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| Phloem |
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| Phloem is a complex permanent tissue, which is specialized for the conduction of food and other organic substances. Phloem is also a heterogenous tissue, made up of four different types of cellular elements, namely, |
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 Sieve tubes |
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 Companion cells |
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 Phloem parenchyma and |
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 Phloem fibres |
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| Of these, the sieve tubes and the companion cells are directly involved in the translocation of the organic substances. Hence, they are commonly described as essential elements. Phloem parenchyma and phloem fibres are described as associated elements since they play only a supporting role in the process. |
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| The sieve tubes, the companion cells and the phloem parenchyma represent the living components of the tissue while phloem fibres represent the only nonliving component of the tissue. |
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| Phloem is commonly described as a living, complex permanent tissue. |
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| They represent the most active food conducting elements in the phloem tissue. The sieve tubes are found arranged parallel to one another from one end of the plant body to another. Each sieve tube is formed by a series of hollow, cylindrical cells called sieve tube cells arranged one above the other. The sieve cells are separated from each other by horizontal perforated plates called sieve plates. The sieve cells communicate with each other through the sieve plates. |
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| Each sieve cell has a thin cell wall, which is composed of only cellulose. The cell has a central mass of dense cytoplasm. The granular cytoplasm forms a number of projections called cytoplasmic strands extending towards the sieve plate. The nucleus is absent. |
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| Phloem Structure |
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| They are more or less spindle shaped cells associated with the sieve tube cells. Each companion cell is found attached to any one lateral surface of a sieve cell. The companion cell and the neighbouring sieve cell together represent a pair of sister cells. The companion cell has a granular cytoplasm, prominent nucleus and one or two small vacuoles. The nucleus of the companion cell is said to be capable of exerting its influence on the adjacent sieve cells. |
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| Phloem parenchyma is represented by a group of living parenchyma cells that are found in-between the sieve tubes. They are meant only for storage of organic food. |
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| Phloem fibres are represented by the dead sclerenchyma fibres that are found in between the sieve tubes. They are meant only for providing mechanical support. |
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| Phloem can be distinguished into two types namely |
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 Primary phloem |
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 Secondary phloem |
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| Primary Phloem |
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| Primary phloem is the phloem that is formed during normal growth in the plant body. It is a derivative of primary meristem. It is found in both monocots and dicots. The primary phloem is further composed of protophloem and metaphloem. The sieve tubes and the companion cells, which appear earlier during normal growth, represent protophloem, while metaphloem is represented by the sieve tubes and companion cells that appear later. However, there is no significant morphological difference between protophloem and metaphloem. |
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| Secondary Phloem |
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| Secondary phloem is the phloem that is formed during secondary growth. It is a derivative of secondary meristem. Secondary phloem is characteristic feature of only dicots. It is also known as bast. It is also of commercial importance since it yields bast fibers. |
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| The following table summarise the characteristics of and difference between xylem and phloem |
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