Reproduction in Plants: Introduction

You observe in nature that a mature mango plant produces mango fruits, the seeds of these fruits germinate and develop into new mango plants.

Similarly, the other organisms also give rise to the young ones of their own kind.

This ability of living beings is called reproduction. Reproduction helps in perpetuation or continuation of species in nature.

 

MODES OF REPRODUCTION (ASEXUAL AND SEXUAL)

There are two major modes of reproduction asexual (apo = without/away, mixis = mixing) and sexual reproduction (amphi-both. mixis = mixing).

In Angiosperms vegetative Propogation (asexual) and sexual reproduction through production of lower are the main modes of reproduction.

In Lower organisms there are many different methods of asexual reproduction such as, binary ission, budding, sporulation and fragmentation.

Asexual reproduction is more common than sexual reproduction in lower organisms.

 

Asexual Reproduction

it is also called as Apomixis.

The origination of word is apo = without/away, mixis = mixing.

The main feature of asexual reproduction is, it is uniparental (only one parent is involved) and offsprings produced are genetically identical to that parent.

Binary fission

It is common in unicellular organisms. Parent cell devides to produce two equal cells that give rise to two new individuals, e.g.

bacteria and amoeba.

Budding

In Budding, the division is unequal and small cells (buds) are produced that remain attached initially to the parent cell.

Eventually the small cells (buds) get separated and mature into new individual e.g. yeast.

Spore formation

Different types of motile and non-motile spores are produced by algae and fungi. Mostly algae produce motile spores called zoospores e.g. chlamydomonas;

while fungi produce non-motile spores called conidia e.g Penicillium.

 

Fragmentation

Filamentous algae reproduce by fragmentation, i.e. filament gets broken into small fragments and each fragment develops into new individual.

 

 

Asexual Reproduction (Vegetative propagation) in Angiosperms

The reproduction which occurs without involving meiosis and fusion of gametes is called asexual reproduction.

In Angiosperms it occurs through the parts of vegetative organs like root, stem, leaf or buds of a plant, therefore, it is called vegetative propagation or vegetative reproduction.

These vegetative parts which act as propagules have adequate reserve food material and at least a growing point.

The Propagule grows to form an independent plant under favorable conditions. Few Natural Methods of vegetative propagation are given below

Tuberous roots:

In some plants the underground roots store plenty of reserve food and become swollen. Such swollen roots are called tuberous roots.

These roots in some plants have adventitious buds on their surface which sprout under favorable conditions

It produces 'Leafy shoots' (slips) and adventitious roots from the base of shoot.

These sprouts may be separated and planted for commercial cultivation.

Under natural conditions, 'slips' are separated after degeneration of intervening root part.

The tuberous roots are adventitious and may be produced singly or in groups or cluster and are called simple tuberous and fasciculated tuberous roots respectively.

Example:

1. In sweet potato the tuberous roots are produced singly at nodes of a creeping stem.
2. In Asparagus and Dahlia they are produced in cluster.

Underground modification of stem: Stem Tubers

• In Potato plant (Solanum tuberosum), the basal and underground pat of stem produce axillary or extra axillary underground branches.
• The terminal part of such branches becomes swollen due to storage of starchy reserve food.
• These swollen tips of underground branches of stem are called stem tubers.
• A stem tuber has many notches on its surface called 'eyes'.
• Each 'eye' is actually at a node and consists of one or more small axillary buds and reduced scale leaves.
• After termination of dormancy period, under favorable conditions, one of axillary bud from an 'eye' sprouts by suppressing the growth of other buds.
• The tuber is cut into pieces, each at least with one 'eye', and are grown separately for commercial cultivation.

 

Other Stem modifications: Rhizome, Corm and Bulb

Rhizome

It is a prostrate, dorsiventral, thickened brownish stem, which grows horizontally under the surface of the soil.

It shows distinct nodes and internodes. It possesses a terminal bud and axillary buds in the axil of each scale leaf present at node.

 

Bulb: It is a condensed, disc like underground stem, which itself does not store food material.

The upper surface of disc like stem is slightly conical and bears centrally placed apical bud and many concentrically arranged, overlapping scale leaves.

Inner scale leaves or leaf bases store food and are thick and fleshy, while outer few scaly leaves remain thin and dry and are protective in function.

Lower surface of disc-like stem produces adventitious roots.

The discoid stem with compactly arranged fleshy leaves above and fibrous roots below is commonly called bulb.

 

Corm : Corm is a short, stout, fleshy, upright and thickened underground stem. It bears many buds in the axils of scale leaves which develop into daughter corms.

Subaerial modification of stem: Runner

Runner is a slender, prostrate, sub aerial branch, which creeps horizontally on the soil.

It is produced by the plants like Cynodon (doobgrass), Fragaria (strawberry) and Oxalis etc.for vegetative propagation.

Runner develops from the lower axillary bud of stem and is thin elongated cylindrical wire like structure with long intemodes.

It creeps on the ground and becomes rooted at the nodes. Shoots are produced from upper side of nodes.

After getting detached from parent, such shoots grow as independent plants.

A parent plant produces many such runners which spread in all the directions.

 

In addition to runner the other modifications are stolon, offset and sucker. These three are also subaerial modiications for vegetative propogation.

Modifications of Leaf

In plants like Bryophyllum, Kalanchoe, Begonia etc., vegetative propagation takes place with the help of their leaves.

In Bryophyllum, leaf is succulent with crenate or notched margin. Adventitious buds called epiphyllous buds (or foliar buds) are produced at the notches at the tip of lateral veins.

These buds start sprouting on the leaf to form leafy shoot and adventitious roots.

When such sprouts fall on the wet soil, they develop into independent plants.

In some species of Bryophyllum, the new plants are formed from the leaves only when the leaf is separated from the parent plant and falls on wet soil.

In Begonia, the foliar buds are produced on the surface of leaf

Artificial Methods of Vegetative Propagation.

Agricultuists and Horticultuists have developed several methods/techniques for the propagation of desired varieties of economically important plants.

These are called Artificial Methods of Vegetative Propagation.

The three main techniques are, cutting, grafting and budding.

 

Cutting

Cuttings are the small pieces of the plant parts like root, stem or leaves, which are able to develop into new plants when placed in moist soil.

A cutting must have the pimary meristem. Cutting is the most convenient and a cheap method of propagation.

Grafting

Grafting is the technique of joining together the parts of two different plants in such a way that they unite and continue their growth as one plant.

In Grafting the plant rooted in the soil and on which the part of other plant is inserted, is known as stock while the other which is inserted on stock is called scion (graft).

Budding

Budding is basically a kind of grafting in which a single bud with a small part of bark and living tissue is grafted on the particular stock.

In grafting the cambium plays important role. The cambia of both, stock and scion fuse together and make the union of these two plants successful.

As monocots do not have inter or intra-fascicular cambium and do not show secondary growth, grafting is not possible in monocots.

Micropropogation

To produce large number of plant propagules, tissue cutture technique is used.

For this shoot apical meristem is used as an explant and many shoot apices can be produced which are called micropropagules.

These micropropqgules are used to produce large number of gentically identical plants i.e clones within short time period.

Significance of Vegetative Propogation

Vegetative propagation is very significant as it is easy and cheaper method of multiplication.

The plant, like banana, pineapple, grapes, etc., which do not produce viable seeds under natural conditions, can be propagated by only this method.

It results in formation of genetically similar plants, thus the improved variety can be multiplied on the same line for several generations.

The yield can be increased by grafting the high yielding variety on the stock of variety with low yield but which are better adapted to particular region.

By this method rapid propagation of the plants can be done where the seeds are with long dormancy period.

In grafting process some desired character of stock (like disease resistance, vigor, etc.) can be transferred to the scion.

It helps in production of the clones of economically useful and rare plants.