THE MAGIC of tissue culture is in finding the right nutrient medium for a plant organ, tissue or cell to grow. Scientists jealously guard the protocols developed by them. The nutrient mixture induces changes within the growing cell mass, known as a callus. A change in the mixture can induce or arrest the differentiation of cells into stems and roots.

Micropropagation, the basic tissue culture technique, can produce large numbers of genetically identical clones of a plant in a very short time. Shoot tips, axillary buds found at the base of leaf stems, groups of small flowers or inflorescences, leaf sections and root tips are the parts commonly used. After the tissue is cultured, it is induced to produce roots and is transferred to land. Acclimatisation of the cultured plant is crucial to its survival. Plants grown in test tubes face considerable difficulty in surviving in real nature.

Somatic embryogenesis is the fastest method of clonal multiplication. Certain somatic cells (non-reproductive cells) develop, for reasons not completely understood, into embryo-like structures that can be encapsulated in a gel to constitute artificial seeds.

Protoplast fusion or somatic hybridisation is a technique that uses protoplasts or cells from which the outer wall has been removed. Two protoplasts of genetically divergent species can be fused to produce hybrids. This would be impossible in nature. Scientists, for instance, have produced a cross between a potato and a tomato, called pomato. Genetic traits such as disease resistance can also be transmitted from one generation of a plant to another through this method.

Crosses between plants of different species seldom succeed. But scientists can sometimes remove the embryo of a cross before it aborts and then grow it in a tissue culture medium. This technique is known as embryo rescue.

Scientists have also discovered that when the anthers -- the male reproductive organs of a plant -- are cultured at a particular stage of development, embryos with half the number of chromosomes (haploids) are developed. Two haploid embryos can then be fused together to produce plants in which both sets of chromosomes are identical.