Stem cells divide by mitosis to make new cells, and some of the new cells can then differentiate into specialised cell types. This is how an organism gets the range of cells it needs.
In an embryo, stem cells divide and differentiate to make all the different cell types, such as nerve, muscle and blood cells. This is needed for development, which is how a single fertilised egg becomes a whole organism.
In adult animals, stem cells are used for growth and for repair. For example, stem cells in bone marrow divide to replace worn-out blood cells, and stem cells in the skin replace cells that are damaged or lost.
In plants, meristems carry on dividing throughout life. Their cells differentiate into the tissues that make the plant grow taller and its roots grow longer, and they also help the plant repair damage.
