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Vegetative Reproduction or Propagation in Flowering Plants
Vegetative propagation or reproduction is the process of multiplication in which a portion or fragment of the plant body functions as a propagule and develops into a new individual. Vegetative propagation is a form of asexual reproduction in plants. Among the flowering plants, a number of herbaceous and perennial plants propagate vegetatively in nature. Root, stem, leaves, and buds are the common structures that take part in vegetative propagation and develop into a completely new plant. The methods of vegetative propagation are grouped into two different methods: natural and artificial.
Characteristics of Vegetative Reproduction
- Apart from sexual and asexual reproduction, vegetative reproduction is a comparatively simple and primitive process.
- This reproduction takes place simply by cell division or by separation from the mother’s body.
- Mitotic cell division takes place and only one individual is required.
- Offspring bears similar parental qualities.
Classification of Vegetative Propagation in Plants
Natural Vegetative Propagation
In this method, vegetative propagules detach from the mother plant and develop into new plants under suitable conditions. This type of reproduction involves roots, stems, or leaves. It takes place by the following methods:
1. Propagation by Adventitious Bud
The adventitious buds are the subnormal buds found at the points along the stem. They arise from the dormant buds in the leaf axils of the young stem and persist for an indefinite number of years within the cortical-cambial zone.
Different types of adventitious buds are
- Root bud: In the case of Trichosanthes, Ipomoea, the adventitious buds of the roots develop and form new individual plants.
- Stem bud: New individual plant grows from the developing bud of the Dahlia, Rosa’s stem.
- Leaf bud: In the case of Bryophyllum, Begonia, an adventitious bud grows from the leaf and then matures and detaches from the mother body, giving rise to new individual plants.
In the case of Bryophyllum, buds on marginal notches of intact leaves form plantlets while staying attached to the plants. In the case of Begonia, injured leaves develop into new plants. The margins of succulent and fleshy leaves of Bryophyllum contain adventitious buds in their notches. When these leaves fall on the ground, the buds develop into small plants. In the case of the ‘Walking fern’ (Adiantum caudatam), its leaf tips give rise to new plants whenever they come in contact with soil.
2. Propagation by Roots
The tap roots of Albizia, Murraya, etc., and the adventitious roots of Asparagus, Dahlia, sweet potato, etc., serve to propagate plants vegetatively. Adventitious buds developed from both tap roots and adventitious roots are planted in the soil, to give rise to new plants.
3. Propagation by Stems
(a) Underground modified Stems:
Underground stems are modified plant structures that derive from stem tissue but exist under the soil surface. Plants use under¬ground stems to multiply their numbers by asexual reproduction and to survive from one year to the next, usually over a period of dormancy. Some plants produce stems modified to store energy and preserve a location of potential growth to survive a cold or dry period which normally is a period of inactive growth and when that period is over the plants resume new growth from the underground stems.
Some plants which store up food in their underground parts of stem only to tide over unfavourable conditions and assume various shapes and sizes are called under¬ground modified stems. Underground modified stems are of the following four types:
(i) Tuber: These are fleshy, swollen, rounded, or oblong distal portions of underground axillary or adventitious branches. The tips of these branches become enlarged in the form of tubers, due to the accumulation of surplus food material manufactured by the aerial shoots. Each tuber possesses a number of depressions called eyes. Each eye represents a node. It has a scale leaf in the form of a ridge. Each eye contains 1-3 dormant buds. The stem-tuber lacks adventitious roots, e.g., Potato (Solanum tuberosum).
(ii) Rhizome: These are fleshy, horizontally growing, perennial, underground stems, which continues to grow for an indefinite period producing new leaves or shoots during favourable period. The aerial leaves or shoots die during unfavourable conditions and are replaced by new ones on the arrival of the next favorable period. A rhizome is differentiated into nodes and internodes. The nodes bear scale leaves that protect axillary buds. It also bears adventitious roots at the nodes on its lower side. Both apical buds and axillary buds are present e.g., Ginger (Zingiber officinale).
A special type of rhizome is the rootstock which is a rhizome growing up vertically instead of horizontally. Very common examples of rootstock are found in bananas and aroids (e.g., Alocasia indica).
(iii) Bulb: This is another underground modified shoot in which the axis, i.e., the stem is extremely shortened to a convex disc with compressed internodes, and consisting of a terminal bud and numerous fleshy scale leaves with axillary buds. The base of the discoid stem bears fibrous adventitious roots. The terminal bud develops into a leafless hollow floral axis called a scape. Bulbs are of two types:
- Tunicated bulbs: In this type the fleshy leaves commonly occur surrounding the short stem in a concentric manner. The outer leaves become dry and membranous to form the coat or tunic, e.g., Onion (Allium cepa).
- Scaly bulbs: These bulbs lack a tunic or covering sheath. The fleshy scales are narrow and overlap one another on the margins only. Such a bulb is never a compact body, e.g., Lilium sp.
(iv) Corm: This a stout, solid, fleshy underground rootstock with a large apical bud. It bears a number of circular nodes with scales, which represent thin sheathing bases of fallen dead leaves. The nodes bear axillary buds. A large number of adventitious roots are also borne at the base of the corm, e.g., Amorphophallus campanulatus.
(b) Subaerial Modified Stem:
Some weak herbaceous plants produce modified branches that grow and finally give rise to small daughter plants. In this way, a colony of plants is produced. This modification is meant for vegetative propagation. There are four types of subaerial modified stems as discussed in the following:
(i) Runners: These are subaerial, weak, slender, lateral branches, that grow horizontally along the soil surface. They grow from basal axillary buds of the short, erect shoots. A number of runners arise from one erect shoot and spread in different directions and ultimately bear new crowns and a tuft of adventitious roots at nodes, e.g., Marsilea quadrifolia, Oxalis comiculata, etc. A special type of underground runner or creeping stem is sobole.
(ii) Suckers: Sucker is an underground runner, which arises from the underground base of the aerial shoot, grows horizontally in the soil, and ultimately comes out to form a new aerial green shoot. Each sucker has one or more nodes with scale leaves and axillary buds. It also bears adventitious roots at the nodes, e.g., Chrysanthemum coronarium.
(iii) Stolons: These are elongated arched or horizontal runners with long internodes, which develop adventitious roots on coming in contact with the soil at the nodes. A stolen bear’s scale leaves and axillary buds at the nodes. The axillary buds may either form a secondary stolon or may grow up as an erect short aerial stem, e.g., Fragaria vesca.
(iv) Offsets: These are just like runners but the inter-nodes are shorter and stouter than the runners. An offset arises from an axillary bud at the base of the cluster of leaves. It runs horizontally for a short distance and terminates in a bud that develops adventitious roots and a cluster of leaves, e.g., Eichhornia, Pistia, etc. Offsets are lateral shoots used for plant propagation. Offsets are part of the branch by which plants can reproduce asexually and formed a new daughter plant.
(c) Modified Aerial Stems:
In some plants, stems undergo an extreme degree of modifications and take up specific appearances to perform some special functions. Due to extreme modifications, their stem nature is very difficult to identify, except for the origin and the position. These are, therefore, called metamorphosed stems. The metamorphosed stems are of the following types:
(i) Stem tendril: It is a metamorphosed stem or branch, slender and coiled, meant for the support of a weak stem. These arise as modifications of axillary or terminal buds. Tendrils may be branched or unbranched. According to their origin tendrils may be apical (e.g., Vitis vinifera), axillary (e.g., Passiflora sp.), or extra-axillary (e.g., Cucurbita sp.).
(ii) Thorn: These are hard, straight, and pointed structures with direct vascular connections with the stem proper. Thorns may be modifications of axillary bud as in Duranta repens or terminal bud as in Carissa carandas. Thorns may be simple or branched and may even bear leaves, flowers, and fruits.
(iii) Phyllodade or Cladophyll: These are green flattened stems or branches, which appear leaf-like, and have the function of photosynthesis in the absence of normal green leaves. The true leaves are either feebly developed or modified into spines. Phylloclades are succulents due to the storage of water and food and are characteristics of some xerophytes, e.g., Opuntia dillenii.
(iv) Cladode: This is a type of phylloclade consisting of one internode only. These are green cylindrical or flattened stems or branches of limited growth. They perform the function of photosynthesis. The true leaves are reduced to scales or spines to reduce transpiration. The cladodes arise in the axils of scaly or spiny leaves at the nodes on the normal stem, e.g., Asparagus racemosus.
(v) Bulbil: These are modifications of axillary vegetative or floral buds into swollen structures which serve for the propagation of these plants. In Dioscorea bulbifera, the bulbils are modified axillary buds whereas in Agave, these are modified floral buds. These get detached from the parent plant, fall on the ground and germinate into new plants.
(vi) Pseudo-bulb: It is also a modified stem as seen in aerial orchids. Here generally one internode of the stem becomes modified into a fleshy and tuberous structure stores moisture in excess and survives during unfavourable conditions, at the return of favourable conditions it germinates to form a normal plant body.
(vii) Thalamus: The thalamus or the floral axis on which floral leaves are present is a modified and condensed stem. The presence of nodes and internodes on it is distinct in the thalamus of Gynandropsis pentaphylla and in some other plants.
(viii) Turions: Turions are fleshy buds that develop in aquatic plants for propagation as well as perennation, e.g., Utricularia, and Potamogeton.
Advantages of Vegetative Propagation
- Vegetative propagation is a more rapid, easier, and cheaper method of propagating plants as compared to seeds.
- It is the only known method of multiplication in plants like banana, seedless grapes, oranges, rose, jasmine, sugarcane, potato, and many others.
- The most important advantage of vegetative reproduction is the preservation of desirable characteristics in the plants, which is not possible in plants raised from seeds.
- The good qualities of plants can be preserved for a long time.
- Superior quality flowers and fruits can be produced by the method of grafting.
- Disease-free plants can be produced through micropropagation.
- Transgenic plants (genetically modified plants) can be produced through the use of tissue culture.
Disadvantages of Vegetative Propagation
- There are no variations. Therefore, the plants may show degeneration and in such plants, there is less adaptability to the changed environment.
- Good qualities cannot be introduced nor can bad characteristics be eliminated in plants multiplied through vegetative propagation.
- A diseased parent transmits the disease to all its daughters.
- Vegetative propagules are not as efficiently protected as the seeds are.
- There is no dispersal of vegetative propagules. Therefore, it causes overcrowding.
Differences between Vegetative Reproduction and Sexual Reproduction:
|Vegetative Reproduction||Sexual Reproduction|
|1. This type of reproduction only occurs in plants.||1. This type of reproduction occurs in both plants and animals.|
|2. Only vegetative parts are involved in this reproduction.||2. Sex organs are involved in this reproduction.|
|3. In this reproduction fertilization is not occurred, that was why gametes are not formed.||3. In this reproduction, the union of two gametes and the formation of a zygote happens.|
|4. Due to this reproduction variation does not occur.||4. Due to this reproduction variation occur for evolution.|
|5. The nature of this reproduction is simple and occurs rapidly.||5. The nature of this reproduction is complex and takes more time.|