Meristem or building tissue refers to a type of plant tissue that consists of undifferentiated cells and can be involved in cell division growth. Primary meristems exist both in the aboveground part of the plants at the extreme end of the shoot tips (sprout apical meristems) and at the extreme end of the lateral shoots (subapical meristems) as well as in the subterranean part at the root tips (root apical meristems). The cells of the meristems usually have thin cell walls with little cellulose. In contrast to differentiated cell types, they theoretically have the unlimited ability to divide. Meristems are often involved in plant growth. Sprout and root meristems constantly give freshly divided cells to the plant body, which then form into different organs such as shoot, root and leaf.