Tissue culture is a method of propagation using little bits of plant tissue. These bits are seperated from the plant and placed under sterile conditions in an artificial medium. Given the right conditions the cells will continue to grow and divide. Cells in culture don't grow old, but can be kept in this condition indefinately, thus saving valuable space for the breeder. There are two types of tissue culture:
1. Meristem culture. The meristem is the where the plant is actively growing, and the cells are dividing. The meristem is usually free of viruses and other pathogens, so meristem culture is used to obtain a disease free clone.
2. Callus culture. The plant tissue is induced to form undifferentiated growth or a callus, then induced to differentiate into the various organs by finding the proper balance of hormones. In meristem culture organs are already differentiated, so epidermal cells give rise to more epidermal cells. In callus culture any cell could become an epidermal cell or any other organ. Auxin is used to promote roots, and cytokinin promotes shoots.