CDS (II) 2020 General Knowledge · Question 108 of 120

Correct — D, lignified wall and are dead. Sclerenchyma is the plant's hard supporting tissue, and the two facts in this option are the two that define it. Its cells lay down an extremely thick secondary wall impregnated with lignin, the rigid polymer that makes wood woody, so the wall material is cellulose and hemicellulose stiffened with lignin rather than cellulose alone. That wall is laid down from the inside, and by the time it is complete it has closed the cell almost entirely and cut off the exchange the protoplast needs, so the living contents die away. What remains is a hollow, rigid tube — dead cells that go on giving the plant mechanical strength long after they stop living. Husks, nutshells, the gritty stone cells of a pear, and the commercial fibres of jute, coir and flax are all sclerenchyma.

• (a)cellulose wall and are living — Describes parenchyma — thin-walled living cells of cellulose that fill up soft tissue, store food and, when they contain chloroplasts, photosynthesise. Wrong on both counts for sclerenchyma.
• (b)lignified wall and are living — Half right and the most tempting distractor. The wall really is lignified, but heavy lignification is precisely what kills the cell — the thickened wall seals the protoplast off, and a mature sclerenchyma cell has no living contents left.
• (c)suberized wall and are dead — Correct that the cells are dead, wrong about the substance. Suberin is the waxy waterproofing of cork, which is what makes cork impervious to water and gas; sclerenchyma is stiffened with lignin, which gives hardness rather than waterproofing.

Simple permanent tissues in plants come in three kinds, distinguished by their walls. Parenchyma has thin cellulose walls and living contents, and does the storing, packing and, in green tissue, the photosynthesising. Collenchyma has walls unevenly thickened with cellulose and pectin at the corners, stays alive, and provides flexible support to young stems and leaf stalks. Sclerenchyma has uniformly thick lignified walls and is dead at maturity, providing rigid support. Sclerenchyma occurs as long fibres and as short, irregular sclereids or stone cells.

The four options are the four combinations of two variables — what the wall is made of, and whether the cell lives — so the reliable method is to settle each variable separately rather than to recognise a phrase. Wall: lignin, because that is the substance associated with hardness in plants, as against cellulose for softness and suberin for waterproofing. Life: dead, because the wall thickens until nothing can pass. Once both are settled only one option survives. A useful physical check on the answer: jute and coir keep their strength after the plant has been retted and dried, which is only possible because the strength was never in the living contents.

• Sclerenchyma cells have thick walls of cellulose and hemicellulose heavily impregnated with lignin, and are dead at maturity.
• The two forms of sclerenchyma are fibres, which are long and slender, and sclereids or stone cells, which are short and irregular.
• Parenchyma is thin-walled cellulose and living; collenchyma is unevenly thickened with cellulose and pectin and also living.
• Suberin, not lignin, is the waxy substance in the walls of cork cells and is what makes cork impervious to water.
• Jute, coir, flax and hemp fibres, coconut husk and the gritty stone cells of pear flesh are all sclerenchyma.

• Choosing 'lignified and living' — thick lignification is what kills the cell.
• Confusing suberin with lignin; suberin waterproofs cork, lignin hardens wood and fibres.
• Assuming a dead cell is a useless cell; sclerenchyma does its entire job after the protoplast has gone.

As this wall-plus-status item, as 'which plant tissue is dead at maturity', or as a tissue-to-function match across the three simple tissues.

• practice — not a real PYQ

  Flexible support to a young stem is provided chiefly by

  • (a)parenchyma
  • (b)collenchyma
  • (c)sclerenchyma
  • (d)phloem

  Answer(b) collenchyma — living cells with corners thickened by cellulose and pectin, giving strength without losing flexibility.

• practice — not a real PYQ

  The waxy substance that makes cork impervious to water and gases is

  • (a)lignin
  • (b)suberin
  • (c)pectin
  • (d)chitin

  Answer(b) suberin — deposited in the walls of cork cells. Lignin hardens sclerenchyma, pectin cements plant cell walls and chitin belongs to fungi and arthropods.