Modern plant tissue culture is performed under sterile conditions with filtered air. Plant materials from the environment are naturally contaminated with microorganisms, so they must be sterilized in chemical solutions like sodium hypochlorite or mercuric chloride. Explants are then placed on solid or liquid culture media composed of inorganic salts, organic nutrients, vitamins, and plant hormones to induce cell growth and differentiation. Solid media contain a gelling agent like purified agar.