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

Step 1. Recall what these two cell types have in common. Both sclerenchyma cells (sclereids and fibres) and tracheids are described in the chapter as dead cells with lignified secondary walls, in contrast to the living cells of parenchyma and collenchyma.

Step 2. Explain the mechanism of wall thickening. As these cells mature, they lay down a secondary wall over their original thin primary wall, and this secondary wall is uniformly and strongly thickened with lignin - a rigid, water-repelling polymer. In sclerenchyma cells this thickening is so extensive that the internal lumen (the space that once held the living protoplasm) is reduced almost to nothing; in tracheids it takes the form of annular, spiral, reticulate, scalariform or pitted patterns.

Step 3. Explain why this thickening causes the cell to die. Once this heavy, largely impermeable lignified wall is complete, the cell's protoplasm can no longer exchange materials, gases and water with its surroundings the way a living cell needs to; deprived of its ability to sustain normal metabolism, the protoplasm degenerates and the cell dies, leaving behind only its rigid wall. …

Explain the shared cause (heavy lignified secondary-wall thickening cutting off the living protoplasm) and then the functional payoff of …

• Stating only 'they are lignified so they die' without explaining the actual mechanism - the wall thickening occluding the lumen and blocking normal metabolic exchange. …