Developmental Biology in botany deals with the process of reproduction and all the series of developmental stages required to form a new plant from reproductive structures.
The vital characteristics of living organisms by which new individuals are formed and so formed individuals are similar to their parents are called reproduction. It is of two types- Asexual and sexual reproduction.
The reproduction in which new individuals are formed from a single parent without the involvement of gametes and their fusion is called asexual reproduction. It is also called apomixis. The individuals of the new generation are exactly similar to their parent and are known as clone (unit- remete).
Asexual reproduction is of following types : Agamospermy, spore formation, and vegetative propagation. Here we discuss vegetative propagation.
It is the type of reproduction in which new plants are produced directly from any vegetative part of the parent plant. The part of the plant which produces new plant is called propagule. Vegetative propagation is of following types:
The vegetative reproduction which occurs naturally without human effort is called natural vegetative propagation. This type of reproduction takes place in various ways-
a) By roots: E.g sweet potato,Dahlia, Asparagus e.t.c. by modified tuberous roots. The leafy shoot arises from buds of the root are called slips. Some plants that propagate by common roots are Dalbergia, Alnus, Rosa e.t.c.
b) By stems: Different types of stems are capable of producing new plants such as:
⇒Underground stems :
1) Rhizome : Thickened, prostrate underground stem storing food is called rhizome. Example- Banana, ginger, turmeric e.t.c.
2) Corm : Swollen, unbranched, vertically grown underground stem is called corm. Example- Colocasia, Crocus e.t.c.
3) Bulb : Underground, condensed stem containing fleshy leaves is called bulb. Example- Onion, Garlic e.t.c.
4) Tuber : Swollen ends of underground stem storing food is called tuber. Example- Potato,Marsilea e.t.c.
5) Sucker : The short underground stem branches that arise at the base of the aerial shoots is called a sucker. Example- Mint,Chrysanthemum e.t.c.
Creeping and rooted stems which are neither aerial nor underground are called sub-aerial stems.
1) Runner : The narrow, green, prostrate stem which has distinct nodes and internodes is called a runner. Eg- Cynodon, Centella e.t.c.
2) Stolen: The stem which grows laterally from the base of a stem and runs horizontally is called a stolon. Eg- Strawberry e.t.c.
3) Offset : The stem which is found in hydrophytes and is like the runner but has only one internode is called offset. Eg- Eichhornia, Pistia e.t.c.
⇒ Aerial stems:
Normal stem also gives rise to plants. Eg- Sugarcane, rose e.t.c.
c) By leaves: Plants likeBryophyllum, Begonia, Adiantum e.t.c. reproduce vegetatively with the help of leaves.
d) By bulbils: The modified multicellular floral or vegetative fleshy buds are known as bulbils. Eg- Garlic, Pineapple,Cycas, Oxalis e.t.c.
The vegetative reproduction which occurs by human efforts is called artificial vegetative propagation. This method is applied for the quick production of new plants and combination of the qualities of two varieties of plants.
a) Cutting: The portion of any plant organ such as stem, root or leaf, used for vegetative propagation is called cutting. Stem cuttings are most commonly used for this purpose. Factors such as the optimum length and diameter of the cutting, age of the parent plant and season are to be considered while selecting a cutting for each species. Some of the plants propagated by stem cuttings are sugarcane, rose, Hibiscus e.t.c.
b) Layering: Layering is a technique for plant propagation in which a portion of an aerial stem is encouraged to grow roots while still attached to the parent plant and then removed and planted as an independent plant. Layering is done on the one-year-old branch during a rainy season. A soft basal branch is defoliated and small injury or cut is giving in the middle. The injured part is either pegged down in the soil or covered by grafting clay to produce adventitious roots on it. Until the production of roots, it is supplied with water. After the production of roots, it can be separated from the parent plant. Layering is of different types- Soil layering (Serpentine or Mould) and air layering (Gootee).
⇒ Serpentine layering is the layering in which a single branch is pegged in soil at many places to produce many plants from a single branch. Mould layering is the layering in which a branch is pegged in soil at one place and its apical bud is exposed in the air. The place which is pegged in the soil gives rise to roots and it can be cut to plant separately.
⇒ Air layering is the type of layering which is done in the air on woody non-flexible branches. A healthy branch is defoliated and given an injury. The injured region is then covered by grafting clay and the clay is covered again by a rug which is tied with the help of a rope. This is called gootee. It is then supplied with regular water until the formation of roots. Application of a root-promoting substance to the exposed wound is sometimes beneficial. Grafting clay is the mixture of cow dung, soil, finely cut hay and water. Air layering is also called gootee.
c) Grafting: Grafting is the technique of joining two plants in such a way that they unite and later develop a composite plant. Grafting is done between the two closely related dicotyledonous plants having vascular cambia. The rooted supporting portion of one plant, called stock, is joined with a twig of another plant's shoot system, called scion.
There are numerous uses for grafting:
- To create new plants
- To create dwarf plants
- To increase hardiness
- To increase disease resistance
- To change the natural plant form
A grafted plant consists of a scion- a short piece of stem with two or more buds, and the stock- which is the lower portion of the graft. There must be a contact with the cambium layer of the scion and the stock for successful growth to occur.
It is also essential that the graft doesn't dry out. This is prevented by wrapping the grafted area with a wax, grafting compound or rubber/ polyethene banding.
The scion and stock must also be compatible plants. Like apple to apple, orange to orange (Same genus).
The main reason for grafting is to asexually propagate of orchard trees, shade trees, and roses. There are several grafting methods commonly used by propagators:
Whip and tongue grafting is commonly used to propagate fruit trees especially apple and pear. The root of a young seedling tree is used for the rootstock. The scion is dormant twig containing three or four buds- it is the diameter of a pencil. The stock and scion are cut at a slant angle. The whip or tongue is made by making a single cut one-half inch deep into the sloping cut- both the scion and the rootstock have a tongue cut. Insert the scion tongue cut in the rootstock cut. Line up the cambium layers and bind with graft tape. Seal with wax or grafting compound to prevent moisture loss.
Cleft grafting is often used to join a smaller scion to a larger stock . It is usually done in the late winter. The branch or tree is cut straight across. With a knife or grafting tool, a split of the slit is made in the cut end of the limb or rootstock. Next a scion piece 3-5" long with one or two buds is cut and the end sharpened into a thin wedge.
Budding is similar to grafting except that the scion is reduced to a single bud with a small portion of bark or wood attached. The single bud scion is joined with the root stick to form a new plant. It is done in the spring or fall when the bark separates easily from the wood. It is faster, easier, and more economical than grafting. No wax is needed and cambium doesn't need to be aligned. Less scion is needed here.
The other types of grafting are crown grafting, side grafting, wedge grafting e.t.c.
d) Approach grafting : It is also known as inarching. It is used to join together plants that are otherwise difficult to join. The plants are grown close together and then joined so that each plant has roots below and growth above the point of union. Both scion and stock retain their respective parents that may or may not be removed after joining. The graft can be successfully accomplished any time of the year.
The distinguishing feature of approach grafting is that two independently growing, self-sustaining plants are grafted together. These self-sustaining characteristics of both plants which are to be grafted ensure the survival of both even if the grafting attempt is, for some reason, not successful. However, odds of being successful are greatly enhanced because of the active growing condition of both plants involved and absence of a time limitation required for the healing of the graft union to occur before the dependent scion does from the lack of sustenance.
e) Micropropagation: It is a process of plant propagation in which a plant tissue is excised and cultured in an artificially prepared nutrient medium under controlled laboratory conditions. The ability of every living plant cell to produce the entire plant is called totipotency. This is being exploited industrially to multiply plants which are difficult to propagate by conventional means.
1) It is easier, quicker, and less expensive method of propagation.
2) Characters of the parent plant are preserved. A superior variety can be produced by vegetative means.
3) It is the only method of reproduction in plants which do not reproduce viable seeds such as seedless grapes, rose, banana e.t.c.
4) It is easier to get rid of pathogens by vegetative propagation.
5) It is possible to raise a large number of selected strains by vegetative propagation.
6) By tissue culture, a large number of disease-free identical plants can be grown in a very short period.
1) The quality of the plants cannot be improved.
2) Offspring do not show genetic variations.
3) Unwanted characters can not be eliminated from plants.
4) Diseases in the parent plants are transmitted to the offsprings.
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The vital characteristics of living organisms by which new individuals are formed and so formed individuals are similar to their parents are called reproduction.