The ingredients of plant tissue culture media include plant hormones, inorganic nutrients, organic nutrients and vitamins. Plant hormones are added to regulate growth. In tissue culture, they are mainly used to stimulate adventitious regeneration of roots, shoots and embryos, outgrowth of axillary buds, and formation of callus. Moreover, cytokinin and auxin are often required to achieve quantitative growth (increase of cell number and volume). In tissue culture, usually only cytokinin and auxin are added. Plant hormones are typically added within the range 0.1–10 μM (0.02–2 mg.l-1). A major part of the research efforts in plant tissue culture concern modification of the concentrations and types of plant hormones. The doseresponse curves of plant hormones are generally bell-shaped. At a too low concentration there is no effect, and at a too high concentration the added hormone is inhibitory. The promotive effect only occurs at intermediate concentrations. To detect these concentrations, usually first a broad range is taken (0, 0.1, 1, 10, 100 μM), and after that a narrow one. It should be remembered that hormones act in a logarithmic way.