Meristem

Meristem is a plant tissue consisting of actively dividing, incompletely differentiated cells that can give rise to various specialized tissues. These cells are typically small with dense cytoplasm, large nuclei, and relatively thin walls; they retain high metabolic activity and a high capacity for division. Meristems are located at positions of growth, notably the tips of roots and shoots, and along the stems and roots where growth occurs.

Apical meristems at the tips of shoots and roots drive primary growth, producing protoderm, ground meristem, and procambium, which differentiate into the epidermis, cortex and pith, and vascular tissues, respectively. Lateral meristems, including the vascular cambium and cork cambium, produce secondary growth, adding girth by generating secondary xylem, phloem, and periderm. Intercalary meristems occur in some monocots, especially grasses, enabling rapid regrowth of elongating tissues at nodes or leaf bases.

Meristem activity is regulated by plant hormones and gene networks. Auxin and cytokinin balance affects cell proliferation and organ initiation. In shoot apical meristem, a stem cell population is maintained by a regulatory loop involving WUSCHEL and CLAVATA genes; KNOX genes help maintain meristem identity in shoots. Environmental cues can modify meristem activity, influencing branching and organ formation.

Meristems are central to plant growth, regeneration, and adaptation, and are a focus of plant biology and tissue culture.