NDA (I) 2018 General Ability Test · Question 74 of 150

Correct — D, Growth occurs in plants due to division of cells of these tissues. Meristematic tissue is the plant's growing tissue. Its cells are small, thin-walled, densely packed with cytoplasm, have a prominent nucleus and little or no vacuole, and above all they retain the power to divide throughout the plant's life. Every new cell in a plant traces back to a meristem, and the cells produced there later enlarge and specialise into permanent tissues. Growth by cell division at fixed growing points is precisely what this statement asserts.

• (a)These are dead tissues and form wood — Meristematic cells are very much alive — a dead cell cannot divide. Wood is formed from the secondary xylem, whose conducting elements are indeed dead at maturity, but that xylem is itself produced by a living meristem, the vascular cambium.
• (b)They provide flexibility to plant due to their thickened walls — This describes collenchyma, a permanent tissue whose cells are thickened at the corners and which gives flexibility to young stems and leaf stalks. Meristematic cells have thin walls, not thickened ones.
• (c)These are present in the bark of a tree only — Meristems occur in three positions, not one. Apical meristem sits at the tips of roots and shoots and adds length; lateral meristem, the cambium, lies along the sides and adds girth; intercalary meristem sits at the base of leaves or above the nodes, which is how grass regrows after it is cut.

Plant tissues divide into meristematic and permanent. Meristematic tissue consists of cells that keep dividing, and it is found at three kinds of site — apical at the growing tips, lateral or cambial along the sides, and intercalary at the bases of leaves and internodes. Cells cut off from a meristem gradually lose the power to divide and take on a fixed shape and function, a process called differentiation, and the tissues they become are the permanent tissues — parenchyma, collenchyma and sclerenchyma among the simple ones, and xylem and phloem among the complex.

This item is one question wearing four costumes, and each wrong option is really a different tissue's description handed to the meristem. Option (a) belongs to sclerenchyma and to mature xylem, option (b) to collenchyma, option (c) to a partial memory that cambium lies near the bark. The way to keep them apart is to learn each tissue by its wall and its state — meristem thin-walled and living, parenchyma thin-walled and living with large vacuoles, collenchyma unevenly thickened and living, sclerenchyma thickened with lignin and dead. Then attach each meristem to what it produces: damage the apical meristem and the plant stops growing taller, and it is the lateral meristem that thickens a trunk year by year to give the rings a botanist counts.

• Meristematic cells are living, thin-walled, densely cytoplasmic and capable of continued division.
• Apical meristem increases length; lateral meristem or cambium increases girth; intercalary meristem sits at the bases of leaves and internodes.
• Cells produced by a meristem become permanent tissues through differentiation.
• Collenchyma gives flexibility through unevenly thickened walls; sclerenchyma gives strength and its cells are dead.
• Wood is secondary xylem, produced by the living vascular cambium.

• Attributing to the meristem a property that belongs to collenchyma or sclerenchyma.
• Thinking meristematic tissue is dead because the wood it eventually produces is.
• Assuming meristems occur only at growing tips, and forgetting the lateral and intercalary kinds.

NDA asks which statement about a named tissue is correct, or which meristem is responsible for growth in a stated direction.

• practice — not a real PYQ

  Which tissue is responsible for the increase in the thickness of a tree trunk?

  • (a)Apical meristem
  • (b)Lateral meristem
  • (c)Intercalary meristem
  • (d)Sclerenchyma

  Answer(b) Lateral meristem — the cambium adds new tissue along the sides and so increases girth.

• practice — not a real PYQ

  A grass lawn regrows quickly after mowing chiefly because of the activity of

  • (a)apical meristem
  • (b)intercalary meristem
  • (c)vascular cambium
  • (d)cork cambium

  Answer(b) intercalary meristem — it sits at the base of the leaf, below the cut, and keeps pushing the blade upward.