In plant tissue culture, plant cells, tissues, or segments are grown for generating large numbers of new cells, tissues, or plants. Plant tissue culture is also used to study development of plant cells, and it provides mechanisms of genetic engineering and produces valuable chemicals found in plant cells. Much smaller pieces of tissue are used by plant tissue culture in the cloning process. Moreover, tissue culture procedures are required for the management of genetically modified cells used to grow transgenic whole plants. In plant cells and tissues grown in vitro, some vitamins may behave as limiting factors for cell growth. Vitamins frequently used in cell and tissue culture media are thiamine, nicotinic acid, pyridoxine, and myo-inositol. The most common gelling agent used for preparing semisolid and solid tissue culture media is agar. In plant tissue culture, there are four important classes of growth regulators: auxin, cytokinin, gibberellin, and abscisic acid.