![]() ![]() In gymnosperms, the sieve elements display more primitive features than in angiosperms, and instead of sieve plates, have numerous pores at the tapered end of the cell walls for material to pass through directly. After injury, a unique protein called “P-protein” (Phloem-protein), which is formed within the sieve element, is released from its anchor site and accumulates to form a ‘clot’ on the pores of the sieve plate and prevent loss of sap at the damage site. The sieve plates also act as a barrier to prevent the loss of sap when the phloem is cut or damaged, often by an insect or herbivorous animal. Sieve plates are relatively large, thin areas of pores that facilitate the exchange of materials between the element cells. Sieve PlatesĪt the connections between sieve member cells are sieve plates, which are modified plasmodesmata. There are two main types of sieve element: the ‘sieve member’, which is found in angiosperms, and the more primitive ‘sieve cells’, which are associated with gymnosperms both are derived from a common ‘mother cell’ form. They are unique in that they do not contain a nucleus at maturity and are also lacking in organelles such as ribosomes, cytosol and Golgi apparatus, maximizing available space for the translocation of materials. The sieve element cells are the most highly specialized cell type found in plants. The sieve elements are elongated, narrow cells, which are connected together to form the sieve tube structure of the phloem. Each of the components work together to facilitate the conduction of sugars and amino acids, from a source, to sink tissues where they are consumed or stored. The structure of the phloem is made up of several components. Where there are areas of high and low pressure, the photoassimilates and water are consistently moved around the plant in both directions. As the concentration of sugars reduces in the solution, the amount of water influx from the xylem also drops this results in low pressure in the phloem at the sink. When the sink receives the sugar solution, the sugars are used for growth and other processes. the roots, growing tips of stems and leaves, flowers and fruits). The high turgor pressure causes the water and sugars to move through the tubes of the phloem, in to the ‘sink tissues’ (e.g. Water is drawn passively from the adjacent xylem over the gradient to create a sugar solution and a high turgor pressure within the phloem. When there is a high concentration of organic substance (in this case sugar) within the cells, an osmotic gradient is created. The next step, translocation of the photoassimilates, is explained by the pressure flow hypothesis. The sugars are moved from the source, usually the leaves, to the phloem through active transport. ![]()
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