Dorsifixed
Versatile
8. This structure helps in microsporogenesis (a process of development of male gamete).
Between both the lobes there is a layer of connective sterile tissue which does not
participate in reproductive process.
The area between microsporangia and outer anther layer is called anther wall which is
made up of four layers:
1) Epidermis- Outermost layer which helps in protection.
2) Endothecium-This layer has radially arranged cells and these cells have callose
thickening.
3) Middle layer-Three to four layers of loosely arranged thin walled parenchyma cells.
These three layers helps in dehisence.When anther matures, these three layer become rigid
except the point of dehisence from where the anther ruptures.
4) Tapetum- It is innermost layer, cells are pyramid shaped and multinucleated with thick
cytoplasm.Tapetum is a nutritive layer which provide nourishmment to the
developing microspore.Tapetum also secrete callase which dissolve callose of
microspore tetrad.
Tapetum produces Ubisch bodies which are coated with sporopollenin which makes exine
of the pollen grain.
Sporopollenin is the most resistable substance found in the living world.
Tapetum helps in formation of pollenkit and normally found as powdery substance.
Ex- Hibiscus.
9. microsporogenesis
The process of formation of microspores (male gamete) in
microsporangium is called microsporogenesis.
As the anther is young all these four microsporangia are filled with
sporogenous cells called archesporial cells but as anther grows :
1)Its size also increases.
2)The size of microsporangium increases
3)The space between the cells increases and microsporangium now called
pollen sac.
Sporogenous cells are capable of forming microspores. Not all the
sporogenous cells converts in to microspores rather few degenerates
which is utilized by these sporogenous cells and it is now called
microspore mother cell (MMC)/(PMC) which will undergo meiosis to
form microspore.
The pollen grains of a tetrad grow and separate from one another.
Usually the arrangement of microspores in a tetrad is tetrahedral or
Isobilateral. However, decussate, linear and T shaped are also found.
12. Pollen Grain
The pollen grains represents the male gametophyte.
Now, this pollen grain undergoes one mitotic division in which
karyokinesis takes place first which is followed by cytokinesis.
So, now nucleus will divide first and two nuclei would form and this
is followed by cytokinesis which is not equal to both the nuclei.
The separation of cytoplasm (cytokinesis) is not equal. One part of
cell would get more cytoplasm while other will get less cytoplasm.
The larger part will be called as vegetative cell or tube cell while
the smaller cell is call generative cell.
In most of the angiosperms, the pollen grain released at two
celled (tube cell + generative cell)stage.
Anther wall get fibrous and from point of dehiscence the anther
get rupture and pollen grains are released.
Allergy to pollen grains is called Hay fever.
14. The hard outer layer is called exine which is made up of
sporopollenin. It can withstand high temperature, strong acids and
alkali.
The inner wall of pollen grain is called intine which is made up of
cellulose and pectin.
When pollen grain matures it contains two cells vegetative cell and
generative cell.The vegetative cell is bigger, has abundant food
reserve and large irregular shaped nucleus.The generative cell is
small and floats in the cytoplasm.
Pollen grains are normally shed at 2 celled stage but in some
species they may shed at 3 celled stage (generative cell divides
mitotically to give rise to the two male gametes).
Pollen grains are rich in nutrients. Pollen grains have to land on the
stigma before they lose viability if they have to bring about
fertilization.
Pollen grains are stored at liquid nitrogen (-196 degree Celsius)
which can be used as pollen banks.
15. The Pistil, Megasporangium(Ovule) andEmbryosac
The pistil is centrally situated female reproductive organ of the
flower.The free unit of gynoecium is called pistil.
The pistil may be made of single or many ovules. It may be simple
or compound.A simple pistil is always is always monocarpellary.
Such a pistil is found in the flowers of pea, bean, Gold mohur etc.
In monocarpellary or simple pistil the ovary is single chambered.
The compound or polycarpellary pistil may be of two types:
a) Apocarpous :- In this condition, there are many carpel's but they
are all free and each carpel forms a separate ovary. Ex- Rose,
Lotus, Strawberry.
b) Syncarpous :- In this condition, there may be two or more carpel's
but they are always more or less fused with one another to form a
single ovary. Ex- Mustard, Lady’s finger.
In Angiosperms the ovule found is Anatropous and Bitegmic
16.
17.
18. Anatropous &Bitegmic ovule
• Funicle : It helps in the attachment of
ovule with placenta. Ovule is attached to ovary
with placenta.
•Hilum :The body of the ovule fuses with
Funicle and the point of attachment is called
Hilum.
• Nucellus : It provide nourishment to the
developing embryo. Sometime the nucellus is less
= Tanuinucellate and if nucellus is plenty =
Crassinucellate.
• Micropyle end :The integument
leaves a narrow passage known as Micropyle at
one end of ovule. Pollen tube enters to the
embryosac through Micropyle aperture.
•Chalazal end:The place of origin of the
integuments usually lies at opposite end of
Micropyle, known as chalaza.
• Integuments : Protects the inner
tissues of ovule.
• Embryosac : Contains embryo which
develop in to seed.
Cells of nucellus towards Micropyle end
get change into MMC (2n) and
undergoes meiosis to give rise to
megaspore.
19. megasporogenesis
The process of formation of haploid megaspores from the diploid
megaspore mother cell is called megasporogenesis.
Generally, a single megaspore mother cell (MMC) differentiate in
the micropylar region of nucellus.
The MMC undergoes meiotic division which results in the
production four haploid megaspore.
In majority of angiosperms, only one of the megaspores is
functional while the other three degenerates.
During Megasporogenesis, one nucellar cell at Micropyle end
form archesporial cell.
This archesporial cell divides to form a primary parietal cell ( stops
further division) and primary sporogenous cell which leads to the
formation of MMC (2n) which undergoes 2 successive meiotic
division to form 4 haploid megaspore which are arranged in linear
tetrad.
20. Out of these four megaspore, three megaspore towards Micropyle
degenerate and one towards chalaza enlarges to form functional
megaspore.
The functional gametophyte forms female gametophyte or
Embryosac.
Nucleus of megaspore divides mitotically (only karyokinesis, 3
times) to form 8 daughter nuclei and all haploid.
Three nuclei migrate to the Micropyle end and get surrounded by
cytoplasm to form egg apparatus (middle cell forms the egg and
other two forms synergids or helping cell).
The other 3 nuclei migrate towards chalazal end and get
surrounded by cytoplasm and form 3 antipodal cells.
2 nuclei migrate to the middle and form 2 polar nuclei (both are
haploid or may fuse to form a secondary nucleus (2n) with a central
cell).
Thus, 8 nucleated, 7 celled embryosac is formed.
This is Monosporic (as it develops from one megaspore), 7 celled (3
antipodals+1 egg) and 8 nucleated embryosac is called polygonum
type of embryosac.
22. Types of ovule
There are following distinct types of ovules:
A) orthotropousovule : It is a straight or erect ovule. In this type the
micropyle lies at one pole while the chalaza and Hilum lies at
opposite pole.Thus, the micropyle, chalza and Hilum lies at same
vertical plane. Ex- betel, long pepper, etc.
23. B) Circinotropous ovule :This upright position of ovule is achieved by
complete rotation of the body.
C) Anatropous ovule :This is an inverted type of ovule so that the
integuments on one side is partly fused with the Funicle, forming a
region known as Raphe.This type of ovule is more common and
found in Pea, Gram, Castor etc.
D) Hemianatropous ovule : It is transverse type of ovule where chalazal
and micropyle are at perpendicular to Funicle.
E) Amphitropous ovule :This is intermediate between orthotropous and
anatropous ovule.The ovule is bent half way so that the long axis
of the ovule comes to lie at right angel to the funiculus. Ex- Poppy.
F) Campylotropous ovule :The body of the ovule get curved or bent
round like a horse shoe shape so that the micropyle Funicle and
chalaza come to lie at one pole but there is no fusion of the Funicle
with the integument. Ex- Mustard.
24. The transfer of pollen grains from the anther to the stigma is
termed as pollination.
It may be of two types:
A) Selfpollination
B) crosspollination
Incaseofselfpollinationthepollengrainsaretransferredfromtheanther tothe
stigma orstigmasofthesameflower.Itisalsotermedasautogamy.
Incaseofcrosspollinationthepollengrainsarecarriedinvariouswaystothe
stigmaorstigmas ofanotherflowereitheronthesameplantoronthe
differentplantofsamespecies.Itisalsoknownasallogamy/Xenogamy.
25.
26.
27. Adaptations for self pollination
Homogamy: Maturation of anther and stigma simultaneously.To ensure that self
pollination takes place, the anther and stigma of the bisexual flower or those of
the unisexual flower must mature at the same time. Ex- Mirabilis.
CLEISTOGAMY:This is a condition in which flower remains closed.There are some
bisexual flower which never open.As a result, the pollen grain are distributed on
the stigma of the same flower. Such flowers are small, inconspicuous and lacks
colours and fragrance. Ex- Oxalis,Viola, Commelina.
These plants produce two types of flowers :
A) Cleistogamousflower: (closed) adaptationfor self pollination.
B) Chasmogamous flower : (open) adaptation for crosspollination.
HOMOGAMY CLEISTOGAMOUS CHASMOGAMOUS
28. GEOCARPY:As soon as fertilization takes place in ground nut plant , the stalk of
the ovary elongates and starts growing towards the surface of the ground.
Pedicle burries its tip containing ovary in ground and ovary give rise to
underground fruit.
In vinca rosea, anther are present at the mouth of tubular corolla and at
maturity the stigma passes through the mouth of corolla tube resulting in self
pollination.
Arachis hypogea
30. In Mirabilis Jalapa (4o’clock plant), stamen bend on stigma on maturation
of anther.
In Solanum tuberosum, curving/coiling of style on anther. In sunflower,
self pollination occurs if cross pollination fails.
31. Adaptation for cross pollination
In majority of cases self pollination is prevented in a number of different ways
which are as follow :
Unisexuality/ Dicliny :
A) Dioeciousflowers: In this case the flower are always unisexual but male and female
flowers are borne on two different plants. Ex- Betel, Pepper vine, Mulberry.
B) Unisexualflowers : In these also there are male and female flowers, but they are
both borne on same plant.Thus in unisexual flower self pollination cannot
take place. Ex- castor, cucumber, pumpkin.
Bisexuality/Dichogamy : Maturation of stamen and pistil takes place at different
time.This condition is known as Dichogamy due to which self pollination is
prevented.
A) Protandrous:Those flowers in which stamens mature earlier than the pistil. Ex-
Jasmine,Coriander, Sunflower.
B) Protogynous: Those flowers in which pistil mature and stigma become receptive
earlier than the stamen in the same flower. Ex-Tobacco, Rose, Grasses.
32. •Heterostyly
Heterostylic flowers like primroses
show very distinct adaptation to
prevent self pollination.
In this case stigma and stamen occur
at two distinct positions and the pollen
grain produced are adapted to stick to
the stigma of the another flower.
The length of stamen and style are
different in different flowers.
oxalis
Evening prime rose
Pinflower Thrumflower
33. Pre-potency-Pollen grains of another flower germinates rapidly than the pollen
grains of the same flower over the stigma. Ex- apple, grape.
self-incompatibility: This is a genetic mechanism that 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.
Ex-Tobacco, Potato.
Hercogamy: In certain flowers, stamens and pistil come to maturity at the same
time, but still self pollination is avoided by means of following adaptation which
form a kind of physical barrier to self pollination :
A) The stamen and pistil may lie at some distance from each other.
B) In Aristolochia the flowers are trumpet shaped and hang somewhat
downwards and corolla act as a barrier.
C) In many orchids and calotropis the pollen grains are held together by a
delicate membrane which act as a physical barrier and the entire mass is
known as pollinium, is formed at a place from where it can not reach the
stigma by itself.The pollinia remain fixed in their position by adhesive disc and
can only be carried away by insects.
D) The anther may be inserted with in the corolla tube and the style is exerted or
the vice versa and thus self pollination is avoided.
35. Advantagesofcrosspollination
Cross pollination is more common in plants than self pollination
because of following advantages:-
It leads to the production of stronger and healthier seeds than self
pollination.
It provides mixing/blending of characters of two parents. so, it
provides an opportunity for the production of the individuals with
new and useful characters which is most likely to survive and
produce well develop plants.
It has been utilised by men to improve his crops and offer greater
variety. Many different kinds of fruits and vegetables are primarily
due to cross pollination.
Ex- A strong, healthy, disease resistant crop is desired, and one
which will produce good wheat grains for milling and making
bread. Many other characters have been modified as a result of
cross pollination.
36. Viability of pollen grains
It is a period for which pollen (male gamete) possess the ability to fertilize the
egg.
Ex-Wheat, rice= 30 min.
Most plant= few months
Longest pollen= Zostera
Larger pollen= Mirabilis Jalapa
Smallest pollen= Myosotis.
m y o s o t i s
S m a l l e s t p o l l e n
Largerpollen
37. Anemophily(Windpollination)
It is a mode of cross pollination or transfer of pollen grains from a
mature anther to the stigma of a pistil which is accomplished
through the agency of wind.
Pollen grains are produced very very large in numbers.
Pollens are light weight, dry dusty so that they can be easily blown
away by wind. In case of Pinus, the pollen grains may be winged
which are found hundreds of kilometers away from the parent
plants.
The anther should be well exposed or we can say the flower should
be well exposed above the leaf.
In anemophilous flowers, anthers are versatile. Non-essential
parts are either absent or reduced.
Stigma is exposed, sticky, feathery or hairy.
Flowers are not bright coloured and without nectar and fragrance.
Anemophily is highly wasteful as it is non-directional.
Ex- Maize, Wheat, Rice, Grass.
38.
39. Hydrophily(waterpollination)
It is the mode of cross pollination or transfer of pollen grains from
the mature anther of a flower to the stigma of another flower
which is accomplished through the agency of water.
In many aquatic plants with emergent flowers, pollination occurs
by wind or insects, e.g., Lotus,Water Lily, Water Hyacinth.
Flowers are small and inconspicuous.
Floral parts are unwettable.
Nectar and odour are absent.
Pollen grains are light and unwettable due to presence of mucilage
cover.
Stigma is long, sticky but unwettable.
Hydrophily is of two types— Hypohydrophily and Epihydrophily
40. Hypohydrophily Epihydrophily
Pollination takes place under the
water.
Plants are submerged rooted.
Zostera has the longest pollen
grain needle like without exine.
These pollen grains have same
specific gravity as water so they
remain in the water body.
The stigmas are also long.The
needle like pollen grains have great
chances to touch the long stigmas
and coil around the it to perform
pollination.
Ex-Zostera (MarineAngiosperms)
or Sea grass.
Plant is Dioecious and submerged
rooted.
The male flowers are arranged in a
cluster called spadix while female
flowers are produced on a long pedicel
which grows until the flower reaches to
the surface of water.
After maturation, the male flower
detaches from the cluster and
moves up to the surface of water
and the stamens are exposed.
At the same time female flower are
uncoils and reaches up to the
surface of water.
The pollens are released on the surface
of water and pollination takes place and
as soon as it happens the female flower
again recoils.
Ex-Vallisneria.
42. Entompphily (by insects)
The flowers are brightly coloured but if flowers are not coloured then some
other parts must be coloured. Like in Bougenvellia, the bract is coloured, in
Euphorbia, the leaf is coloured, in Mussaenda, the sepal is coloured.
Scent/ fragrance is present. Like Jasmine.
Flowers are conspicuous, if small then in clusters.
May produce edible pollen grains. Like pappaver, Rosa.
Nectar glands are present at such positions that the insect encounters anther
and stigma.
Flower shape and color may match an insect.The male moth sits on a flower
conceiving it as a female moth to copulate it, but inturn pollinates the flower.
Like Orchid flower.
In Yucca flower-Yucca moth (PronubaYuccasella) drills a hole in the ovary of
flower to lay her egg.The moth seals the hole with pollen grains from different
yucca flowers.
Now this plant and moth has a mutual relationship, means they can not survive
without each other.
Moth will get a safe place to lay her eggs and their larvae can feed on material
inside so that their survival is confirmed and in turn flower will get pollinated as
pollen grains are just near to the ovule.
44. Post pollination changes
Fertilization was discovered by STRASBURGER.
A) Pollen-Pistil Interaction/ Identification :
Pollen grains of various plants may land over stigma.
The pistil recognise the pollen grains, whether it is of the
right type (compatible) or of the wrong type (incompatible).
Pistil accepts the right pollen and promotes its growth
leading to fertilization where as the wrong pollens are
rejected from germination.
The events from pollen deposition on stigma, pollen
germination and entry of pollen tube into ovule for
fertilization are referred as pollen-pistil interaction.
45. Secretion from stigma : Stigma secretes a liquid mainly
consist of water and sugar.This liquid is absorbed by pollen
grains through germ pore and at a result of the intine
stretches but due to exine it cannot stretch, from germ pore
pollen tube germinates.
Normally, only one pollen tube emerges known as
Monosiphonous and if there are many pollen tube emerges
then known as Polysiphonous.
Growth of PollenTube: In most of the Angiosperms, Pollen
grains are released at two celled stage, one bigger vegetative
cell and a smaller generative cell.
After formation of pollen tube, the generative cell undergoes
one mitotic division and results in formation of two male
gametes.
Now, the whole cytoplasmic content including vegetative cell
and two male gamete will pass through the pollen tube.
46. The style from which pollen tube is going to pass may be
hollow or solid.
In case of solid style, it may chance that pollen tube get little
squeezed and may result in back flow of cytoplasmic content.
To avoid this situation callose plugs are formed so that even if
it get squeezed no content will go back.
47. There are three possibilities that pollen tube enters in the embryosac –
A) If pollen tube enters through micropylar end and reaches to embryosac-
Porogamy.
B) If pollen tube enters through chalazal end end and reaches to embryosac-
Chalazogamy.
C) If pollen tube enters through integuments and reaches to embryosac-
Mesogamy.
Porogamy is most common amongAngiosperms.
48. Entry of pollen tube in to embryosac : One of these two
synergids starts to degenerates and the filliform apparatus of degenerating
synergid guides the pollen tube to enter in to embryosac.
As soon as the pollen tube enters in degenerating synergid, it ruptures and two
male gametes are released and wall of degenerating synergid also dissolves.
49. Fertilization : 1st male gamete (n)+Egg (n)= Zygote (2n) (Embryo)
Triple Fusion : 2nd male gamete+2 polar nuclei=Primary
Endosperm Nucleus (PEN) [3n].
The developing embryo will get nutrition from endosperm.
Fertilization andTriple fusion together known as Double
fertilization.
Double fertilization was explained by Nawaschin.
Fertilization takes place first, hen triple fusion takes place i.e. if
embryo is formed then only nourishment is required.
If fertilization fails then endosperm will not form.
FERTILIZATION FORMATION OF ENDOSPERM NUCLEUS(3n)
DEVELOPMENTOF EMBRYO FORMATION OF NUTRITIVETISSUE
50.
51. Post fertilization changes
The process of fertilization results in formation of diploid
zygote and triploid PEN respectively.
The diploid zygote represents the first cell of sporophytic
generation and it develops in an embryo.
The triploid PEN forms the endosperm for the nourishment of
the embryo.The ovule changes into seed, the integument
changes into seed coat and ovary forms the fruit.
Normally, till these changes takes place all these nucellus get
used up, but sometimes after formation of seed some part of
nucellus still remains i.e., persistent nucellar containing seed
is called perispermic seed. Ex-Beet root,Black pepper.
52. Formation of endosperm
In angiosperms, the endosperms develops in three different ways:
A) Free nuclear type :This is the most common type of endosperm
development.The endosperm nucleus give rise to a number of free nuclei.The
coconut water from a tender coconut id free nuclear endosperm having
thousands of free nuclei(only Karyokinesis).
B) CellularType : Division of PEN is immediately followed by wall formation so
that endosperm is cellular. Ex- white part of coconut.
C) HelobialType :It is intermediate between nuclear and cellular and is common
in monocots.The first division of PEN is followed by wall formation and after
that free nuclear division continues. Ex- Asphodelus.
53. Formation of embryo
DEVELOPMENT OF DICOT EMBRYO
The zygote divides by a transverse division producing a Basal cell
towards micropyle and a Terminal cell towards chalaza.
Basal cell undergoes many transverse division to form a long suspensor
(6-10 celled).The function of suspensor is to push the proembryo deep in
to the endosperm.
The terminal cell divides in various planes to form proembryo.
The cell of suspensor towards proembryo functions as hypophysis,
which divide to form 8 cells. 4 cells towards proembryo form root cortex
and other 4 cells towards suspensor give rise to root cap and root tip.
The terminal cell of suspensor towards micropyle swells and forms
haustoria.
The proembryo forms an octant (8 celled stage), of this octant 4 lower
cells towards the suspensor forms hypocotyl and 4 cells towards chalaza
give rise to plumule (shoot tip) and cotyledons.
In beginning the embryo is globular, as it grows it become heart shaped
and then get curved into a horse shoe shaped.
54. A typical dicotyledonous embryo consist of an embryonal axis
and two cotyledons.The portion of the embryonal axis above the
level of cotyledons is the epicotyl, which terminates with
plumule or stem tip.The cylindrical portion below the level of
cotyledons is hypocotyl that terminates at it lower end in the
radicle or root tip.The root tip is covered with root cap.
The seed is the final product of sexual reproduction in
angiosperms.
1)Radicle develops from
hypophysis.
2)Hypocotyl develops from the
lower half of proembryo.
3)The two cotyledons and
plumule develops from upper
half proembryo.
56. Development of Monocot Embryo
The zygote greatly enlarges in size and divide by two
transverse division to form 3 celled proembryo.These 3 cells
are- Basal cell (towards Micropyle end),Middle cell, Terminal
cell (towards chalazal end).
The large basal cell does not divide and is transformed in to a
large basal cell.Terminal cell undergoes number of divisions
in various plains and form a single cotyledon (Scutellum).
The middle cell undergoes transverse and vertical division and
give rise to suspensor cells, Coleorrhiza (Radicle covered
with scaly structures), Coleoptile (Plumule covered with scaly
structures) and hypocotyl.
In Monocots like Colocasia no suspensor is formed.
57. •The cotyledon here is very thin &
Papery and almost negligible reserve
food material, but still we are saying
it cotyledon. Why?
•Normally, Cotyledons have reserve
food material but here the reserve
food material has to pass on the
developing embryo.
•Whenever the endosperm is
digested, the digested endosperm
first comes to cotyledon then
through cotyledonary node it is
passed on to the developing
embryo.
58. Development of Seed
Zygote develops into embryo.
Triploid PEN give rise to nutritive endosperm which provides
nourishment to embryo.
Nucellus is used up during development of embryo.
Integuments of Ovule dries up.The outer integument become hard and
form Testa, while the inner forms membranous Tegmen.
Micropyle remains in form of a small pore on the surface of seed. Oxygen
and water enters the seed through this pore at the time of germination.
Hilum marks the point of attachment of ovule to Funicle/Stalk.
Dormancyofseed-Dormancy, is that phase in the lifecycle of a seed , when
it fails to germinate even though the environmental condition which are
normally considered favourable at present.
SeedViability-It is the capacity of germination under suitable conditions. It
varies in different kinds of seed. In few species, seeds loose their viability
with in a few month but large number of plants have several years of
seed viability.
Ex- Lupinusarcticus– Its seeds are excavated from Arctic tundra.They
remain in dormancy for 10,000 years. Phoenixdactylifera(date palm)
recently 2000 years old viable seeds excavated near dead sea.
59.
60. Albuminous Seed/Endospermic Seed : If after formation of embryo, endosperm still
remains as food storage tissue. Ex-Wheat, Barley, Maize, coconut etc.
Ex-Albuminous/Non-Endospermic Seed : Endosperm is completely used up by growing
embryo. For further development of embryo during germination, nutrition is stored in
cotyledons. Ex-Gram, Pea, Bean etc.
Occasionally, in some seeds such as black pepper, remains of nucellus also persistent.This
residual persistent nucellus is the perisperm and such seeds are called perispermic seed.
61. Classificationoffruits
Fertilized ovary changes in to fruits and their study is called
Pomology.
They can be classified as follow:
SIMPLE FRUITS: Single ovary of single flower.
(Unicarpellary, Multicarpellery Syncarpous)
AGGREGATE FRUITS: Multiple pistil of single flower.
(Multicarpellery Apocarpous)
COMPOSITE FRUITS:Fruitsdevelopsfromcompleteinflorescence.
Fruits canalsobeclassifiedas:
TRUE FRUIT/EUCARPS:Simplefruit.Ex-Mango,Pea.
FALSE FRUIT/ PSEUDOCARPS:AggregateandCompositefruits.
Ex-Apple,Pear,Brinjaletc.
62. The process of formation of embryo without the fusion of
gametes is known as Parthenogenesis.
Fruits are formed without fertilization is called
Parthenocarpic fruits. Such fruits are either seed less or
contain non-viable seeds.
Parthenocarpy can be induced by the application of low
concentration of Auxins and Gibberellins.
The best varieties of Banana, Pineapple, grapes etc are
Parthenocarpic.
Parthenocarpic fruits have higher proportion of edible parts
than normal fruits.
Ethylene is a ripening hormone whileAbssicicAcid (ABA) is a
dormant hormone.
63. A p o m i x i s
A few flowering plants have evolved a mechanism to produce seeds without
fertilisation called Apomixis.
Apomixis is a form of asexual reproduction that mimics sexual reproduction.
The term Apomixis was first coined by Winkler in 1908.
Plant which shows Apomixis are known as Apomictic plants.
There are several methods of apomictic development of seeds are as follow:
Agamospermy: Formation of embryo from an unfertilized ovule i.e. Haploid
gametes are not formed . Seeds and embryo are formed without meiosis and
fertilisation.
Adventive Polyembryony: Extra embryo develop from a diploid cell of nucellar
tissue or integument. Ex-Citrus, Opuntia.
RecurrentAgamospermy:A diploid embryosac is formed with diploid egg.All
the cells of the embryosac are diploid as it is formed directly either from nucellar
cell (Apospory) or diploid megaspore mother cell (diplospory).
Non-RecurrentAgamospermy: Embryo develops from haploid female gamete
without fertilisation.(Parthenogenesis)
Polyembryony: Development of more than one embryo in a seed. It was
discovered by Leeuwenhoek in 1729 in Orange.
64. Artificial Hybridization
It is useful to enhance quality of plants in plant breeding. Its
purpose is to combine characters of two plants.
Selection of plants.
Selection of plants for male and female gametes. After selection
the male reproductive part is removed and the process is called
emasculation. Emasculation is only required when flower is
bisexual. If flower is big than it is done manually and if flower is
small than it is done by suction pump.
Bagging is done to prevent any undesirable pollen grain to fall on
stigma. So, female part is covered with a bag.
Collection of a desired pollen grains through bagging.
Dusting of desired pollen grains on stigma of emasculated flower.
Fertilisation and seed formation.
Testing of seeds by germinating in soil. If seed satisfies the new
character then it is sold commercially.
