CDS (I) 2023 General Knowledge · Question 43 of 120

Correct — C, Xylem parenchyma. Xylem is a complex tissue, which means it is built from more than one kind of cell, and it has four of them: tracheids, vessels, xylem fibres and xylem parenchyma. Of those four, only the parenchyma cells are alive at maturity. The other three die and lose their contents, and that is not a defect but the design. Water rises through a tall tree by transpiration pull, a tension transmitted through a continuous column of water held together by cohesion and stuck to the walls by adhesion. A pipe carrying water under tension is best empty of living contents and reinforced with lignin, so tracheids and vessels shed their protoplasm and become hollow conducting tubes, and xylem fibres thicken into pure mechanical support. Xylem parenchyma stays alive to do what living cells do — store starch and fats, help with short-distance lateral movement of water and solutes, and in some species grow into the vessels as tyloses to block them. The one line to carry away is that conduction in xylem costs the plant no energy at the point of transport, which is why the conducting cells can afford to be dead.

• (a)Tracheid — Dead at maturity. Tracheids are elongated cells with tapering ends and lignified walls; water passes from one to the next through pits, and the cell contents are gone.
• (b)Vessel — Dead at maturity, and the more efficient of the two conducting elements. Vessel members are stacked end to end with their end walls perforated, forming a continuous open tube.
• (d)Xylem fibre — Dead at maturity, with thick lignified walls and a narrow lumen. Its job is mechanical strength rather than conduction.

Plant tissues are either simple, made of one cell type, or complex, made of several working together. Xylem and phloem are the two complex tissues of the vascular system. Xylem carries water and dissolved minerals upward and is composed of tracheids, vessels, fibres and parenchyma. Phloem carries the products of photosynthesis and is composed of sieve tubes, companion cells, phloem parenchyma and phloem fibres — and in phloem it is the conducting elements that stay alive, the mirror image of the arrangement in xylem.

Two habits make this family of questions easy. First, remember xylem and phloem as opposites: xylem conducts upward with dead cells and no energy cost at the point of transport, phloem conducts in both directions with living cells and an energy cost. Second, remember that parenchyma is the general-purpose living packing tissue of the plant and turns up inside both complex tissues, which is why 'parenchyma' is the safe answer whenever a question asks which part of a conducting tissue is alive. The physics behind the design is worth one line as well: a column of water under tension can only hold together because of cohesion between water molecules, and that same cohesion is what makes a dead, rigid, lignified tube the right pipe for the job.

• Xylem is composed of tracheids, vessels, xylem fibres and xylem parenchyma.
• Only xylem parenchyma is living at maturity; tracheids, vessels and fibres are all dead.
• Water rises through xylem by transpiration pull, aided by the cohesion of water molecules and their adhesion to the vessel walls.
• Xylem parenchyma stores food and assists in the lateral movement of water; in some species it forms tyloses that block vessels.
• In phloem the position is reversed — the conducting sieve tubes are living cells, supported by companion cells.

• Assuming the conducting cells must be alive because conduction is a life process; in xylem the transport is passive.
• Mixing xylem and phloem components — sieve tubes and companion cells belong to phloem.
• Treating the xylem fibre as a conducting element; it is structural.

As a which-element-is-living item, as a composition item listing the parts of xylem or phloem, or as a mechanism item on how water rises.

• practice — not a real PYQ

  Which one of the following is a component of phloem rather than of xylem?

  • (a)Tracheid
  • (b)Vessel
  • (c)Companion cell
  • (d)Xylem fibre

  Answer(c) Companion cell — it accompanies the sieve tube in phloem, the tissue that carries the products of photosynthesis.

• practice — not a real PYQ

  Water rises to the top of a tall tree chiefly because of which one of the following?

  • (a)Root pressure alone
  • (b)Transpiration pull, with cohesion and adhesion of water
  • (c)Active pumping by living vessel cells
  • (d)Capillarity in the phloem

  Answer(b) Transpiration pull, with cohesion and adhesion of water — the tension created by evaporation at the leaves is transmitted down an unbroken column of water in the xylem.