The vascular tissue system includes all the tissues of the plant that function to transport materials long distances. There are two tissue types. Both are complex tissues with more than one cell type. Xylem transports water up the plant. Phloem transports photosynthate around the plant.
Xylem always includes tracheary elements. These are dead at maturity, and always have secondary walls. Tracheary elements have pits. If a tracheary element only has pits, then it is a tracheid. If a tracheary element also has perforations, then it is considered to be a vessel element. Conifers only have tracheids. Flowering plants can have both.
Phloem always includes sieve elements. Sieve elements are alive at maturity, but are simplified. Their nuclei, vacuoles and other cell stuctures degenerate. All sieve elements have pores connecting adjacent sieve elements. If the pores are restricted to specific walls shared by vertically arranged sieve elements, then the sieve element is a sieve-tube element. Sieve-tube elements always associated with companion cells. Sieve-tube elements are only found in flowering plants. The sieve elements found in conifers are sieve cells which are associated with albuninous cells. Other cell tyes found in phloem include parenchyma and fibers.