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Revision Notes for Class 7 Science Chapter 12 Reproduction in Plants
Class
7 Science students should refer to the following concepts and notes for
Chapter 12 Reproduction in Plants in Class 7. These exam notes for
Class 7 Science will be very useful for upcoming class tests and
examinations and help you to score good marks
Chapter 12 Reproduction in Plants Notes Class 7 Science
Reproduction in Plants
A
characteristic feature of all living organisms is that they give birth
to young ones of their own kind. What would happen if new generations of
species were not produced from their parents? The earth would turn out
to be barren place with no life. Since the life span of an organisms is
limited, they have devised ways and means to multiply their number. This
ability of all living organisms to produce new individuals of their own
kind is called reproduction. However, reproduction is not essential for
an organisms’s survival, but it ensures that the organisms leaves
behind more individuals of its own kind, so that the species does not
perish from earth.
There are different types of plants growing around us in park, school, garden and in our neighbourhood.
Many of these plants bear flowers and fruits. Such plant body consists of two main parts:
i) Vegetative Part: These are concerned with nutrition and growth. They comprise of the roots, stems and leaves.
ii) Reproductive Part:
This is essentially concerned with reproduction of the plant. It
comprises of the flower. A plant may have both male and female parts
borne on the same flower or they may be borne by different flowers.
MODES OF REPRODUCTION
The three most common methods of reproduction among living organisms are
1. Asexual reproduction
2. Vegetative reproduction
3. Sexual reproduction
Asexual Reproduction
This
type of reproduction involves the production of a new organism by a
single parent. It occurs when there is plenty of food available and
conditions are suitable for growth. Asexual reproduction is common in
unicellular organisms but is also observed in multicellular plants and
animals. There are many forms of asexual reproduction.
a) Fission b) Fragmentation c) Budding
d) Sporulation e) Regeneration
a) Fission:
Fission means division of cell into two parts. During fission, two
daughter cells of equal size are formed from one parent. This is the
simplest and most common method of reproduction seen in unicellular
organisms such as Amoeba, Paramoecium, Euglena and bacteria.
i) Binary fission:
In binary fission, the fully grown parent cell splits into two halves
to produce two daughter cells. Binary fission takes place when food is
abundant. In Amoeba (Fig. 1) a unicellular organisms that lives in ponds
and puddles, first the nucleus divides into two equal nuclei and then
the cytoplasm divides. This results in two daughter amoeba which grow
attain full size and split again.
The
binary fission may be longitudinal or transverse. Paramoecium (Fig. 2)
shows transverse binary fission. The parent cell or individual no longer
exists after binary fission in complete.
ii) Multiple fission:
During unfavourable conditions like lack of water, excessive heat or
high temperature, a thick protective wall develops around the Amoeba
cell, called the cyst (Fig. 3). The
Amoeba
splits many times within the cyst to form many small Amoebae. This is
called multiple fission. When the cyst finally breaks, several daughter
cells are released at once.
b) Budding:
Budding is another method of asexual reproduction in which a small bulb
like outgrowth appears on the body of the organism called the bud. It
grows and may break away from the parent to form a miniature organism.
Yeast, corals, sponges and Hydra reproduce by budding (Fig. 4).
In
yeast, a unicellular organisms, a little cytoplasm accumulates at one
end of the cell. The nucleus of the parent cell divides into two and one
is sent into the bud. Bud formation occurs very rapidly in yeast cells,
to form a chain of yeast cells under favourable conditions. All these
chains of buds don’t detach from the parent individual. So a sort of
colony is formed.
In Hydra, a multicellular
organisms a bud appears on the body wall which grows into a full Hydra
in a day or two, develops tentacles and mouth and detaches from the
parent body.
In corals and sponges, buds don’t separate out
but remain attached to the parent organism. They grow to full size and
reproduce again and again, producing a colony.
c) Spore Formation:
During unfavorable conditions many fungi (mushrooms), ferns, mosses and
bacteria such as yeast, rhizopus and mucor, reproduce through spore
formation. A spore is a tiny spherical unicellular body protected by a
hard and thick wall (Fig. 5). Spores are microscopic structures. These
spores help in overcoming conditions unfavourable for reproduction, in
which the plants can survive eg. lack of food and external temperature
conditions by forming spores. The resistant spores of various organisms,
e.g. the bread mould (fungi) float in the air and settle on covered
bread or any other suitable food. It germinates and grows under
favourble conditions like nutritions, temperature, moisture, etc. to
produce hyphae. Hyphae penetrate the bread, as can be seen as white
thread like structure
on
the bread during rainy season some hyphae are erect and possess the
swelling at their tips, called sporangium. The spores are produced by
multiple fission inside the sporangium.
The white powder like
substance that grows on leather goods and shoes during the rainy season
are also fungi whose spores germinate to produce new colonies on leather
given favourable conditions of temperature and moisture.
In case of mosses and ferns, spores are produced inside the special structure called capsules.
d) Fragmentation:
There are several filamentous organisms like Spirogyra or flatworm that
increases their population by fragmenting their filaments. The
organisms breaks up into two or more fragments after maturation, and
each fragment starts growing into a new individual (Fig. 6).
e) Regeneration: The ability of certain plants and animals to re-develop, a lost limb or body part, is called ‘regeneration’, e.g. Amoeba and Paramoecium can
regenerate
lost parts. In fact, if an Amoeba is cut or crushed into many small
pieces, with a portion of nucleus in every piece, each piece can develop
into a full Amoeba. Regeneration is more common in plants than in
animals.
If a Hydra (Fig. 7A), earthworm or Planaria is cut into pieces, each piece can develop into small Hydra, earthworm or Planaria.
A starfish (Fig.7B ) if caught by the enemy, loses its arms and
escapes. Later on, it can develop or regenerate than arm. Similarly, a
wall lizard can regenerate and its lost tail. A grasshopper can grow a
new leg in place of broken one. The power of regeneration is restricted
only up to heating of wounds etc., in man. Humans are not able to
regenerate a lost part naturally.
Vegetative Reproduction
Like
spores, higher plants can also survive unfavourable conditions. Under
such conditions, the aerial parts of a plant usually die but the
underground parts like root or stem remain dormant or inactive. When
conditions are favourable again, these dormant parts grow again to
produce a new plant. Similar is the case with Colocasia, sweet potato,
potato etc., which do not produce any seeds. Man has encashed this
property of such plants to maximum advantage. Thus, when new plants are
produced from parts of the parent plant such as the root, stem or
leaves, without the help of any reproductive organs, it is known as
vegetative reproduction. Different methods are used in horticulture for
fruit yielding and ornamental plant, like, cutting (stem), eg. in sugar
cane, layering and grafting in mango, roses etc.
a) Vegetative reproduction by stem: Some
plants send out a side branch from the main plant, and a root grows
down into the soil. The stem grows in length and creeps along the ground
(runners) and forms roots at intervals, thus forming new plants
(Fig.8). When the new plant has grown enough, the link between the old
and the new plant withers away, eg. some species of grass and
strawberry.
Some
plants reproduce vegetatively beneath the soil, e.g., onion, potato,
ginger, Colocasia etc., Modified underground stems also store food.
Underground stems are capable of producing several new plants from their
buds and thus help the plant to multiply. Tuber, bulb, corn, rhizome
are the different modifications of an underground stem. (Fig,8).
i) Tuber: Seeds
of potato are not viable. Potato tuber is the swelling of the branch of
an underground stem. The leaves on the branches and stems growing above
the soil make food in the form of starch. This starch is passed down
the stem via the phloem tissues, into the underground branches to make
new tuber The tubers have ‘eyes’ present on the them. The ‘eye’ is made
up of a bud and scale leaves. When sown, each eye is capable of
developing the root stem and leaves, as it contains meristematic tissue.
ii) Bulb: Bulbs
are actually swollen large underground buds covered with scale leave It
is a shoot modification that has a very short stem and apical and
axillary buds. It stores food for the growth of new leaves and flowers.
For example, onion, lily, tulip, garlic etc.
iii) Rhizome: It
is horizontally growing underground stem, swollen with stored food. A
rhizome bears distinct nodes, internodes scaly leaves and adventitious
roots. It has buds on its surface from which grow new plants. Ginger and
turmeric are examples (Fig.9).
iv) Corm: Some
plants like Colocasia, Crocus, Gladiolus etc., consist of a short
swollen solid fleshy underground stem (Fig. 10). It is in fact a
condensed form of rhizome and bears one or more bud The food made by the
leaves is stored in the corm at the end of the growing season, until it
is used for the growth of a new plant, the following season. Daughter
corms develop from the sides of the parent corm, which later break off
to form new plants.
