Permanent Tissues-Complex Tissues-Phloem Homework Help
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Permanent Tissues – Complex Tissues – Phloem
Phloem: Phloem is responsible for transporting food materials synthesized in leaves to other parts of the plant. In angiosperms, the phloem is composed of four different elements namely, sieve tube elements, companion cells, phloem parenchyma and phloem fibres. In gymnosperms, phloem has albuminous cells and sieve cells. They lack sieve tubes and companion cells.
Sieve tube elements: These are long, tube-like structures, longitudinally arranged and associated with the companion cells. As the name suggests, their end walls are perforated in sieve-like pattern to form sieve plates. A mature sieve tube element lacks nucleus and is composed of peripheral cytoplasm and large vacuole. However, the functions of sieve tubes are controlled by nucleus of companion cells.
Companion cells: These are specialised parenchymatous cells. They are closely associated with sieve tube elements and connected by pit fields present between their common longitudinal walls. The main function of companion cells is to maintain the pressure gradient in the sieve tubes.
Phloem parenchyma: These are long, tapering cylindrical cells composed of dense cytoplasm and nucleus. The cell wall is made up of cellulose and consists of pits through which plasmodesmatal connections exists between the cells. The main function of phloem parenchyma is to store food material and other substances like resins, latex and mucilage.
Phloem fibres: These long, unbranched, pointed needle-like apices that are composed of sclerenchyma cells and are usually found in secondary phloem. In mature cells, these fibres lose their protoplasm. The phloem fibres of jute, flax and hemp are used for commercial purposes.
Protophloem:Protophloem is the first formed primary phloem that consists of narrow sieve tubes.
Metaphloem:This is secondary phloem that consists of bigger sieve tubes.