Vascular plant tissues
Vascular tissues are made up of more than one type of cells. Vascular tissues include the xylem and phloem, both of which are conductive tissues.
Xylem is composed of four types of cells which are the tracheids, vessel elements, parenchyma and fibres. Xylem, as a whole, has two major functions, the conducting of water and mineral salt from root to leaves, and to provide mechanical strength (support) to the plant body.
Tracheids are elongated and lignified, like sclerenchyma fibres they have tapering end walls that overlap with adjacent tracheids. Thus they have mechanical strength and give support to the plant. Tracheids are dead cells with empty lumens when mature.
Vessels are cylindrical, tube-like structures; they are the characteristic conducting units of the xylem. They are very long and tubular structures formed by the fusion of several cells end to end in a row. Vessels are dead at functional maturity and are perforated.
Phloem resembles xylem in possessing tubular structures modified for translocation. The phloem consists of five cell types which are sieve tube cells, companion cells, parenchyma, fibres and sclereids.
Sieve tubes are made of elongated cells placed end to end to form a tube. The end walls have pores in them forming the sieve plates. Sieve tube cells are alive at functional maturity. The main function of the sieve tubes is to carry food, especially carbohydrates.
Companion cells are parenchyma cells adjoining the sieve tube cells. They assist the sieve tube cells in the conduction of food. They are living cells with elongated nucleus.