Meristems are areas of plant tissue that are rich with undifferentiated cells that can undergo mitosis. These regions create the active growing areas of the plant. Because plant cells are enclosed in cell walls, they are rather rigid at maturity. This means that once a cell differentiates into its final form, its location, size, and shape are fixed. As a result, the only way a plant can grow is to make additional cells.
The tips of the shoots and roots are the places where plant growth occurs. Root tips grow outward into the soil to access more water and nutrients. Shoot tips grow to increase the plant's height and its access to sunshine. Since this growth must come from the production of new cells, we find meristems undergoing mitosis in these locations.