The growers now use a plant propagation technique called plant tissue culture. Many full-sized plants can be produced by starting with only a small piece of plant tissue – about the size of a grain of rice. The small piece of the plant is placed in a sterile environment on a medium containing the nutrients needed for growth. This callus can be divided over and over to produce more cell clumps growing on sterile growth media.
Eventually, the callus can be induced to form roots and shoots by changing the concentration of plant hormones in the growth media. Both the callus tissue and the plants that formed in a small-scale tissue culture experiment. The yellowish clump is callus tissue; the green leaves are the many, small African violet plants which eventually formed. They also found that the relative concentration of these hormones in the growth media determined what grew – more callus tissue and/or plant shoots and/or plant roots.
Ultimately entire plants (clones) were produced from small pieces of callus tissue by manipulating the hormone ratios in the media. Today, many horticultural plants and trees start their life as a clump of cells on tissue culture media. Original Link: https://www.yahoo.com/entertainment/over-garden-gate-plant-tissue-103602160.html