Reproduction in Plants

The complex process by which an organisms produce their offsprings is

called reproduction. It is of two categories:

1. Asexual reproduction
2. Sexual reproduction
Asexual Reproduction
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Offsprings arise from single parent & inherit genes of that parent only.
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Doesn’t involve meiosis or fertilisation.
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Offsprings are genetically identical to parents.
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Found in lower graded living beings.
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It occurs in many different forms. Vegetative propagation, fragmentation ,
regeneration, binary fission, and budding are some of the forms of asexual
reproduction.
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Consists of two categories:
(I) Spore production (II) Vegetative reproduction
Spore Production
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Found in lower graded plants (e.g. bryophytes (mosses and liverworts),
lichens, fungi and algae)
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Modified somatic cells of plants produce organs containing numerous
spores usually known as spore case (sac) or sporangium.
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Sometimes sporangium may contain a single spore.
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Exospores may form outside sporangium. Some of these are known as
conidium.
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Mucor and Penicillium are two types of fungi. Mucor reproduces asexually
by forming enormous numbers of microscopic spores inside sporangium.
Penicillium reproduces by conidia formation.
Vegetative Reproduction
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Takes place without formation of spore or gamete.
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Any organ may be modified or the fragmentation of any portion of the body
may occur.
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Two types of vegetative reproduction takes place:
1. Natural vegetative
2. Artificial vegetative
Natural Vegetative Reproduction
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Vegetative reproduction that takes place naturally by different methods:
1. Segmentation
2. Through root
3. Modified Stem
Segmentation
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Generally this type of reproduction is seen in lower plants. For example, if
fragmented each part of Spirogyra, Mucor etc. grows into a new plant and
live independently.
Through Root
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Some plants are grown from the roots. Example- Segun, Patol, etc. Some
roots are fat and juicy. Buds are grown on the surface and from these buds
new plant grows. Example-Sweet potato. This is also known as budding
where a new organism develops from a bud of an existing organism.
A picture is provided in the next slide for reference.
Modified Stem
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In some cases stems are modified. Some of these modified stems have no
resemblance with true stem for their appearance and positions. This
modification occurs in order to protect from adverse condition, to ensure
storage of food or for vegetative reproduction.
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These modifications include: Tuber, Rhizome, Bulb, Stolon, Offset, Bulbil
and from leaves.
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Tuber: Stem tubers are formed by outgrowths from the lowest axillary buds
which turn downwards into the soil. Eventually the tip of the underground
stem fills with starch and swells rapidly to form a tuber. Tubers are
distinguished by their origin and the presence on their surfaces of scale
leaves and axillary buds, which form the eyes. From each eye an individual
plant grows. For example- Potato.
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Rhizome: Rhizome lies parallel inside the soil. Like stem it bears distinct
node and internode. Internode (portion of stem between two successive
nodes) bears scale leaves and axillary (shoot located in axil of leaf) buds.
They become fattening and juicy by storing food. In favourable condition
these buds grows into individual plants. For example-Ginger.
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Bulb: These are very small stems. New plants grow on their axillary and
terminal buds. Example-Onion, Garlic etc.
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Stolon: You may have seen arum stolon. These are branches from arum
(kochu) and modified for reproduction. Stolon produces buds at the terminal
end and thus helps in reproduction. Example-Arum, Minit (pudina).
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Offset: Water hyacinth, Spirodela etc. form a very short runner called an
offset. Runners grow out horizontally from axillary buds forming several
new plants. After some days the daughter plants are detached from mother
plants and grow individually. Example- Water hayacinth.
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Bulbil: The improper development of axillary buds of some plants forms
round-shaped structure called bulbil. After sometimes, bulbil is separated
from the plant and drops on soil and finally produces new plants. Example-
Yam.
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From leaves: From a grounded leaf of a Bryophyllum numerous young
bryophyllum grows. Example-Stone-chips (Bryophyllum). All the process of
vegetative reproduction cited above occurs naturally. The daughter plant
that grows from vegetative reproduction has the same characteristics as
the parent. So, no new character appears. So, for production of better
quality cash crops, artificial vegetative propagation method is used.
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Examples of tuber and offset are given below:
Artificial Vegetative Reproduction
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Plants produced from seeds sometimes show less productivity and inferior
quality. In these plants artificial vegetative propagation is a useful tool to
restore the parental quality. Let us know about artificial vegetative
propagation:
1. Grafting
2. Cutting
1. Grafting: The process by which a straight, young and fresh stem
develops root and thus enables the stem to live individually is known as
grafting. A cut is made in the bark of the stem, where roots to be developed.
Now the cut is to be covered by soil and cow dung, and finally with the help
of cellophane tape or polythene to protect the fall off soil and cow dung.
This part of the stem should be kept moist by regular watering. If kept in
this condition, root will grow in a couple of days. The stem with root,
separated from mother plant, grows into new plant after planting.
2. Cutting: You have observed that stem cuttings of rose if put into moist
soil grows new leaves after a few days. Each stem of this kind grows into a
new rose tree.
Sexual Reproduction
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The parts of a flower are receptacle, calyx, petal, stamen (androecium) and
carpel (gynoecium). Some flowers may have extra parts under the calyx,
such as china rose (joba). This extra parts is called epicalyx.
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Some flowers may or may not have stalks (pedicel). Flower with stalk is
known as stalked and without stalk is known as unstalked flower.
Different parts of a flower

Sepals: In many flowers there is an outermost whorl (ring) of sepals, known

collectively as the calyx. Sepals are usually green and look like small
leaves. Sepals may be segmented or joined. They enclose and protect the
central region of the flower (from sunlight, rains and insects) when it is in its
budding stage.
Petals: In most of the flowers the reproductive organs are surrounded by a
whorl of petals. Petals are known collectively as the corolla of the flower.
Petals may be joined as in datura or may remain separated, eg, china rose.
Some flowers have different coloured and scented petals. Petals of this
type attract insects which come to collect the nectar and by doing so
transfer pollens from flowers to flowers. This is the second whorl of the
flower.
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Stamens: Surrounding the gynoecium is a whorl of stamens. Stamens are known
collectively as androecium. These are the male reproductive organs. A stamen
consists of a stalk (filament) bearing an anther. Each anther is made up of four
pollen sacs in which pollen grains are formed. Pollen grains contain the male
gametes. They take part in reproduction directly. This is the third whorl of the flower.
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Carpels: The gynoecium is at the centre of a flower. A gynoecium may be
structured with one or more carpels. When a gynoecium is formed with many more
carpels which are completely merged with each other, then it is called, syncarpous
and when they are separated, it is called polycarpous. The gynoecium is typically
made up of an ovary, style and stigma as in the centre of flower. One or more
ovules are in a flower, arranged inside the ovary. Within the ovule, the female
reproductive cell ovum is produced. This ovum, like an androecium, is directly
involved in the process of reproduction.
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Sepal and petals are known as the accessory whorl while the stamens and
carpels are known as the essential whorl of a flower.
Inflorescence: The mode of arrangement of flowers on the floral axis is
known as inflorescence. The inflorescence is of two types. For example-
(a) Recemose- The growth of the floral axis is unlimited.
(b) Cymose-The growth of the floral axis is limited.
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Inflorescence is very important for pollination.
Diagram of a typical flower
Structure of an ovule
Pollination
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Pollination is the transfer of pollen grains from anthers to stigmas. It is the
precondition of seed and fruit production. Pollination is of two types-self
pollination and cross pollination.
Self Pollination Vs Cross Pollination
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Agents for pollination: In most cases the transfer of pollen occurs through
an agent. The carrier that carries pollen up to stigmas is known as agents
for pollination.
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Wind, water, insects, birds, snails as well as man also act as agents of
pollination. Insects and birds fly from flowers to flowers in search of nectar
or in attraction of their brilliant colours. During this act pollen grains get
stuck to the body of the carrier. As insects and birds do this, they transfer
pollen from one flower to another unknowingly.
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To get the help of pollinating media some structural modifications are seen
in flowers. These modifications are known as adaptation. The adaptations
are different for various pollinating media.
Adaptations of wind vs water vs animal pollinated flowers
Fertilisation
The sexual union of the motile and small male gamete with the comparatively bigger,
non motile female gamete is known as fertilization.
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Pollen grains are transferred to the stigma by pollination. The stigma produces a
sticky fluid which nourishes the pollen grains and stimulates each other to burst open
and develop a long, hollow, tubular outgrowth known as pollen tube. This tube
pushes its way between cells of the style and grows towards the ovule and finally
reaches the embryo sac.
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By this time, two male gametes are formed inside the pollen tube. The apex of the
pollen tube ruptures (bursts open) releasing male gametes. Ovule contains embryo
sac. Female gamete or ovum develops inside embryo sac. One of the two male
gametes discharged from the pollen tube unites and fertilizes the egg. The other
male nucleus (gamete) unites and fuses with secondary diploid nucleus and
develops into cereal grains.
Fruit Formation
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The process of growing these fruits start by the transformation of ovaries of
gynoecium (female part of a flower containing one or more carpels) of
flower. The transformation occurs in the ovary after pollination and
fertilization. The ovules transform into seeds. After fertilization, the ovary
alone or in combination with other floral parts turns into fruits.
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When only the ovary turns into fruit, it is known as true fruit, eg, Mango,
Jackfruit etc. When the fruits are developed from different floral parts other
than the ovary, they are called false fruit, eg, Apple, Dellenia (chalta) etc.
Types of fruits
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All (both true and false) fruits are divided into three main classes on the basis
of their origin and nature. These are - simple fruit, aggregate fruit and multiple
fruit.
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Simple fruit: When the ovary of a single carpel or more than one united
carpels of a flower develop into a fruit, it is known as simple fruit, eg, Mango.
Depending on the nature of pericarp, simple fruits are again divided into two
groups- dry fruits and fleshy fruits. Dry fruits: Fruits having thin and dehiscent
pericarps when they are ripe, are known as dry fruits. Example- Bean, Lady's
finger, Mustard etc. Fleshy fruits: Fruits with thick and succulent pericarps (the
part of a fruit formed from the wall of the ripened ovary) are known as fleshy
fruits. The pericarp of fleshy fruit does not split open when it ripens. Example-
Mango, Black berry, Banana etc.
Continued
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Aggregate fruit: When the fruit develops from the merger of several
ovaries with many free carpels of a flower, it is known as aggregate fruit.
The number of fruitlets formed corresponds with the number of carpels
present in the gynoecium of the flower. A cluster of fruitlets is produced at a
time and placed on a single stalk. For example -Custard apple, Champa,
Nayantara, Akanda etc.
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Multiple fruit: When all the flowers of an inflorescence together form a
fruit, it is known as multiple fruit. Examples of such fruits are- Pineapple,
Jackfruit etc.
Structure of Seed
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A seed is an important part of a flowering plant. They give rise to a new plant.
They may be of different shapes, colours and sizes. They may be round,
wrinkled, winged or hairy. They are in a dormant condition until they receive
adequate sunlight, water, and soil. The growth of the plant from a seed is known
as germination. A seed has three parts:
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Seed Coat
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Endosperm
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Embryo
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Sometimes perisperm may be present.
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Both endosperm and embryo are the products of double fertilization, whereas the
seed-coat develops from the maternal, ovular tissues.
Continued
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There is a small pore, called micropyle, near the pointed part (end) of the seed. Radicle
comes out through it. Seed at this stage may be pealed off by slight pressing of the
finger tips. Felled off seed is yellow in colour. Two cotyledon of the seed will be open if
further pressure is applied to it. At the junction of two cotyledon there is a white coloured
structure, known as embryonic axis.
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The area of embryonic axis which is attached to the cotyledons is known as nodal zone.
The part of embryonic axis above the nodal zone is called epicotyl and the part below
the nodal zone is known as hypocotyl. The upper apex of the embryonic axis is known
as plumule while the lower end of embryonic axis is called radicle. Radicle, plumule and
cotyledon are collectively known as embryo and their outer covering is called seed cost.
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Plants with the seed having only one cotyledon are called as monocotyledons. Plants
with the seed having two cotyledons are called as dicotyledons.
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Seed coat consists of two layers, such as-
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Outer layer: The outermost layer of seed coat is hard and thick. It is known
as testa.
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Inner layer: The innermost thin layer of seed coat is known as tegmen.
Seed Germination