An extinct group of plants that flourished in the mid- to late Devonian (360–350 million years ago) and contained the ancestors of modern gymnosperms (conifers and cycads). They were shrubs and trees, some up to 12 m tall, with frondlike leaves. Progymnosperms evolved a vascular cambium that was bifacial, capable of producing not only xylem on its inner face (as in the more primitive lycophytes and sphenophytes) but also phloem on its outer face. Moreover, as the cambium was pushed outwards during radial growth of the stem, the cambial cells were able to divide radially and so function indefinitely. These features permitted the growth of wider trunks, with more efficient vascular systems and stronger wood. Also, the first true simple leaves appear among certain members of the group, such as Archaeopteris (see telome theory). However, reproduction was by spores, not seeds. See also euphyllophyte.