SEXUAL REPRODUCTION IN FLOWERING PLANTS

Reproduction ensures continuity of species generation after generations because the older
individuals undergo senescence and die. Flowering plants shows sexual mode of
reproduction and bears complex reproductive units as male and feminine reproductive units
along with accessary structures.

Flower may be a modified stem which functions as a organ and produces ova and/or
pollen. A typical angiospermic flower consists of 4 whorls of floral appendages attached
on the receptacle: calyx, corolla, androecium (male organ consisting of
stamens) and gynoecium (composed of ovary, style and stigma).
Pre-fertilisation: Structures and Events

Several structural and hormonal changes result in formation and development of the floral

primordium. Inflorescence is made that bears floral buds then flower.

In flowers, male (androecium) and feminine (gynoecium) differentiate and develops within which

male and feminine gametes are produced.
Stamen, Microsporangium and Pollen Grain
Stamen consists of long and slender stalk called filament and customarily bilobed anthers.

Each lobe contains two theca (dithecious).

The anther is four-sided structure consisting of 4 microsporangia, two in each lobes
Microsporangia develop further and become pollen sacs which contain pollen grains.

Microsporangium is usually surrounded by four layered walls- the epidermis,
endothecium, middle layer and tapetum. Innermost layer tapetum nourishes the developing
pollen grains.

Sporogenous tissues- it's compactly arranged homogenous cells which are present at
centre of every microsporangium when the anther is young
Microsporogenesis- the method of the formation and differentiation of microspores (pollen
grains) from microspore mother cells (MMC) by reductional division is termed
microsporogenesis.
Pollen grain represents the male gametophytes. Pollen grains are manufactured from 2 layered Wall,

 1. Exine-Made of sporopollenin- most resistant organic matter known.t can withstand high
temperatures and powerful acids and alkali. No enzyme can degrade sporopollenin

 2. Intine
Thin and continuous layer

 Made of cellulose and pectin

 3. Germ pores
apertures on exine where sporopollenin is absent

 forms plant structure.

 4. A cell wall surrounds cytoplasm of pollen grain.

 MATURE POLLEN

 A mature pollen include 2 cells with nucleus (Vegetative and Generative)

 VEGETATIVE CELL
Abundant food reserve
Large irregular nucleus
.Responsible for the event of pollen grain

 GENERATIVE CELL

 Small

 Involves in syngamy (fuse with an egg)

 Pollen grains of the many species e.g Parthenium cause severe allergies and bronchial

 diseases in some people and ends up in chronic respiratory disorders- asthma,
bronchitis, etc.

 Pollen grains are rich in nutrients and are used as pollen tablets as food supplements.

 Viability of pollen grain varies with species to species and will land on stigma before this
period to germinate. Pollen grains of enormous number of species are stored in nitrogen at
temperature - 196, called pollen bank.

 The Pistil, Megasporangium (Ovule) and Embryo sac

 Gynoecium may consists of single pistil (monocarpellary) or over one pistil
(polycarpellary) which can be fused (syncarpous) or free (apocarpous).

 eg Multicarpellary and syncarpous pistil- Papaver
Multicarpellary and apocarpous pistil- Michelia

 Each pistil has three parts the stigma, style and ovary. Inside the ovary is ovarian cavity
locule). The placenta is found inside the ovarian cavity. Megasporangia (ovules) arise from
placenta
Megasporangium (ovule)

 Ovule could be a small structure attached to placenta.
Funiclestalk by which ovule is attached to placenta
.Hilum- junction between ovule and funicle
Integuments- protective envelops

Micropyle- small opening at the tip of ovule into where plant structure enters
Pollination - transfer of pollen grains from anther to stigma.

 a) Autogamy- transfer of pollen grain from anther to stigma of same flower.

 i. Cleistogamous - flower which don't open. cleistogamous flowers are autogamous as
there is no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce
assured seed-set evenm within the absence of pollinators. e.g Viola (comnon pansy), Oxalis, and
Commelina.

 ii. Chasmogamous- exposed anther and stigma.

 b) Geitonogamy - transfer of pollen grains firom anther to stigma of various flower of same
plant. Geitonogamy is functionally cross-pollination involving a pollinating agent, genetically
it is kind of like autogamy since the pollen grains come from the identical plant

 c)Xenogamy- transfer of pollen grain from anther to stigma of various plant's flower of
same species.

 Agents of pollination includes abiotic (water, wind) and biotic (insects, butterfly, honey bee
etc. sizable amount of pollen grains are produced by plants using abiotic mode of pollination

as most of pollen grains are wasted during transfer.
Adaptations in flowers for Pollination

I. Wind Pollination

pollen grains:-light, non- sticky, winged
anther - well exposed
stigma: large and feathery
flower: one ovule, arranged as inflorescence

Ex: corn cob, cotton, date palrm

IL. Water Pollination

Bryophytes, Pteridophytes, Algae

pollen grains: protected by mucilaginous covering

Ex: H2O plants-Vallisneria, Hydrilla
Sea grass-Zostera

Main features of wind and water pollinated plants
produce pollen grains in large no.
-do not produce nectar

IIL. Insect Pollination
- Flowers: large, colourful, fragrant, rich in nectar
Pollen grains : sticky
Stigma : sticky

Certain rewards to pollinators:

nectar and (edible) pollen grains as foods
provide safe place for laying eggsEx: Amorphophallus, Yucca
Outbreeding Devices- the assorted mechanisms take discourage self-pollination and
encourage cross pollination as continued selfpollination results in inbreeding depression. It
includes

Pollen release and stigma receptivity not synchronized.
Anther and stigma are placed at different position.
Inhibiting pollen germination in pistil.

Production of unisexual flowers.
Pollen pistil interaction - the pistil has ability to acknowledge the compatible pollen to initiate
post pollination events that results in fertilisation. Pollen grain produce plant structure through
germ pores to facilitate transfer of male gametes to embryo sac.

Artificial Hybridization

Crossing diff styles of species- hybrid individual- with desirable characters of the

parent plants
desired pollen grains for pollination- stigma protected against contamination
Emasculation : removal of anther
Bagging: flower covered- bag made from butter-prevent contamination of stigma

from unwanted pollen

Bagged flower- attains receptivity- mature pollen grains- dusted on the stigma - rebagged-
fruits allowed to develop

Double Fertilisation- after entering the one in every of the synergids, each pollen grain releases two male gametes. One male gametes fuse with egg (Syngamy) and other male gametes fuse with
two polar nuclei (triple fusion) to supply triploid primary endosperm nucleus (PEN).
Since two styles of fusion takes place in an embryo sac the phenomenon is named double
fertilisation. The PEN develops into the endosperm and zygote develops into embryo.

Post fertilisation events include endosperm and embryo development, maturation of
ovules into seeds and ovary into fruits.

Endosperm- the first endosperm cell divides many time to forms triploid endosperm
tissue having reserve food materials.

Two styles of endosperm development:
() Free nuclear type (common method)
(i) Cellular type

(a) Non-albuminous- endosperm completely utilized- before maturation of seeds. e.-g pea,
groundnut

(b) Albuminous- a little of endosperm remain in mature seeds. e.g wheat, maize, castor

Embryo- Embryo develops at the micropylar end of the embryo sac where the zygote is
located.

Embryogeny- early stages of embryo development.The zygote gives rise to the proembryo
and subsequently to the globular, heart-shaped and mature embryo.

Embryo consists of:
embryonal axis
-cotyledons
plumule
radicle

Monocotyledonous Seed
Scutellem= Cotyledon
-Coleorrhiza: undifferentiated sheath covering radical & plant organ


-Coleoptile: sheath covering plumule
Seed
-Fertilized and mature ovule develops into seed.

Seed consists of
-cotyledon)
-embryonal axis
Seed coat- double layered- formed by integuments

Testa (outer coat)
Tegmen (inner coat)

Micropyle- small opening on reproductive structure, it facilitates entry of H20 &02 into seeds (for
germination)
Hlum- scar on reproductive structure
Seed-Albuminous/ Non-Albuminous
-Perisperm: remnants of nucellus that's persistent. Ex: Black pepper
Dormancy: state of inactivity

The wall of ovary develops into wall of fruit called pericarp. In true fruits only ovary
contributes in fruit formation by in pome thalamus also contributes in fruit formation.

Apomixis
Form of asexual reproduction- mimics sexual reproduction- seed formed without
fertilisation
Formation of apomictic seeds:
diploid cell (formed without meiosis)- be converted into embryo without fertilization
cells of nucellus (2n) surrounding embryo sac- protrude into embryo sac- be converted into
embryos. Ex. Citrus and Mango.
Polyembryony

Occurrence of quite one embryo during a seed
Often related to apomixes. Ex: Citrus, groundnut