CDS (II) 2024 General Knowledge · Question 2 of 120

Correct — B, Water and dissolved minerals. Xylem is the plant's upward plumbing. Water absorbed by the root hairs enters the xylem and travels to the stem and leaves, and the mineral ions the roots have absorbed from the soil — nitrate, phosphate, potassium, and the trace metals — travel dissolved in that same stream. Option (a) is therefore true but incomplete, and incompleteness is what this item is testing: minerals do not have a separate transport tissue of their own, they ride with the water. What xylem does not carry is the sugar the leaf makes; that is the phloem's job, and it moves in both directions rather than only upward.

• (a)Water — True as far as it goes and that is the trap. Water does move in the xylem, but so do the mineral ions dissolved in it, and the fuller option is on offer.
• (c)Gases — Gases move in and out of a leaf through the stomata and diffuse through the air spaces between cells. No conducting tissue carries them up the stem.
• (d)Food — Food — the sugar made in the leaves — is carried by phloem, not xylem, and phloem can move it upward or downward to wherever the plant needs it.

Vascular plants have two conducting tissues. Xylem carries water and the minerals dissolved in it from the roots upward; its conducting elements, the tracheids and vessels, are dead hollow tubes at maturity, which is possible because the lift is driven by physics rather than by the cells themselves. Phloem carries the products of photosynthesis, and its sieve tubes are living cells assisted by companion cells.

Two facts settle every xylem-phloem question: xylem moves water upward, phloem moves food in either direction. The reason minerals belong in the xylem answer is that the plant has no third pipeline for them — a root hair absorbs an ion from the soil solution and that ion is swept along by the transpiration stream. The lift itself is not pumped: evaporation from the leaves pulls on a continuous water column held together by cohesion between water molecules and stuck to the vessel walls by adhesion. One printed oddity in this stem is the grammar — the paper writes 'Xylem in the plants are responsible', where the singular tissue takes a plural verb. It changes nothing that is being tested.

• Xylem conducts water and dissolved minerals from the roots to the stem and leaves; phloem conducts the food made in the leaves.
• Tracheids, vessels and xylem fibres are dead at maturity; xylem parenchyma is the only living element of xylem.
• The upward pull comes chiefly from transpiration, with cohesion between water molecules and adhesion to the vessel walls keeping the column unbroken.
• Root pressure also pushes water up, but it matters mainly at night, when transpiration is low.
• Transport of food in the phloem, unlike the flow in xylem, can be upward or downward as the plant's demands change.

• Choosing the bare 'Water' option when a fuller and equally true option is available.
• Assuming minerals have a transport tissue of their own separate from water.
• Mixing up the two tissues: phloem carries food, not water.

Either as a straight function question like this one, or one step deeper — which xylem element is living, or which combination of processes lifts water in a tall tree.

• practice — not a real PYQ

  Which one of the following is transported by the phloem in a plant?

  • (a)Water absorbed by the roots
  • (b)Mineral ions from the soil
  • (c)Food prepared in the leaves
  • (d)Oxygen released in photosynthesis

  Answer(c) Food prepared in the leaves — phloem translocates the sugar made in photosynthesis, and it can move upward or downward.

• practice — not a real PYQ

  Which one among the following elements of xylem consists of living cells at maturity?

  • (a)Tracheid
  • (b)Vessel
  • (c)Xylem parenchyma
  • (d)Xylem fibre

  Answer(c) Xylem parenchyma — tracheids, vessels and xylem fibres are all dead at maturity, so parenchyma is the only living component.