ASEXUAL REPRODUCTION
Reproduction is the process of formation of new individuals of a species from the preexisting ones. Plants show two types of reproduction, asexual and sexual.
Asexual reproduction is the mode of formation of new individuals from the parts of the parent without the meiotic formation of gametes and their fusion. The method of reproduction which do not involve meiosis and fertilization are known as apomixis or asexual reproduction. Asexual reproduction if of two types:
- Agamospermy, a method in which seed is formed but without gametic union.
- Vegetative propagation includes methods where vegetative parts of the body such as rhizomes, tubers, bulbs etc., are used for multiplication.
AGAMOSPERMY
It is a form of vegetative reproduction by plants in which seeds are formed but without fusion of gametes. It also includes processes like apospory and apogamy. Apogamy is the formation of sporophyte directly from a gametophyte without involving the formation and fusion of gametes whereas apospory is the formation of gametophyte directly from sporophyte without meiotic formation of spores.
In agamospermy, the seeds possess an embryo developed not from a diploid zygote but by an abnormally formed diploid egg. In such cases, not only the egg but the entire embryo sac is diploid because it develops without meiosis either from a megaspore mother cell or any cell of the nucellus.
The diploid egg of an embryo sac, thus formed, directly develops into an embryo without fusion with the male gamete. The process of development of egg into an embryo without fertilization is called parthenogenesis. Besides this, embryos are also produced directly from the cells of the nucellus. This is called adventive polyembryony. It results in the formation of many embryos in each seed. The example is Citrus seed which contains 2 to 40 embryos.
VEGETATIVE PROPAGATION
Vegetative propagation can be defined as regeneration or formation of new individual from any vegetative part of the plant body. The methods of vegetative propagation involve separation of a part of plant body which develops into a new plant.
Methods of natural vegetative propagation:
Different plant parts are variously modified for vegetative propagation. Some of these are as follows:
Vegetative propagation by stems: The modified stems like bulbs rhizomes, runners, corms, tubers, offsets etc., help the plant to multiply under favourable conditions.
- Bulb: It is a modified shoot that has a very short stem and possesses apical and axillary buds. Some of these buds grow to form shoots. For example, Allium (onion), Tulipa (tulip), Narcissus etc.
- Runners: These are creeping stems which produce roots at nodes. Runners break at places and each piece grows as an individual. For example, Cynodon, Oxalis, Centella etc.
- Rhizomes: These are underground horizontally growing stems. They have distinct nodes, internodes, and axillary buds. The branches grow from the buds, which later separate to form independent plants. The common examples include Zingiber, Marsilea
- Corm: Corms are unbranched swollen underground stems which bear many buds. Under favourable conditions, the buds sprout to form new plants. For example, Gladiolus, Colocasia, Freesia, Crocus etc.
- Tubers: It is a modification of underground stem tip. Each tuber bears several eyes on its surface in small depressions. In order to raise a new crop, the tuber is cut into pieces, each piece bearing at least one eye. The central bud of each eye develops into a new shoot. For example, potato.
- Offset: An offset is a short, thickened prostrate stem developing from the base of the main stem. It bears a tuft of leaves at the apex. Offsets are separated from the mother plant by cutting them close to the main stem and planted directly if they are well rooted. For example, Pistia, Eichhornia are propagated by offsets.
Vegetative propagation by roots:
The roots help in vegetative propagation. Some common examples of root tubers are sweet potato (Ipomoea batatas), Asparagaus, Dahlia etc. These roots develop adventitious buds, each of which forms plant. When placed in soil the buds present on the roots grow into leafy shoots called slips. Slips are separated and planted to form new plants.The roots of some woody plants produce shoots which grow into new plant. For example, Murraya, Albizzia, Dalbergia Sisso etc.
Vegetative propagation by leaves
Leaves of a number of plants develop adventitious buds for vegetative propagation. For example, leaf tip of Walking Fern (Adiantum caudatum), marginal notches in Kalanchoe and Bryophyllum. In Bryophyllum, daigremontianum, the marginal buds sprout while the leaf is attached to the plant. In some other plants, the buds develop when the leaf is injured or detached or placed over a moist cell. For example, Begonia (elephant ear).
Bulbils: They are fleshy buds which develop into new plants after falling on the soil for example Agave, Oxalis, Ananas sativa (pineapple), Dioscorrea, lily, Chlorophytum. Some of them are modified floral buds. For example, Agave. In Dioscorrea, they develop in the axils of leaves while in Oxalis, they occur above the base of the fleshy root.
Turions: They are flesh buds that develop in aquatic plants for perennation and propagation. For example, Potamogeton, Utricularia.
Aerial shoots: A stem segment of Opuntia and other cacti develop into a new plant after falling on the soil. A segment of sugarcane having at least a single node is used in agriculture to produce new plant.