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Describe the structure of the xylem and the phloem.

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The Xylem and Phloem Describe the structure of the xylem and the phloem. Xylem is a vascular tissue that functions in the transport of water and dissolved substances upward in the plant body. It forms a continuous pathway running through the roots, the stem and to the leaves. In the flowering plants, the xylem usually includes two unique types of cells: tracheids and vessel elements. It also includes numerous parenchyma and sclerenchyma cells, especially fibres. The parenchyma cells are the only living cells in the mature, functioning xylem. The cytoplasm and the nuclei of tracheids, vessel elements and sclerenchyma cells disintegrate at maturity, due to the addition of lignin, leaving the thick cell walls. Vessel elements, made from vessel cells, are conductive elements, which are more highly specialised than tracheids. They are characteristic of the flowering plants and do not occur conifers. In general, vessel elements are shorter and wider than tracheids. The vessel elements form when a column of parenchyma cells loses their end walls. The vessel elements are stacked on top of one another to form a continuous tube. ...read more.


Companion cells. Usually one or more specialized, elongated parenchymatous cells, called companion cells are closely associated with the sieve elements in most flowering plants. The companion cells are derived from the same original cell as the associated sieve element. Mature companion cells retain both their cytoplasm and their nuclei. It has been suggested that the nucleus of the companion cell controls both its own cytoplasm and the cytoplasm of the adjoining sieve element, after its nucleus has disintegrated. Relate the structure of the xylem and phloem to their function. Xylem: The function of the xylem is to transport water, and because water is important to the plant it needs to be in continual supply. This is helped by the way the xylem is formed. It is made from vessel elements, which in turn are a column of parenchyma cells that have lost their ends, forming a long drainpipe tube. This long continual drainpipe means that the water can flow in an uninterrupted manner. The xylem needs to be well protected. ...read more.


Phloem: In the phloem the pores allow continuous flow. Being in the middle of the vascular bundle means that it is well protected. Being alive means that when loading and unloading the sucrose it can be done by means of active transport. If it were dead this would not be possible. There is also an increased amount of mitochondria, which produce ATP. This means that active transport can happen, and happen quicker because the ATP is situated near by. The sieve plates are design so that they clot when the phloem is broken. This is helpful because it means that the flow of assimilates can be stopped if necessary and none will be lost. As there is no lignin the phloem it does not have a main structural role. Although the plates do provide horizontal support, forming a ladder like effect. The companion cells are an important part of the phloem. They have all the organelles, unlike the sieve elements, which are deficient in their nuclei. This means that the companion cells act as the 'brain' for the sieve element. It is also the companion cell that starts the process of loading the sucrose at the source. Rebecca Worley Biology ...read more.

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