A Simple Type of Wood in Two Early Devonian Plants

Wood (secondary xylem) derives froma lateral meristem called vascular cam-bium. It is considered essential for pro-duction and maintenance of trees but is equally important in hydraulic functioning of smaller growth forms. Wood includes longitudinally elongated cells (water-conducting elements and fibers) and transversally oriented cells (rays); it is characterized by radial rows of cells resulting from the periclinal divisions of cambium initials. A vascular cambium evolved independently in several Devonian and Carboniferous plant lin-eages [supporting online material (SOM) text]. Unlike the cambium initials of all other lineages, the cambium initials of the lignophytes (pro-gymnosperms and seed plants) frequently divide anticlinally (1), which results in circumferentia