Plants keep pockets of stem cells working at their tips for life
A meristem is plant tissue made of stem cells that never stop dividing and can become any cell type. Every leaf, flower, root and ring of wood traces back to one. The Swiss botanist Carl Wilhelm von Nägeli named them in 1858, from the Greek word meaning to divide.
Meristematic cells are small, with thin walls, dense protoplasm that fills the whole cell, little or no vacuole and no gaps between neighbours. Instead of working chloroplasts they hold proplastids that mature later. When they divide, some daughters stay as stem cells while others specialise and usually lose the power to divide. Nägeli introduced the term in his 1858 Contributions to Scientific Botany.
There are three main kinds. Apical meristems at the tips of shoots and roots drive growth in length; intercalary meristems in the middle of stems and leaves allow regrowth; and lateral meristems, or cambium, thicken woody plants. The tips produce three primary tissues: protoderm, which becomes the epidermis; procambium, which forms the first xylem and phloem and later the cambium layers; and ground meristem, which makes the cortex and pith. Later, the vascular cambium adds the secondary xylem and phloem that make wood, and the cork cambium replaces the epidermis with bark and cork; herbaceous plants skip this secondary growth. In some arctic plants, apical meristems sit low or midway up the stem, probably as an advantage in harsh conditions.
At the shoot tip, a slowly dividing central zone keeps a reserve of stem cells that feed a busier peripheral zone, where leaf and flower primordia start one at a time at a steady interval called a plastochron. In Arabidopsis thaliana, three CLAVATA genes limit the size of this reserve, and the WUSCHEL gene, active just beneath the stem cells, stops them differentiating; the two form a feedback loop tied to the hormone cytokinin.
The root tip works differently, surrounding a slow-dividing quiescent centre with two pools of stem cells, all shielded by a root cap that constantly sheds its outer cells.
Source: Meristem
