P lant tissues normally grow in an organized fashion in which specific cell types differentiate from nonspecialised meristematic cells. Plant developmental processes can be modified by culture in vitro in a suitable nutrient medium and with the application of plant growth regulators. The interactions of the main growth regulators can be complex, but at the simplest level auxins can cause cell enlargement and division, cytokinins cause cell division, gibberellins cause elongation, and abscisic acid inhibits growth ( 1 ). When meristems are cultured, it is possible to maintain organization and multiply such meristems, and this is the basis of micropropagation. If a suitable combination of growth regulators is chosen, however, cultured tissues can be grown in a disorganized and undifferentiated way to form callus. Single isolated wall-less cells, protoplasts, can also be released from tissues following enzymatic degradation of cell walls, and these can be cultured to form callus. F