CLASS: XII STREAM: SCIENCE

SUBJECT: BIOLOGY (BOTANY)

TEACHER: Mrs.NAMRATA B. MORAL

CHAPTER: 1 (REPRODUCTION IN ORGANISMS)

INTRODUCTION :

The period from birth to natural death of an organism represents its life span.

The life span of an organism may be few minutes to several thousand years. Life

span of Mayfly is one day, while giant tortoise is considered the longest living

animal (about 100 to 150 years).

Life spans of some organisms are

African elephant: 60 to 70 years Indian elephant: 48years

Rose: 7 to 8 years Dog: 10 to 13 years (varies in different

Breeds of dog)

Butterfly: 1 to 2 weeks Crow: 15 years

Banyan tree: 200 years Banana tree: 25 years

Cow: 18 to 22 years Horse: 25 to 30 years

Fruit fly: 40 to 50 years Rice plant: 3 to 7 months

Irrespective of long or short lifespan of organism, death is a certain

phenomenon. So, there must be a process in living organism that ensures

continuity of species. This process is reproduction.

Reproduction is defined as a biological process in which an organism gives rise

to young ones or offspring similar to itself.

Reproduction performs the following functions:

(i)Maintains life on earth.

(ii)Enables continuity of the species, generation after generation.

(iii)Create genetic variations among populations.

Based on whether there is participation of one organism or two in the process

of reproduction, it is of two types.

ASEXUAL REPRODUCTION: When the offspring is produced by a single

parent without the fusion of gametes, the reproduction is called as

asexual reproduction. It involves only mitotic cell division. Meiosis does

not occur in asexual reproduction.

Offspring produced by asexual reproduction are identical copies of their

parents. Such a group of morphologically and genetically similar individuals

are called clone.

Asexual reproduction in Animals: Asexual reproduction is common in single

celled organisms and animals with relatively simple body organisations.

In lower organisms like Protists and Monerans, the parent cell divides

into two, to give rise to new individuals. Thus in these organisms cell

division is itself a mode of reproduction.

Reproduction in Protists Reproduction in Monera

Fission: It is the division of the parent body into two or more daughter

individuals identical to the parent. It can occur by binary fission and multiple

fission.

Binary fission: It is the division of the parent cell into two nearly equal

sized daughter individuals, each of which grows into an adult. I t occurs

in single-celled animals like bacteria and protozoa, e.g. Amoeba,

Paramecium.

The diagram shows Binary fission in Amoeba.

Multiple fission: It is the division of the parent body into many small

daughter individuals, each of which grows into an adult, e.g.

Plasmodium (the malaria parasite), Amoeba (during unfavourable

conditions), etc.

Multiple fission

Budding: It is the mode of asexual reproduction in which one or more

unequal and small projections called buds are produced that remain

attached initially to the parent cell, but eventually get separated and

mature into new organisms, e.g. yeast, Hydra, etc.

Budding in Hydra and Yeast are external forms of budding.

Internal budding takes place in Sponges with the formation of gemmule. A

mass of cells enclosed within a common opaque envelop, rich in food,

germinates into new plant, and is called gemmule. Each gemmule has a group

of cells called archeocystes surrounded by the protective covering. During

adverse conditions, a mass of archaeocytes comes out via micropyle that

grows into a colony.

Sporulation: Sometimes the products of multiple fission become

individually surrounded by resistant coats, i.e. cyst walls before their

release from parent, the process is known as sporulation and the

encysted products are known as spores. It helps to overcome

unfavourable conditions. Sporulation is generally found in monera,

protista and Amoeba.

Some of the common spores are:

Zoospores: The formation of zoospores takes place during favourable

condition. The protoplasm divides longitudinally by simple mitotic division

forming two daughter protoplasts. Second longitudinal division divides it

into four and so on. Zoospores are the spores of certain algae, fungi, and

protozoans, capable of swimming by means of flagellum.

Conidia: These are asexually produced spores that are born externally to

the cells that produce them (Penicillium).

Fragmentation: It is a mode of asexual reproduction in which parent body

breaks into distinct pieces, each of which regenerates into an offspring. It is

found in sponges, sea anemones (coelenterates) and echinoderms. It is also

found in algae (Spirogyra), fungi etc.

Fragmentation in Spirogyra

Asexual reproduction in plants: In plants, asexual reproduction is known as

vegetative propagation. It is the process of formation of a new plant from

detached vegetative parts of the plant.

The vegetative structures or the units of vegetative propagation are known as

vegetative propagules.

Vegetative propagation in plants are of two types:

Natural vegetative propagation: Under favourable conditions this can

take place by the following vegetative propagules-

Roots: The buds present on the roots grow into leafy shoots called slips

above the ground and adventitious roots at their base. Each slip gives

rise to a new plant, e.g.sweet potato, Dahlia, guava, yam, Tinospora,

etc.

Underground stems: Underground modified stems posses buds which

grow into new plants. Some of its types are:

Suckers: The arise from the base of the erect shoot, grow horizontally in

the soil and then come out to form new aerial shoots, which become

independent when suckers break off, e.g. mint, Chrysanthemum, etc.

Rhizome: These are modified stems with buds and sufficient food

storage. A piece of rhizome containing bud can give rise to a new plant,

e.g. ginger, turmeric, banana, etc.

Rhizome of ginger

Rhizome of banana plant

Bulbs: They are highly reduced underground stems with a number of

buds, e.g. onion, lilies, garlic, etc.

Tubers: They are the modified underground stem branches having

several buds, e.g. each eye of the potato is a bud, which grows into a

new potato plant.

3. Creeping stem: Vegetative propagation by the means of creeping stems

can occur with the help of the following vegetative propagules:

Runners: They are modified stems, which produce adventitious roots at

nodes. Each node gives rise to an aerial shoot which becomes a new

plant, e.g. Oxalis, Centella, grasses, etc.

Runner of Oxalis Runner of grass

Offset: They are one internode long runners, which develop tuft of

leaves at the apex, e.g. Pistia (water lettuce), Eichhornia (water

hyacinth), etc.

4. Aerial stems: Aerial modified stems of cacti develop new plants when the

stem segments fall on the ground e.g. Opuntia.

Opuntia

5. Leaves: Some plants develop adventitious buds on their leaves which get

detached and develop into new plants, e.g. Bryophyllum, Kalanchoe,

Begonia, etc.

Bryophyllum leaf

6. Bulbils: These are fleshy buds produced in the axil of foliage leaves in

place of axillary buds. They grow t new plants when they fall on the ground,

e.g. Agave, Allium sativum, etc.

Bulbil of Agave Bulbil of Allium sativum

Artificial vegetative propagation: Artificial vegetative propagation is a type

of plant reproduction that involves human intervention. The most common

type of artificial vegetative propagations are as follows:

Cutting: A part of the plant, typically a stem or leaf, is cut off and

planted. Adventitious roots develop from the cuttings and a new

plant forms.

The process of cutting and growing new plant

Layering: It is a kind of plant propagation in which a portion of an

above-ground shoot is allowed to grows root while still attached to

the parent plant and then detaches as an independent plant.

The process of layering

Grafting: It is a horticulture technique whereby tissues of plants are

joined so as to continue their growth together. The rooted supporting

portion of one plant called the stock, is joined with the stem of another

plant called the scion, e.g. it is done in plants like rose, apple, plum,

peach, mango, etc.

Process of grafting

Micropropagation: It is a method of propagation of plants by growing

plantlets in tissue culture and then planting them out. It includes

culturing the cells, tissues and organs in laboratory which is called

tissue culture. Initially, this form an undifferentiated mass of cells

called callus. Later on this callus differentiates to form many small

plantlets.

The process of Tissue Culture