sentences:  The stele of plants undergoing secondary growth is surrounded by a vascular cambium, a ring of meristematic cells which produces more xylem on the inside and phloem on the outside.Since xylem cells comprise dead, lignified tissue, subsequent rings of xylem are added to those already present, forming wood.Fossils of plants from the early Devonian show that a simple form of wood first appeared at least 400 million years ago, at a time when all land plants were small and herbaceous.