Xylem and phloem are called as complex tissues because they are formed of more than one type of cells. Xylem is composed of tracheids, tracheae, xylem parenchyma and xylem fibres. Tracheids and tracheae (vessels) are called as conducting elements.
Phloem is made-up of four types of cells :
• Sieve elements
• Companion cells
• Phloem parenchyma
• Phloem fibres
(i) Sieve elements : In lower vascular plants, single celled structures called sieve cells are present while the Angiosperms have the multicellular sieve tube members. They are long tubular channels. They are formed of elongated living cells without nucleus and arranged end to end in vertical rows. The end walls of the individual sieve tubes are perforated by number of pores. The end walls of sieve cells are known as sieve plates. These plates connect the adjacent sieve cells to form a continuous long distance channel for the transport of food materials.
(ii) Companion cells : Sieve tube members of the Angiosperms are accompanied by highly specialised parenchyma cells called as companion cells. They are in contact with cytoplasm of the sieve tube members by plasmodesmata in their thin walls.
Companion cells have nucleus, richly granular cytoplasm and vacuoles. They control the activities of sieve tube members.
(iii) Phloem parenchyma : The phloem parenchyma cells are living, thin walled and the most simple ones. They contain starch, tannins and crystals. These cells perform the function of storage and lateral translocation of food substances.
(iv) Phloem fibres : The phloem fibres or sclerenchyma cells are the components of phloem. The fibres may be septate or non-septate and may be dead or non-living at maturity. They provide mechanical support to the plant body.
