Xylem Tissue
Xylem is the primary water conducting tissue in plants. It consists of three intergrading cell types: tracheary elements, fibres, and parenchyma. Tracheary elements and fibres are dead at maturity. The xylem comprises almost all of the woody tissues of trees, and in addition to its function as transport tissue, it supports the plant. There are two types of tracheary elements: vessels and tracheids. These elements are the primary conductive cells of water and solutes in the xylem. Vessels are found in the angiosperms (and some gymnosperms)occuring end to end to create conductive “tubes”. Woody species have vessels of many sizes. Xylem of angiosperms may also have tracheids. It is actually difficult to say for certain whether you are looking at a tracheid or a vessel if you do not see an endwall. Vessels are usually shorter and wider than tracheids, but the determining feature is the presence of a perforation plate in the vessel. A perforation is a hole in the wall (most commonly the endwall). Do not confuse this term with a pit. Pits are not holes but rather an area of thinness in the wall which does not have secondary cell wall thickening. Primary cell wall and the middle lamella is still present. The primary wall is permeable so conduction can carry on. Pits are common features of the walls of both vessels and tracheids. Tracheids do not have perforations, but have many pits. Tracheids have very tapered ends. It can be difficult sometimes to distinguish between tracheids and fibres. In fact, they do intergrade and are then called fibre-tracheids.
There is evidence that tracheids were the first vascular cells to evolve, and they occur in almost all vascular plants. Vessels are found almost exclusively in angiosperms. In angiosperms, tracheids tend to be formed initially, and as the plant matures, vessels are formed.