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1.4.12.1: Natural and Artificial Asexual Reproduction - Biology


Learning Objectives

  • Discuss the mechanisms, advantages, and disadvantages of natural and artificial asexual reproduction

Natural Methods of Asexual Reproduction

Natural methods of asexual reproduction include strategies that plants have developed to self-propagate. Many plants—like ginger, onion, gladioli, and dahlia—continue to grow from buds that are present on the surface of the stem. In some plants, such as the sweet potato, adventitious roots or runners can give rise to new plants (Figure 1). In Bryophyllum and kalanchoe, the leaves have small buds on their margins. When these are detached from the plant, they grow into independent plants; or, they may start growing into independent plants if the leaf touches the soil. Some plants can be propagated through cuttings alone.

Artificial Methods of Asexual Reproduction

These methods are frequently employed to give rise to new, and sometimes novel, plants. They include grafting, cutting, layering, and micropropagation.

Grafting

Grafting has long been used to produce novel varieties of roses, citrus species, and other plants. In grafting, two plant species are used; part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Both are cut at an oblique angle (any angle other than a right angle), placed in close contact with each other, and are then held together Figure 2. Matching up these two surfaces as closely as possible is extremely important because these will be holding the plant together. The vascular systems of the two plants grow and fuse, forming a graft. After a period of time, the scion starts producing shoots, and eventually starts bearing flowers and fruits. Grafting is widely used in viticulture (grape growing) and the citrus industry. Scions capable of producing a particular fruit variety are grated onto root stock with specific resistance to disease.

Cutting

Plants such as coleus and money plant are propagated through stem cuttings, where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start producing a root even when placed only in water. For example, leaves of the African violet will root if kept in water undisturbed for several weeks.

Layering

Layering is a method in which a stem attached to the plant is bent and covered with soil. Young stems that can be bent easily without any injury are preferred. Jasmine and bougainvillea (paper flower) can be propagated this way Figure 3.

In some plants, a modified form of layering known as air layering is employed. A portion of the bark or outermost covering of the stem is removed and covered with moss, which is then taped. Some gardeners also apply rooting hormone. After some time, roots will appear, and this portion of the plant can be removed and transplanted into a separate pot.

Micropropagation

Micropropagation (also called plant tissue culture) is a method of propagating a large number of plants from a single plant in a short time under laboratory conditions (Figure 4). This method allows propagation of rare, endangered species that may be difficult to grow under natural conditions, are economically important, or are in demand as disease-free plants.

To start plant tissue culture, a part of the plant such as a stem, leaf, embryo, anther, or seed can be used. The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated mass known as callus, from which individual plantlets begin to grow after a period of time. These can be separated and are first grown under greenhouse conditions before they are moved to field conditions.


Asexual Reproduction: Types of Asexual Reproduction in Organisms | Biology

Some of the important types of asexual reproduction in organisms are: 1. Fission 2. Budding 3. Fragmentation 4. Zoospores and 5. Conidia!

1. Fission:

It is that type of asexual reproduction in which a fully grown parental organism divides into two or more than two daughter cells.

In this, the reproductive unit is whole parental body.

Types of Fission:

On the basis of number of daughter cells produced, fission is of two types:

(a) Binary Fission:

It is division of adult parental body into two nearly equal daughter cells during favourable conditions.

It is the simplest and most common method of asexual reproduction found in protists (Amoeba, Euglena), bacteria, and green algal forms (Chlamydomonas) and planarians (flat worms).

In binary fission, the karyokinesis (division of nucleus) is followed by cytokinesis, so that nothing is left with the parent. Daughters feed, grow and repeat the process. The organisms undergoing binary fission are called immortal as after binary fission nothing is left with the parental body so there is no natural death.

On the basis of plane of cytokinesis, the binary fission is of three types:

(i) Irregular or simple binary fission (Fig. 1.3 A):

Here cytokinesis may take place along any plane but its plane is always perpendicular to that of karyokinesis e.g., Amoeba. So nothing is left with parental Amoeba which completely divides into daughter cells, so it is called immortal as it suffers no natural death.

(ii) Longitudinal binary fission (Fig. 1.3 B):

Here cytokinesis takes place along longitudinal axis e.g., in flagellates (Euglena).

(iii) Transverse binary fission (Fig. 1.3 C):

Here cytokinesis takes place along transverse axis e.g., in ciliate protozoans (Paramecium).

In Opalina and Pelomyxa (both protozoans), the peculiar binary fission called plasmotomy occurs in which a multinucleate adult parent undergoes cytokinesis to form two multinucleate daughter cells followed by karyokinesis in each daughter cell. In Planaria, the parent undergoes transverse binary fission (Fig. 1.4)

(b) Multiple fission:

It is that type of asexual reproduction in which the parental body divides into many daughter cells simultaneously during the unfavourable conditions to increase the chances of survival of daughter cells. In this, the reproductive unit is whole parental body.

Multiple fission is found in a number of organisms e.g., algae among plants, Plasmodium malarial parasite (Fig. 1.5), Amoeba (Fig. 1.6) and Monocystis.

During multiple fission, the nucleus of parent divides by repeated amitosis into many nuclei, each nucleus takes a bit of cytoplasm and forms a daughter cell. Some cytoplasm of the parental body remains unused and is called residual body.

For example, during erythrocytic schizogony in the life cycle of P. vivax (Fig. 1.5), the cryptomerozoite enters the RBC, feeds on the haemoglobin and other cellular contents saprozoically and becomes fully grown and is called schizont. Then multiple fission occurs and 12-24 merozoites are formed.

RBC ruptures and merozoites are released which repeat the process. Similarly in an encysted oocyst called sporont present on the stomach wall of female Anopheles host, the multiple fission called sporogony occurs and many sporozoites are formed. Similarly, Amoeba undergoes multiple fission during unfavourable conditions in an imencysted form as well as in encysted form.

In the former, daughter cells get encysted (called encystation), and are called spores, so the process is called sporulation. The cysts also help in perennation and dispersal. In the later case.

Amoeba withdraws its pseudopodia and secretes a three-layered chitinous cyst wall (encystation). On the approach of favourable conditions. Amoeba undergoes multiple fission and produces many small sized amoebulae or pseudopodiospores (Fig. 1.6)

2. Budding:

It is that type of asexual reproduction in which one or more unicellular or multicellular outgrowths, called buds, are formed on or inside the parental body.

Each multicellular outgrowth called bud enlarges, develops the parental characters and then separates to lead an independent life. It feeds, grows, becomes an adult and repeats the process.

Budding is found in sponges (Scypha), coelenterates (Hydra), annelids (Syllis) and tunicates (Salpa) among animals. Among fungi, it is found in yeasts (Fig. 1.8).

In Hydra (Fig. 1.7) and Scypha (Fig. 1.9) the budding is exogenous as the bud is formed on the outer surface of parental body while in Spongilla (a fresh water sponge), the budding is endogenous as a number of buds called gemmules are formed inside the parental body.

Each gemmule (Fig 1.10) of Spongilla is a mass of undifferentiated cells, called archaeocytes, surrounded by a protective coat of amphidisk spicules. Gemmule helps in perennation and dispersal. During favourable conditions, archaeocytes come out of gemmule through micropyle and form a new sponge.

In Scypha (Fig. 1.9), the exogenous buds remain attached to the parental body and may develop secondary buds to form a kind of colony.

Unlike the binary fission, the identity of the parent body is retained after reproduction.

Table 1.2. Differences between Binary and Multiple Fission.

Number of daughters produced

Parent divides in two daughters.

Parent divides in many daughters.

During favourable conditions.

During unfavour­ able conditions.

Nothing is left with parent.

Residual cyto­plasm is left.

3. Fragmentation:

It is that type of asexual reproduction in which the parental body breaks into two or more fragments either by wave action (e.g., sponges) or by death and decay of old parts. Each body fragment develops into an organism. It is found in some flat worms (Microstomum), sea anemones among coelenterates, and echinoderms. In starfish, even one arm with a part of central disc can develop into whole animal.

Advantages:

(a) There is no need of sexual partners.

(b) Rapid rate of reproduction.

Disadvantages:

(a) No chance of new combinations of genes and variatioris. So individuals may not be able to adapt to changing environment.

(b) It generally leads to overcrowding and struggle for existence.

4. Zoospores:

Members of kingdom fungi and algae reproduce through special asexual reproductive structures called zoospores (Fig. 1.12). These are flagellated, motile naked protoplasmic bodies. Zoospores are produced in zoosporangium.

Zoospores may be biflagellate (e.g., Ectocarpus), quadriflagellate (e.g., Ulothrix) or multiflagellate (e.g., Oedogonium). They may be uninucleate (e.g., Ulothrix) or multinucleate called synzoospores.

5. Conidia:

These are non-motile spores produced exogenously by constriction at the tips of special hyphal branches known as conidiophores. The conidiophores may be branched or unbranched. They produce conidia singly as in Phytophthora or in chains as in Aspergillus and Penicillium (Fig. 1.13).


What is Natural Vegetative Propagation

Natural vegetative propagation refers to the natural development of a new plant without human intervention. The most significant feature of plants that undergo natural vegetative propagation is their ability to develop adventitious roots from parts other than roots such as leaves and the stem. A new plant may arise from the extensions of the stems, root or leaves of the parent plant. Plantlets with adventitious roots at the margin of a leaf are shown in figure 1.

Figure 1: Plantlets of Bryophyllum daigremontianum

Rhizomes, bulbs, runners, and corms arise from the stem in natural vegetative propagation. Buds and tubers are the natural vegetative propagation structures that arise from roots while plantlets are the type of structures that arise at the margin of the leaves. Different types of structures involved in the natural vegetative propagation and example plants are described in table 1.

Developmental Structures of Natural Vegetative Propagation in Plants

Developmental structure

Type of Structure

Modified stems that horizontally grow along the ground surface or underground

Ex: Lilies, irises, orchids, and certain grasses

Modified stems that horizontally grow on or under the surface of the ground

Ex: Strawberry and currants

Swollen, underground stems

Ex: Garlic, onion, lilies, daffodils, tulips, and shallots

Vegetative structures developed from root or stems

Ex: Stem tubers – Potatoes and yams

Root tubers – Sweet potatoes and dahlias

Enlarged, bulb-like underground stems

Ex: Taro, gladiolus, and crocus

Plant shoots arisen from underground buds

Ex: Apple, cherry, and banana trees, hazel, shrubs, raspberries, gooseberries, and rose

Vegetative structures developed at the margin of leaves

Ex: Kalanchoe, spider plant, hawkweed, dandelion, some citrus, some orchids, and many kinds of grass


Watch the video: Asexual Reproduction (November 2021).