chiman

You can refer to this article to see how it works:
http://www.suite101.com/article.cfm/18505/105386

Other articles available:
http://www.suite101.com/topic_page.cfm/18505/4966

Just like chaman was saying, auxins will cause roots to develop, while cytokinins will cause shoots to develop. If you adjust the ratio between the two just right, you can turn plant material into a strange-looking mass of undifferentiated plant cells, called a callus culture. This callus culture can survive and grow without light, if you grow it on lab media containing sugar and other nutrients. After it grows so much, you can divide this mass into little clumps. You then expose each little clump of cells to hormones, causing them to develop a shoot and then roots... and thus turn each clump into a small plant. The neat thing is that all of these plants will be genetically identical to the plant material you started with. This technique is called micropropagation.