Isoetes, commonly known as the quillworts, is a genus of plants in the class Isoetopsida and order Isoetales. They are lycopods. Generally in pteridophytes, when the young organs mature, no further growth in diameter takes place. In several extinct groups a special ring of cells, the cambium, produced additional xylem to the inside and phloem cells to the outside (secondary growth as opposed to primary growth achieved by apical activity of the stem and root), resulting in increased diameter and a truly woody plant. This is common in many seed plants today, but in the extinct pteridophytes only two genera (Botrychium and Isoetes) show a slight vestige of secondary growth.