derived from the vascular cambium in plants exhibiting secondary growth. Secondary xylem is regarded as consisting of two systems: the axial (vertical) system, derived from the fusiform initials of the vascular cambium and consisting mainly of tracheary elements and fibres; and the ray (horizontal) system, derived from the ray initials of the vascular cambium and consisting mostly of ray parenchyma forming the *rays
. As well as differing from *primary xylem
in origin, secondary xylem usually also has shorter tracheary elements, often arranged in regular rows, although these differences are not always reliable. Commercial wood is secondary xylem. There are two main types of commercial wood, softwood from gymnosperms and hardwood from angiosperms. However, these terms are somewhat misleading as there are hard gymnosperm woods and soft angiosperm woods. Softwoods contain only tracheids as the water-conducting elements, whereas hardwoods usually contain vessels.