What are complex tissues? Describe various types of complex tissues.

The complex tissues are made of more than one type of cells and these work together as a unit.
The complex tissues in plants are of two types
I) Xylem II) Phloem
I) Xylem :
i) The complex tissue which helps in conduction of water and minerals from roots to stems and leaves is called xylem.
ii) It also provides mechanical strength to the plant parts.
iii) It is composed of four different kinds of elements, namely.
a) Tracheids b) Vessels c) Xylem fibres d) Xylem parenchyma.
a) Tracheids:
i) Tracheids are elongated or tube like cells with lignified walls and tapering ends.
ii) These are dead cells and are without protoplasm.
iii) In flowering plants, tracheids and vessels are the main water conducting elements.

b) Vessel:
i) Vessel is a long cylindrical tube-like structure made up of many cells called vessel members.
ii) Each with lignified walls and a large central lumen.
iii) The vessels are also devoid of protoplasm.
iv) Vessel members are interconnected through perforations in their common walls.
v) The presence of vessels is a characteristic feature of angiosperms.

c) Xylem fibres:
i) Xylem fibres have highly thickened lignified walls and obliterated central lumens.
ii) These may either be septate or aseptate.
iii) They provide mechanical support
d) Xylem parenchyma:
i) It is the living component of xylem, cells are thin-walled and stores the food materials in the
form of starch or fat, and other substances like tannins.
ii) The radial conduction of water takes place by the ray parenchymatous cells.
II) Phloem :
i) Phloem transports food materials, usually from leaves to other parts of the plant.
ii) Phloem in angiosperms is composed of
a) Sieve tube elements b) Companion cells c) Phloem parenchyma d) Phloem fibres.
a) Sieve tube elements :
i) Sieve tube elements are also long, tube like structures, arranged longitudinally and are
associated with the companion cells.
ii) Their end walls are perforated in a sieve-like plates.
iii) A mature sieve element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus.
iv) The functions of sieve tubes are controlled by the nucleus of companion cells.
b) Companion cells:
i) Companion cells are specialised parenchymatous cells, which are closely associated with sieve tube elements.
ii) The sieve tube elements and companion cells are connected by pit fields present between their common longitudinal walls.
iii) The companion cells helps in maintaining the pressure gradient in the sieve tubes.
c) Phloem parenchyma:
i) Phloem parenchyma is made up elongated, tapering cylindrical cells which have dense cytoplasm and nucleus.
ii) The cell wall is composed of cellulose and has pits through, which plasmodesmatal connections exist between the cells.
iii) The phloem parenchyma stores food material and other substances like resins, latex and mucilage.
iv) Phloem parenchyma is absent in most of the monocotyledons.
d) Phloem fibres :
i) Phloem fibres are made up of sclerenchymatous cells.
ii) These are generally absent in the primary phloem but are found in the secondary phloem.
iii) These are much elongated, unbranched and have pointed, needle like apices.
iv) The cell wall of phloem fibres is quite thick.
v) At maturity, these fibres lose their protoplasm and become dead.
vi) Fibres obtained from secondary phloem of jute, flax and hemp are used commercially.