Meristem

Meristems are the growth centers of plants. Unlike animals, which form nearly all their organs as embryos, plants keep adding new organs throughout life, and every new cell comes from a meristem. Meristem cells are small and unspecialized and divide again and again. Some daughter cells stay behind to keep the meristem going, while others are pushed out, stop dividing and differentiate into the dermal, ground and vascular tissues.

Apical meristems at the tips of shoots and roots make a plant longer, which is called primary growth; the shoot apical meristem also produces leaves and, later, flowers. Lateral meristems, the vascular cambium and the cork cambium, form cylinders inside stems and roots and add girth, producing wood and bark. Grasses also have intercalary meristems near the base of their leaves, which is why a lawn regrows after mowing. At the summit of the shoot meristem sits a small pool of stem cells, kept at a steady size by a feedback loop between the WUSCHEL and CLAVATA genes.

A cutting can grow into a whole plant because its buds hold shoot meristems and new root meristems form at its base. Growers also culture tiny meristem tips to raise virus-free plants, and pinching out a shoot tip removes the source of auxin that suppresses side buds, making a plant bushier.