Xylem, a term from from a Greek root with a German termination, is used in vegetable histology (q.v.) for wood (q.v.). Xylem generally originates on the inner side of a prochambial strand or rudimentary stele (q.v.), with the differentiation of one or a few tracheids or tracheae, constituting the protoxylem. These are either spirally or annularly thickened. Other tracheids with pits are then differentiated from the procambium, making, with the protoxylem, the primary xylem. This may often contain also wood-parenchyma and fibres. The subsequent activity of the cambium-ring in exogenous stems gives rise to the annual rings of secondary xylem, most of the elements of which are lignified, whilst no spiral or annular vessels are present in it.