Xylem is the specialised tissue of vascular plants that transports water and nutrients from the plants soil to stems and leaves. It also provides mechanical support and storage. Xylem consists of elongated dead cells, arranged end to end to form continuous tubes. The cells have no organelles and cytoplasm, which creates more space inside the vessel for transporting water. The cell walls contain pits which allows water and mineral ions to move into and out of the vessel. The cell wall also contains a tough, woody substance called lignin, which strengthens the xylem vessel and provides structure and support to the plant. The pits in the cell wall have no lignin so water and ions can easily move into and out of the vessels. Phloem vessels transport dissolved substances, such as sucrose and amino acids from parts of the plant where they are made (sources) to the parts of the plant where they are used (sinks).
An example of a source are leaves as they produce glucose during photosynthesis and the products of photosynthesis such as glucose (sugar) are stored in roots therefore they are sinks. Phloem consists of living cells arranged end to end. Unlike xylem, phloem vessels contain cytoplasm, and this goes through the holes in the sieve plates from one cell to the next. Phloem vessels are made up of two types of cell sieve tube elements and companion cells. The sieve tube elements are living cells and are joined end to end to form sieve tubes. The ends of each cell consist of a sieve plate which contains lots of holes to allow solutes to move from one cell to the next. The sieve tube cells contain no organelles and very little cytoplasm to create more space for solutes to be transported. Due to phloem cells having no nucleus the cell cannot survive on their own therefore contain a companion cell which has a nucleus and high levels of mitochondria. Furthermore, the sieve tube element and the companion cell are connected through plasmodesmata (channels in the cell wall) which allows the two cells to communicate.
Where are fibres found in plant stems and what fibres make up the xylem, phloem and Sclerenchyma?
The xylem fibres are non-living sclerenchyma cells as they lose their protoplast at maturity. Within plant stems xylem fibres are found on the inside of the vascular bundle. Having the stronger xylem vessels and fibres in the centre provides strength to the stem and acts like an internal ‘scaffolding’ to support the stem and prevent it from bending in the wind. Phloem tissues are in the middle of the vascular bundle and sclerenchyma fibres are found on the outside of the vascular bundle. Sclerenchyma fibres can be composed of two types of fibre sclereids and fibers Sclereids are sclerenchyma cells that are different from fibers in a way that they vary in shape. Fibers are elongated cells. Sclereids are usually roughly spherical.
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