describe briefly simple dead mechanical tissue
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Answer:
Selerenchyma has its origin from the Gk word scleras meaning hard. The primary wall is made of cellulose and secondary wall is uniformly laid by a process called lignification. The cells are dead and enclose a central cavity called lumen. Certain places are unthickened and called pits. Selerenchyma is of 2 types fibres and selereids. Fibres: There are elongated, spindle shaped, with pointed needle like tips. They occur in groups have thick secondary walls and posses a narrow lumen. They are associated with roots, stems and leaves. Xylem fibres are found associated with xylem, phloem fibres with phloem, pericyclic fibres in the pericycle, cortical fibres in the cortex. They also surround the Vascular bundles of monocot stems. They help in providing mechanical support. They yield fibres of commerce (jute, hemp), provide protection to the vascular bundles. Steroids or stone cells: Vary in shape, thick and hard, with lignified walls showing concentric layering. Lumen is narrow and pits are also present. The grittiness of pulp of fruits like guava, pears is due to the occurrence of selereids, groups. Based on their shape they are grouped as Brachysclereids (stone cells), Macro Selereids (rod-shaped), Osteo Sclereids (bone shaped), and Astrosclereids (star-shaped). These provide mechanical support and rigidity to regions where they are present.