The growing of new plants from cut portions of the vegetative body of an older plant has been carried out by Man for several years.
Because the propagation is influenced by man, it’s called artificial vegetative propagation, compared to vegetative propagation which occurs naturally without man’s action.
The ability to develop a completely new plant from a portion of an old one has been of great agricultural and horticultural importance.
Artificial vegetative propagation can be carried out with cuttings and by marcotting, layering, and grafting.
Methods of Artificial vegetative propagation
There are different methods to carry out artificial vegetative propagation, depending on the nature of the plant.
A large number of plants in the tropic can be artificially propagated
Examples of artificial vegetative propagation by cutting are hibiscus, Cassava, Rose, Croton, and Coleus
These plants can grow vegetatively from the aerial parts of an older plant, especially during the rainy season.
Cut sections of a stem are called stem cuttings.
It is possible to grow plants from stem cuttings because, in the phloem cells of stems, there is a store of food that is sufficient to provide for the growth of roots and new leafy shoots.
Cut sections of roots are known as root cuttings. In certain cases, root cuttings may be used for vegetative growth.
These root cuttings produce adventitious buds which develop new roots as well as leafy shoots.
Stem cuttings are usually made from twigs that have become woody and, therefore, have a store of food.
Each cutting must have at least two nodes close to the end that are inserted into the ground.
This is because roots grow faster from the nodes. Removal of most of the leaves from a cutting is essential since leaves lose water all the time and cutting without roots can not absorb much water from the soil.
The cutting is placed in moist, sandy soil so that when the roots grow they may have a suitable environment. Cuttings placed in suitable soil usually take root easily.
During the early stages, the cutting must be sheltered from the heat of the sun to prevent it from dying before the roots develop.
Some plants are propagated most successfully from cuttings that are quite green.
They should be taken from the growing tips of twigs, as in the Oleander and Coleus.
green cuttings are very vulnerable to water loss and, in some cases, it is best to place them in the water where they will produce roots.
In such cases, the base of the stem can absorb enough water to prevent the leaves from withering before new roots are produced.
When roots are formed, the cuttings should be transferred to the soil but the new roots are very fragile and must be handled with great care.
Root formation in cuttings can be accelerated utilizing growth hormones that are sold commercially.
However, these hormones are of little or no use in the case of cuttings that are very slow to produce roots, or which normally do not do so.
In some plants, cuttings produce roots so rarely that propagation by cuttings is not practical, hence, another method of propagation called marcotting is used.
This method, though a little laborious, has been proved to be extremely successful in the propagation of various garden shrubs and fruit trees like the lemon, pomelo, and mango.
Using a sharp knife, a complete ring is cut through the bark of a healthy branch up to the region of the cambium.
Another ring is made a short distance away (about 5cm) and the bark between the two rings is removed.
At this stage, water and mineral salts can flow freely up the branch to the leaves because the xylem tissue is intact.
However, food can not pass downwards beyond the band of exposed tissue because the phloem has been removed. This will cause an accumulation of food above the band.
Some earth rich in organic manure is applied all around the band and held in place by coconut husk.
The husk is tied firmly with a string and the arrangement is kept moist.
Eventually, roots will grow from the band. When the roots are strong enough, the branch can be cut off and planted, so giving rise to a new plant.
This method is usually adopted for plants that usually do not produce flowers for sexual reproduction, or which may flower only at lengthy intervals.
Plants that can not be easily grown by cuttings, for example, Bougainvillea, are also propagated in this way.
The lemon rose and jasmine is commonly propagated by layering.
Layering is a slow process but is a definite way of stimulating the growth of vegetative parts.
A healthy lower branch is bent to the ground without breaking it.
The free end of the branch is then tied to strong support.
The branch is slit at one of its nodes close to the soil. Alternatively, a ring of bark about 5cm long is removed.
The portion so treated is pushed into the soil and kept below the soil level by means of two pegs.
It is then covered with moist rich soil and a heavy object, for example, a brick, is placed on it.
Soon roots grow out of the slit node ( or the exposed band of tissue0 and new aerial shoots arise above the soil from this and neighboring nodes.
Once the stem has taken root firmly, a portion near the pegs is cut off.
This portion may give rise to one or more new plants.
Grafting is considered to be a very skilled and refined method of vegetative propagation in plants.
The object of grafting is to cause a variety of a plant having some good qualities (the scion of graft) to grow on a well-established root system ( that of the stock).
A small portion or branch of the scion is inserted into a rooted plant (the stock) of the same or allied species so that the two may grow as one plant.
The scion grows and retains all its qualities, supports it, and supplies it with water and mineral salts.
Grafting is carried out for the propagation of certain fruit trees and ornamental shrubs and trees.
Two common types of grafting are bud grafting and stem grafting.
Bud grafting is commonly called using. The essential feature of budding is the fusion of the cambium of the bud from the scion with the cambium of the stock.
The process of budding is as follows. Seedlings of trees that are to be used as stocks are grown in a nursery until their stems are of a suitable thickness.
A T-shaped cut is made in the stem of stock, down to the cambium layer.
The small flap of bark on each side of the cut is then separated by pulling the free tip of the triangle piece of bark, which is now cut from a branch of a selected mother tree (The scion). the slice of bark must reach down to the cambium layer.
The bark is trimmed to fit into the T-shaped cut and placed carefully in position.
The flaps on each side of the cut are straightened.
The insertion is then held in place by a cord. The bud is sheltered from the heat of the sun and also from the rain.
For the process to be successful, the cambium of the bud must fuse with that of the stock and form part of it.
Consequently, the bud will begin to grow. Food materials, mineral salts, and water pass from the stock to the growing bud.
The latter slowly develops into a new branch that spreads out and bears flowers and fruits.
The essence of the process here again is the fusion of the cambium of the stock with that of the scion. All buds are removed from the stock but not from the scion.
The size of the scion is usually similar to that of a stock.
The ends which have to be united can be cut in various ways to ensure a firm fusion.
They may be cut obliquely or in a V-shaped pattern so that they fit well with each other.
They are held together by moist cotton wool or moist cloth and bound over by a thick cord.
The grafted portion may be smeared with grafting wax to prevent fungal and bacterial infection.