7. Why are xylem and phloem called complex tissues?
Complex tissues are tissues made up of more than one types of cells. Since xylem and phloem are consists of different kinds of elements, they are called complex tissues. Xylem is consists of vessels, tracheid, xylem fibre and xylem parenchyma
1. Tracheids- These are elongated, thick-walled dead cells having pointed ends. These lack protoplasm
2. Vessels- These are long, tubular, and cylindrical structures that are formed from the vessel members, with each having lignified walls and large central cavities. Vessels being dead lack protoplasm.
3. Xylem fibres consist of thick walls with an almost insignificant lumen. These provide mechanical support to the plant.
4. Xylem parenchyma is made up of thin-walled parenchymatous cells that help in the storage of food materials and in the radial conduction of water
Phloem is made up of sieve tube, companion cells, phloem parenchyma and phloem fibres
1. Sieve tube- The 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.
2. Companion cells- These cells are parenchymatous in nature and they help in maintaining the pressure gradient in the sieve tube elements.
3. Phloem parenchyma- These cells help in the storage of food and they are made up of long tapering cells, with dense cytoplasm.
4. Phloem fibres- These are made up of elongated sclerenchymatous cells with thickened walls.