Q.Why the cells of sclerenchyma and tracheids become dead ?

Sclerenchyma (Gk. scleros = hard) is the plant's dead mechanical tissue: its cells lose their protoplasm at maturity and are left as an empty, uniformly and heavily lignified secondary-wall shell, strong precisely because there is no longer any living content to protect inside. It comes in two structurally distinct forms. Sclereids (stone cells) are typically isodiametric, though some are elongated, with a wall so thickened by lignification that the internal lumen is reduced almost to nothing; connected to their neighbours only by simple or branched pits, they give a hard or gritty texture to seed coats, endosperms and certain fruit flesh (pear, guava, strawberry). Five to six shapes are named by silhouette: brachysclereids/stone cells (roughly isodiametric; bark, pith, cortex, hard endosperm, fruit pulp), macrosclereids (elongated rods; legume seed coats), osteosclereids/bone cells (rods with dilated ends; leaves, seed coats), astrosclereids/star cells (arms radiating from a centre; petioles, leaves), trichosclereids (thin, hair-like, branching, crystal-studded; hydrophyte stems/leaves) and filiform sclereids (very elongated, fibre-like, ~1 mm; Olea leaf). Fibres, the second form, are the plant's most elongated dead cells - very long, tapering to a point, lignified, with …