ASEXUAL REPRODUCTION

A single individual (parent) is capable of producing offsprings in asexual type of reproduction. As a result, the offsprings

produced are exact copies of each other and their parents. Such morphologically and genetically similar individuals are

called clones.

Ÿ  It is common among single-celled organisms, plants and animals with relatively simple organisation.

Ÿ  It is also called somatogenic reproduction because propagules are formed from somatic cells of the parent.

Ÿ  It occurs by fission, budding, sporulation, fragmentation, regeneration and vegetative reproduction.

(a) Binary Fission

Ÿ   In this process, the parent organism divides into two halves, each half forming an independent daughter organism.

Ÿ  It means, the parent body as a whole forms the reproductive unit and the parent continues living as two daughter

individuals.

Ÿ   Therefore, the organisms that undergo binary fission are said to be immortal.

(ii)  It occurs in Bacteria (Moneran), Amoeba and Paramecium (Protists).

Fig. : Binary fission in bacteria

   Ÿ   Depending upon the plane of division, binary fission is of the following types:

(i)  Simple Binary Fission (Irregular Binary Fission) : Division can occur through any plane, e.g.; Amoeba.

(ii)   Longitudinal Binary Fission : The plane of fission passes along the longitudinal axis of the organism, e.g., Euglena.

(iii)  Transverse Binary Fission : The plane of this division runs along the transverse axis of the individual, e.g., Bacteria,

Paramecium, Diatoms.

(b)   Budding

Ÿ  In yeast, the division is unequal and a small bud (protuberance) is produced that remains attached initially to the

parent cell, later on the bud gets separated and mature into new yeast (cells).

Ÿ   Budding or sporulation is also shown by oidia of Rhizopus.

Ÿ   Buds are also reported in hydra.

              

(c)   Spores: Members of the kingdom fungi and simple plants such as algae reproduce through special asexual

reproductive structure. The most commonly produced structures are conidia and zoospores.

Ÿ  Zoospores 

(i)    These are motile and flagellated spores produced inside the zoosporangia under favourable conditions.

(ii)   In Chlamydomonas(n), the protoplast of cell divides to form 8-16 zoospores. They are pyramid shaped and anteriorly

biflagellated, resembling the parent cell. The parent cell wall breaks and the zoospores are liberated in water. They enlarge

and behave as adult individuals.

(iii)  Zoospores are also produced in the asexual life cycle of Achlya, Saprolegnia, Phytophthora and Ulothrix.

(iv)  Zoospores of Cladophora glomerata are diploid.

Ÿ   Conidia

   (i)    In Penicillium, these spores are produced at the tips of special hyphal branches, called conidiophores.

   (ii)   There are two types of conidiophores :

          a. Unbranched/monoverticillate

          b. Branched/biverticillate

   (iii)  The branches of conidiophores are called rami and branches of rami are called metulae. Each metula bears 2-6 flask shaped structures called sterigmata (phialides). Each sterigma produces a chain of conidia.

   (iv)  Features of Conidia:

          a. Pigmented

          b. Uni or multinucleated

          c. The conidia in the chain are arranged in basipetal manner.

   (d) Vegetative Reproduction

   (i)    Vegetative reproduction does not involve meiosis and fusion of gametes, therefore it is considered ' as a type of

asexual reproduction .

   (ii)   New plants or individuals are produced from vegetative parts of plants and newly formed individuals are genetically

identical to the parent plant.

   (iii)  It is common method of reproduction in the flowering plants.

There are two types of vegetative reproduction.

   I. Natural Methods of Vegetative Reproduction:

Ÿ  These are methods of plant multiplication occurring naturally in which a somatic part of the plant detaches from the

body of the mother and develops into a new independent plant under suitable environmental conditions.

Ÿ  The detachable somatic part that functions in vegetative propagation is called vegetative propagule.

Ÿ  It carries one or more buds. Natural buds occur over the nodes of the stem. When placed in contact with damp soil,

the buds sprout, producing roots and new plants.

Ÿ  This potential is exploited by farmers, e.g., tubers of potato, rhizome of banana and ginger, bulbs, runners, offsets,

stolons, aerial stems etc.

Ÿ  Some propagules carry adventitious buds, e.g., normal and storage roots, leaves.

It occurs by following means:

STEM

   Underground stems

Ÿ  Different types of underground stems like tuber, Germinating rhizome, bulb and corm can take part in Young shoot

vegetative propagation.

Ÿ  A portion of underground stem bearing bud Roots forms a new plant.

(i)    Tuber : It is terminal portion of underground stem branch which is swollen on account of accumulation of food.

 e.g., Potato, Artichoke

Eye of Potato

(ii)   Rhizome: It grows obliquely or horizontally under soil surface. It is well branched and bears nodes, internodes,

buds and scale leaves. e.g., Banana, Turmeric, Aspidium, Adiantum, Ginger

(iii)  Bulb: Stem is unbranched, highly reduced and disc shaped. The bud is surrounded by many concentric scale leaves.

Leaf bases of inner ones are fleshy and edible and outer ones are dry known as tunic. e.g., Onion, Garlic, Narcissus

(iv) Corm: It grows vertically beneath the soil surface. It bears nodes, internodes, buds and scale leaves.        e.g.,

Colocasia, Gladiolus, Freesia., Crocus, Amorphophallus

(3) Creeping stems:

(i)    Runner: It is elongated, prostrate, sub-aerial branch with long internodes and roots at nodes. e.g., Grasses

(ii)   Offset: Short horizontal branch which is one internode long and produces a cluster of leaves above and the cluster

of roots below is called offset. e.g., Eichhornia (Water hyacinth), Pistia.

Water hyacinth or 'terror of Bengal' was introduced in Bengal because of its beautiful flowers and shape of leaves.

However, it turned out to be highly invasive aquatic weed that not only spread to all water bodies of Bengal but also

throughout India. It drains oxygen from the water, which leads to death of fishes and other animals. It is very

difficult to get rid off them since it can propagate vegetatively by offset at a phenomenal rate and spread all over the

water body in a short period of time.

(iii)  Stolon : It is subterranean long lateral branch arising from base of stem. It first grows obliquely upward and then

bends down to the ground surface.

e.g., Strawberry, Vallisneria Leaflets

(iv)  Sucker : It arises by axillary bud of underground part of stem. This lateral branch creeps below the soil surface,

grows obliquely upward and produces new shoot.

e.g., Chrysanthemum, Pineapple, Banana

(4)   Aerial shoots : Each segment of stem having at least one node can form a new plant.

e.g., Sugarcane, Opuntia

(5)   Leaves: Leaves of several plants having adventitious buds help in vegetative reproduction. In Bryophyllum

adventitious buds arise from the notches present at margins of leaves.

e.g., Adiantum (walking fern), Begonia, Streptocarpus, Saintpaulia and Kalanchoe.

(6)  Bulbils: These are fleshy buds which produce new plant. e.g., Agave, Oxalis, Ananas, Dioscorea, Lily, Chlorophytum

(7) Turions : Fleshy buds in aquatic plants helping in perennation, e.g., Potamogeton, Utricularia.

ROOTS

(i)  Both tap roots and adventitious roots takes part in vegetative reproduction due to the presence of bud, known as radicle

bud.

(ii)   Adventitious buds on root detaches and gives rise to new plant.