Xylem serves as a conduit for the movement of water, minerals and phytohormones through a plant. Xylem is a vascular tissue, though its form varies from plant to plant.
Three types of cells compose a plant's xylem. The most important cells, called tracheary elements, are hollow structures that serve as the passageway for water and other small molecules. Tracheary elements occur in two different forms, tracheids and vessel elements. Though both types have some differences, their essential function is similar. Both are dead before the plant rotates them into use.
The other two types of xylem cells are the parenchyma cells and fiber cells. The essential purpose of both types is to provide structure and support for the tracheary elements. Additionally, they prevent the plant's weight from crushing the tracheary elements, which would halt the flow of water.
Xylem is the most abundant biological material on the planet. It is present in the stalks of plants and flowers and makes up the woody part of trees. Scientists suspect that xylem evolved around the time that plants began colonizing terrestrial habitats. Unlike ancient plants that evolved in aquatic or marine ecosystems, surrounded by water, terrestrial plants had to evolve to transport water throughout the plant's various structures.