A stem cell is an undifferentiated cell of an organism. It can divide to give rise to many more cells of the same type, and certain other cells can arise from it by differentiation.
Stem cells have different functions in different places. Stem cells from human embryos can be cloned and made to differentiate into most types of human cell. Stem cells from adult bone marrow can form many types of cell, including blood cells. In plants, meristem tissue contains stem cells that can differentiate into any type of plant cell, throughout the life of the plant.
Treatment with stem cells may be able to help conditions such as diabetes and paralysis. In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from this embryo are not rejected by the patient's body, so they may be used for medical treatment. There are risks, such as the transfer of viral infection, and some people have ethical or religious objections.
Stem cells from meristems can be used to produce clones of plants quickly and cheaply. Rare species can be cloned to protect them from extinction, and crop plants with special features, such as disease resistance, can be cloned to give farmers large numbers of identical plants.