Xylem tissue is a feature of higher vascular plants (Filicinophyta,
Coniferophyta, and Angiospermaphyta). It developed as plants became less
dependent on water and lived in drier environments. It is the tissue
which conducts water (and dissolved nutrients) from the soil to the leaves.
Before this, more primitive plants like the mosses and liverworts had to
(and still do) rely heavily on surface water to be absorbed through the
First xylem starts as living cells which over time elongate and lose their cytoplasm and then die to become a hollow fluid filled tube.
Xylem vessels can exist in several forms as shown on the diagramon this diagram: The thicker parts represent the thicker accumulations of lignin. Microscopic pictures of xilem vessels show sometimes lignin as a spring shape, or rings, while sometimes the desposits are pitted or reticulated.
The Plant Anatomy Gallery has a selection of micrographs. Some of them show sections of xylem.