There is a primary and secondary xylem. Xylem is
made up of tracheids (long, dead cells that function as
conductors), vessel elements (individual short, wide
cells that make up the vessel, also responsible for water
transport), fibers, and parenchyma cells (act as storage
cells and help in assimilation or wound healing). Both
tracheids and vessels have stiff lignin-containing sec-
ondary cell walls that give them rigidity.
Primary xylem comes from the plants procambium
and contains protoxylem (annular and spiral vessels)
and metaxylem (scalariform, reticulate, and pitted ves-
sels). The secondary xylem comes from the vascular
cambium in plants exhibiting secondary growth, such
as wood.
See alsoPHLOEM.352 xylem
annular spiral scalariformreticulate pittedA tissue found in a plant’s vascular bundle (above the phloem)
and in the center of the plant where secondary growth occurs.