The complex tissues are made of more than one type of cells.
Xylem and phloem constitute the complex tissues in plants.
Xylem functions as a conducting tissue for water and minerals from roots to the stem and leaves.
It is composed of four different kinds of elements, namely, tracheids, vessels, xylem fibres and xylem parenchyma.
Tracheids are elongated or tube like cells with thick and lignified walls and tapering ends.
Vessel is a long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity.
Xylem fibres have highly thickened walls and obliterated central lumens, which may either be septate or aseptate.
Xylem parenchyma cells are living and thin-walled, and their cell walls are made up of cellulose.
Fig. Xylem parenchyma
They store food materials in the form of starch or fat or tannins.
Primary xylem is of two types – protoxylem and metaxylem.
The first formed primary xylem elements are called protoxylem and the later formed primary xylem is called
In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ, this type of primary xylem is called endarch.
In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre, such arrangement of primary xylem is called exarch.
Phloem transports food materials, usually from leaves to other parts of the plant.
Phloem in angiosperms is composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibres.
Sieve tube elements are also long, tube-like structures, arranged longitudinally and are associated with the companion cells.
The companion cells are specialised parenchymatous cells, which are closely associated with sieve tube elements.
Phloem parenchyma is made up of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus, absent in most of the monocotyledons.
Phloem fibres (bast fibres) are made up of sclerenchymatous cells, generally absent in the primary phloem but are found in the secondary phloem, which are much elongated, unbranched and have pointed, needle like apices.
At maturity, phloem fibres lose their protoplasm and become dead.
The first formed primary phloem consists of narrow sieve tubes and is referred to as protophloem and the later formed phloem has bigger sieve tubes and is referred to as metaphloem.