Cloning is plant reproduction without the involvement of spores or seeds. Plants clone themselves naturally through a process called vegetative propagation. When gardeners graft buds from one plant to another or grow cuttings, they are cloning them. Horticulturalists can clone plants by growing tissue cultures to produce specific qualities like taste of fruit, color of flowers or the ability of a plant to resist damage by disease or insects.
All plants have evolved some manner of reproducing themselves so they can pass along their genes. Most plants reproduce using spores or seeds. When a plant sends a lateral runner (called stolon) on top of the ground or a rhizome just beneath the surface, and a new plant pops up, it is a clone of the original--it has the same genes. This is called vegetative reproduction, and it is how many invasive species spread.
An Agricultural Leap
Nobody knows when humankind discovered that if you whack a branch or shoot from many kinds of plants and poke them into the ground that they will grow roots and become a new plant. A plant that is grown from a cutting is a clone of the original plant. It is made possible because the tissues of plant contain hormones that regulate growth. Gardeners now routinely grow clones from cuttings or slips from shrubs or trees. The shrub or tree that grows will be a clone of the original.
Evolution of Grafting
Experienced gardeners routinely remove shoots or buds from one plant and graft then onto other plants. As a grafted bud grows, it will have the same genes as the original. The rootstock is the lower portion of the plant that receives the graft; a shoot or bud grafted onto the rootstock is called the scion.
The Chinese are known to have grafted plants by 1560 BC; Aristotle (384 to 322 BC) and Theophastrus (371 to 287 BC) wrote about grafting techniques and results.
Through grafting it is possible to grow several varieties of apples on one tree. Each apple will contain genes identical to those of its parent tree. The chances of grafting two plants within a species are excellent and it is often possible to graft two species within a plant genus. It is rare that plants more distantly related can be grafted. Cacti are among the easiest plants to graft.
Tissue Culture Cloning
A plant physiologist F.C. Steward cloned a plant from the cell of a carrot at Cornell University in 1958. By 1986 scientists were able to manipulate the gene structure of plants. As it has developed, simple cloning of plant tissue culture is not complicated. A sample of tissue, called explant, is washed with soapy water, sterilized in a bleach solution and rinsed in distilled water. The explant is placed in a jelled growing medium. Commercial growing mediums ordinarily contain micronutrients, amino and nucleic acid bases and often phytohormones, plant growth hormones. These usually include auxins that promote the development of roots. The explant is allowed to grow in an environment with the temperature and light necessary for that kind of plant.
Advantages and Disadvantages
Cloning enables horticulturists to develop cultivars of fruits and vegetables that are tastier, look more attractive, store longer and ship better. Horticulturists use cloning to select and reproduce plants that resist certain diseases and insects. This allows desirable plants to remain essentially the same generation after generation. Cloning is often faster and easier than growing plants from seeds. Some trees don't yield fruits or nuts for 10 years or more and cloning gets around this long waiting period.
The major disadvantage is that genetic uniformity can result in an entire population of plants to get wiped out by one disease.