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.
Phloem is made up of four types of cells:
(i) Sieve elements
(ii) Companion cells
(iii) Phloem parenchyma
(iv) Phloem fibres
(i) Sieve elements: In lower vascular plants sieve cells are present while the Angiosperms have the sieve tube members. They are tubular channels. They are formed of elongated living cells without nucleus and arranged in vertical rows. The end walls of the individual sieve tubes are perforated by number of pits. 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 termed as companion cells. They are strongly attached to th sieve tube members by thin walls and perforation. Companion cells have nucleus, richly granular cytoplasm and vacuoles. They do not have starch.
(iii) Phloem parenchyma: Like the xylem parenchyma cells, the phloem parenchyma cells are the most simple and delicate 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 of sclerenchyma cells are the components of phloem. The fibres may be septate or nonseptate and may be dead or non-living at maturity. They support the plant body and sometime functions as storage tissue.
