Xylem and Phloem

• Xylem tissue transports water and mineral ions in solution. These substances move up the plant from the roots to the leaves.
• Phloem tissue mainly transports sugars in solution, both up and down the plant.
• Xylem and Phloem make up a plant's vascular system. They are found throughout a plant and transport materials to all parts. Where they're found in each part is connected to the xylem's other function - support.
• In a root, the xylem is the centre surrounded by phloem to provide support for the root as it pushes through the soil.
• In the stems, the xylem and phloem are near the outside to provide a sort of scaffolding that reduces bending.
• In a leaf, xylem and phloem make up a network of veins which support the thin leaves.
• Xylen is a tissue made frm several different cell types.
• Xylem vessels are very long, tube-like structures formed from cells joined end to end.
• Xylem has no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily.
• Xylem cells are dead, so they contain no cytoplasm.
• Xylem walls are thickened with a woody substance called lignin, which helps to support the xylem vessels and stops them from collapsing inwards. The amount of Lignin increases as the cell gets older.
• In the xylem water and ions move into and out of vessels through small pits in the walls where there is no lignin.
• Phloem tissue transports solutes, mainly sugars like sucrose, round plants.
• Like Xylem, Phloem is formed from cells arranged in tubes. But unlike Xylem, it is purely a transport tissue.
• Phloem tissue contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells.
• Sieve tube cells and companion cells are the most important cells types in phloem for transport.
• Sieve tube elements are living cells that form the tube for transporting solutes through the plant. They are joined end to end to form seive tubes
• The 'sieve' parts are the end walls in the phloem, which have lots of holes in them to allow solutes to pass through.
• Sieve tube elements have no nucleus, a very thin layer of cytoplasm and few organelles . The cytoplasm of adjacent cells is connected through the holes in the sieve plates.
• The lack of a nucleus and other organelles in sieve tube elements means that they can't survive on their own. So there's a companion cell for every sieve tube element.
• Companion cells carry out the living functions for both themselves and thier sieve cells. For example, they provide the energy for the active transport of solutes.