(primary wood) *Xylem
derived from procambium in the primary plant body. In nonwoody plants the primary xylem, consisting of the *protoxylem
, is the only water-conducting vascular tissue, whereas in mature plants exhibiting secondary growth this function is performed largely by the *secondary xylem
. As well as differing from secondary xylem in origin, the primary xylem usually also has longer tracheary elements, which are often arranged in a random fashion, although these differences are not always reliable. Primary xylem also consists of an axial system only and therefore does not contain rays, although rays may be present between the vascular bundles.