Vegetative propagation or reproduction is common among lower plants and in horticulturally important flowering plants. These may be grouped under natural method and artificial method.
A. Natural methods
In cryptogams (nonflowering plants), the following types of vegetative propagation are observed. I lere particularly among the lower plants, the reproduction generally occurs under suitable conditions of environment i.e., when nutrients are plcntily available and temperature and moisture are congenial.
In unicellular fungi like yeast, a swelling is formed any where on the cell wall which is called bud. The nucleus of the mother cell divides and one of the daughter nuclei passes into the bud.
The bud when becomes fully developed, it remains attached to the mother plant by a narrow neck. Later, it gets detached and develops into a new individual.
Some aquatic lower plants like Spirogyra or such types of algae break up into two or more pieces or fragments. Upon maturation these grow into new individuals. In fungi, the hyphae may break into small pieces called oidia. This can form new progenies. In certain cases, fragmentation may be caused due to accidental breaking: The reason may be external factors like wind, water etc.
3. Gemma –
In case of Marchantia (a bryophyte) multicellular bodies known as gemma cups develop on the dorsal side of the thallus. After death and decay of the parent thallus, the gemmae get detached and grow up into new thalli Several types of vegetative propagation are seen among the higher plants. Any vegetative part of the parent plant such as leaf, stem or root can take part in the formation of new progenies. This may occur under normal conditions or may be induced artificially.
1. By roots:
Some adventitious roots of sweet potato, Dahlia etc. become thick, swollen due to storage of food. The adventitious buds are borne on such structures. The buds produce leafy shoots, called slips. When such roots with adventitious buds are planted in the soil, they produce new plants and thus, vegetative propagation occurs.
2. By stems:
The stems are efficient means of vegetative propagation. This may be of the following two types.
(a) Subaerial stems: Subaerial stems may develop as lateral branches from the mother plant. This may break up from- the parent plant and then, grow into new plants. Example- Runners (Oxalis), sucker – (banana, Chrysanthemum), stolon (Jasmine), offset (Eichhornia)
(b) Underground Stems: In certain plants the underground stems become modified for storage of food during the active phase of the growth. Examples- Rhizome (Ginger), tuber (Potato), bulb (Onion) and corm (colocasia)
3. By leaves:
The fleshy succulent leaves of Bryophyllum bear adventitious buds in their notches located in the margins. When the leaves fall on moist soil, these buds develop into small plants completing the process of vegetative propagation.
B. Artificial methods:
Farmers, gardeners, horticulturists have taken advantage of this type of propagation in plants. They have manipulated the process for their own benefit.
This is a very common method. Here a piece of stem up to a suitable length is taken from the parent plant. This stem piece is called the cutting. It should have few nodes and internodes. The cutting is planted in moist soil with suitable nutrients. After sometime, roots emerge from the nodes of the basal portions of the cuttings and the upper buds give
Underground stems modifications: A. Bulb, B. Corm
Reproductions by leaf Fig.u.8.Grafting by stem cutting (A-C) different stages rise to the shoot. The plants of China rose, sugar cane, Bougainvillea etc are commonly grown by this method.
In this process, a detached part of one plant (i.e. twig or bud) is inserted into the stem or root system of another plant. The former is called scion (short piece of detached shoot containing several dormant buds) and the latter stock (lower portion of the plant which is fixed to the soil by its root system).
The grafted portion is covered by grafting wax to avoid infection. The scion becomes part of the plant into which it is grafted. The new plant developed bears flowers and fruits, characteristic of scion. Mango, rose, orange, guava etc. are generally propagated by graftings.
3. Layering: –
Here, roots arc artificially induced to grow on the branches before they arc detached from the parent plant. There are three types of layering: i) serpentine layering. Branches at the lower portions of the stem are put in the soil at many places to form new plants from them, ii) mound layering: Soft lower branches are selected and a ring of epidermal layer is removed.
This part is then pegged in the soil with the apical portion remaining outside. After an interval of time, adventitions roots develop. Then the ringed portion is cut off to allow new plant grow independently, iii) Air layering: Here, a ring of bark is removed from the aerial branches. It is then covered by grafting clay (water, clay, cowdung) with a little amount of root inducing promoter.
The entire portion is wrapped with polythene bandage. At a particular time interval, roots are developed and when separated it can grow into a new plant. Example – Pomegranate, orange, lemon etc.
This is a quick method of reproduction where survival rate of the progenies is very high. Endangered or threatened plant species can be saved by such propagations.. Plants reproducing vegetatively take a short time to mature. Potato for example, takes three months to mature. Plants with desirable qualities may be developed by this method. The vegetative method of reproduction among angiosperms has a lot of agricultural and horticultural applications.
This process is similar to rooting of plant cuttings and is, in a way, another method of vegetative propagation of plants. However, it differs from the conventional procedure in that it is carried out in aseptic condition and requires an artificial nutrient medium A small plant cutting or explant (usually axillary bud) is sterilized and inoculated into culture vessel containing semi-solid nutrient medium.
The inoculated culture vessel is incubated at room temperature, In a short span of time, a large number of shoots develop from the axillary buds through a process called axillary budroliferation. Each growing point is then subcultured to give rise to shoot.
This phenomenon is called adventitious shoot formation. Each shoot is stimulated by auxin to develop roots the new plantlet or propagule is then transferred tofield.
This method is generally practised for ornamental, fruit and crop plants. This is useful because (i) the healthy propagules can only be obtained (pathogen free) (ii) rapid rates of multiplication can be ensured (iii) Development of plant materials with desired traits and their maintenance in a small space.
Substantial benefits can be expected to occur in the production of crops such as tea, coffee, oil palm, date palm, coconut, fruit yielding plants like papaya, banana, Citrus and apples.
Significant progress has also been achieved in developing protocols of micropropagation of tree species. Mass propagation, in vitro, of teak, Eucalyptus spp., sandal wood, different species of bamboo and many other trees has been successfully done.
Similarly, considerable progress has been made in commercial harvesting of medicinal plants such as Dioscorea deltoidea, D.floribunda, Atropa belladona, Solarium Spp., Rauwolfia serpentina and Digitalis anta by microprogation techniques
Micropropagation of meristematic plant shoot (axillary bud) cutting through plant tissue culture technique; B. Development of shoot and root, one following the other, through plant tissue culture technique (organogenesis).
Some species of potato, Cassavia, sugarcane and banana are severely and chronically affected by viruses. Yields of these crops can be increased significantly by planting disease free stocks.
Potato is of most important and widely grown food crops in the world. But it is susceptible to many viral pathogens, some of which may be present without perceptible symptoms. The pathogens cause gradual decrease in vigour and yield of potatoes.
Eradication of viruses can be caused by employing micropropagation techniques where healthy meristematic buds are cultured. More than 500 plants can be obtained in about three to four months starting from a single meristem. By manipulating the medium composition, light intensity and temperature, plantlets can be induced to produce microtubers.
These disease free microtubers can be grown under controlled conditions in soil to form minitubers. The minitubers can be planted directly in the field to raise a disease free crop.
Apart from the application of micropropagation techniques for generating true-to- type planting material from elite genotypes, micropropagation holds special significance in situations where rapid bulking of extremely limited stock material is required. The desired genetic gains achieved through plant breeding can be multiplied several fold on an economic and rapid time scale
Stage of anther development and microsporogenesis ; (A) T.S. of young anther; (B) Differentiation of four rows of archesporial cells [shaded] in the four microsporangia; (C) Primary sporogenous cells [shaded] and • parietal cells [p]; (D) Microspore or pollen mother cells [P.M.C.] (E) Microsporangia [pollen chambers] showing pollen mother cells [p.m.c] and tapetum [t]; (F) Dyad stage of meiosis in P.M.C.G. Tetrad stage; (H &I) Tetrahedral arrangement and development of pollens; (J) T.S. of a mature anther.