Question

Why are xylem and phloem called complex tissues?

Answer 1

Depending on whether all cells in a given tissue are similar in structure and function, the two types of permanent tissues in plants are – simple permanent tissues and complex permanent tissues. Complex permanent tissues have different types of cells within a single tissue and they have different functions.

Complete step by step answer:
Complex permanent tissue in plants is made up of a variety of cells and they together function as a single unit. However, the overall structure and function of all the cells differ from one another. Xylem and Phloem are two main types of complex permanent tissue found in plants.
Xylem: Xylem is a vascular tissue that is primarily involved in the transport of water and nutrients from roots to stem. Tracheids, vessels, Xylem fibre and Xylem parenchyma are four different types of cells which together form Xylem tissue. Xylem tracheids are tube-like cells which have tapering ends and with a thick wall which has a lining of lignin. They lack protoplasm and are dead. Xylem vessels are long cylindrical cells with lignified walls and have a large central hollow space called a central cavity. They are also dead and lack protoplasm. They have many cells called vessel members which are interconnected through a perforation in common walls. Xylem fibres are also dead cells and are majorly involved in mechanical support. The only living cells of xylem are Xylem parenchyma and they store starch and fat.
Phloem : Similar to xylem, phloem is also a vascular conductive tissue and is mainly involved in the conduction of soluble organic compounds (which are synthesized during photosynthesis) from leaves to the rest of the parts of the plant. Companion cells, sieve tubes, Phloem fibres and phloem parenchyma are the four types of cells which together constitute Phloem tissue. Sieve tubes are elongated living cells which are mainly involved in the conduction of carbohydrate from leaves to all the other parts of the plant. Companion cells are mainly present to keep a check on the conduction through sieve tube cells and to some extent also participate in loading and unloading of the sugar into the sieve element. Phloem parenchyma is found in the finest branches and in the ends of sieve tubes and helps in the transport of food to remote areas of the plant body. Phloem fibres die on maturation and they are mostly used for commercial purposes. For example – Jute, flax and hemp.

Note: The arrangement of these complex permanent tissues in plants changes with the type of plant. For example- Xylem vessels are present only in angiosperms and are not seen in gymnosperms. If we cut out sections of Monocot and dicot root and stem, the difference in the arrangement of these tissues can be seen.