Q.7:- Why are xylem and phloem called complex tissues?
Xylem and phloem are known as complex tissues as they are made up of more than one type of cells. These cells work in a coordinated manner, as a unit, to perform the various functions of the xylem and phloem.
Phloem transports food materials and composed of Sieve tube elements, Companion cells, Phloem parenchyma and Phloem fibres.
• Sieve tube elements are tube-like elongated structures associated with companion cells. The end walls of sieve tube elements are perforated to form the sieve plate. Sieve tube elements are living cells containing cytoplasm and nucleus.
• Companion cells are parenchymatous in nature. They help in maintaining the pressure gradient in the sieve tube elements.
• Phloem parenchyma helps in the storage of food and is made up of long tapering cells, with a dense cytoplasm.
• Phloem fibres are made up of elongated sclerenchymatous cells with thick cell walls.