Structure and Function of Xylem and Phloem

There are two types of conducting tissues in plants, namely xylem and phloem. These tissues constitute the vascular tissues. Xylem conducts mainly water and minerals from the roots up to other parts of the plants. Phloem conducts organic food from the leaves both up and down the plant.

Components of xylem and their functions :

Xylem is a complex and permanent tissue that consists of four cell types; the tracheids, vessel elements, xylem fibres and xylem parenchyma. Tracheids are dead, elongated cells with tapering ends and lignified walls. They have mechanical strength and give support and also involve in conduction of water and minerals. These cells are universally present in xylem tissue of all vascular plants, which are therefore, called tracheophytes. Xylem vessels are dead, very long, highly thick walled tubular structures formed by the fusion of several vessel cells (vessel elements) end to end in a row. Vessel elements are shorter than tracheids but they form pipeline in plant body as they are placed one above the other. Xylem vessels are only found in angiosperms where they are involved in conduction of water and minerals and also provide mechanical support to the body. Xylem fibres are also dead but narrow, highly elongated, highly thick walled cells with tapering ends. They are mainly responsible for mechanical support to the plant body. The only living cells of xylem are xylem parenchyma, which are thin walled, broad cells and are in the storage of water and minerals temporarily during conduction.

Vessels and Tracheids

Components of Phloem and their Function :

Phloem tissues are also complex permanent tissues that are composed of three living cells (sieve tube elements, companion cells and the phloem parenchyma) and one dead cell (phloem fibre). Sieve tubes are the long tube like structures, which translocate solutions of organic solutes (sucrose) throughout the plant body. These are formed by the end-to-end fusion of cells called sieve tube elements. Sieve tube elements are thin walled living cells but do not have nuclei, actually, their nuclei are lost as they mature. These cells are narrow at the centre and wide at the ends. A transverse or oblique pore bearing partition, the sieve plate, separates two successive sieve tube elements. Due to the absence of nuclei, the functions of sieve tube elements are controlled by adjacent companion cells. The sieve elements and companion cells, together form a functional unit that act as conducting channels and are involved in transport of organic solutes. Companion cells provide energy to sieve tube elements for their activities. Like xylem tissue, phloem also possesses parenchyma and fibre cells. Phloem parenchymas are storage cells and phloem fibres are very much thick walled supporting cells.

Diagram of  Phloem

 

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