CDS (I) 2024 General Knowledge · Question 28 of 120

Correct — C, Potato, onion, garlic. All three are stem structures that grow underground. The potato is a stem tuber — a swollen underground stem storing starch, and the giveaway is the 'eyes', which are axillary buds; no root bears buds like that. Onion and garlic are bulbs, and a bulb is defined as a short stem that produces fleshy scale leaves or modified leaf bases. The swollen edible part of a bulb is leaf tissue, but the organ is built on a compressed disc-shaped stem at the base, so a bulb still counts as a modified underground stem. Every other set on offer includes at least one true root or a structure that is not a stem at all.

• (a)Potato, artichoke, round gourd — The round gourd is a fruit borne above ground, and the globe artichoke eaten as a vegetable is an immature flower head — neither is an underground stem, whatever is true of the potato.
• (b)Onion, garlic, carrot — Carrot is a storage taproot — a true root swollen with food. Two bulbs and a root cannot make a set of modified stems.
• (d)Potato, garlic, turnip — Turnip, like carrot, is a storage taproot, so this set breaks on its third member even though the first two are stems.

Plants store food in organs derived from roots, stems or leaves, and telling them apart is a standard morphology exercise. True root storage organs include the taproots of carrot, turnip, radish and beetroot. Stem storage organs include the tuber of the potato, the rhizome of ginger and turmeric, the corm of colocasia, and the bulb of onion and garlic. Leaf-derived structures are rarer as vegetables. The distinguishing marks of a stem are nodes, internodes, buds and scale leaves — none of which a root ever has.

The reliable test in the exam hall is to look for buds. A potato's eyes are buds in the axils of scale leaves, which is decisive; a carrot has nothing of the kind, only lateral rootlets scattered over its surface. The bulb is the case that trips people, because the part you eat is leaf, and it feels wrong to call an onion a stem. Keep the definition in mind — a bulb is a short stem bearing fleshy scale leaves — and the organ as a whole sits on the stem side of the line, which is what this stem, worded as 'underground stem or modified stem', is asking for. Garlic is a bulb of the same genus, Allium, as the onion, built the same way with cloves in place of concentric scales.

• Potato is a stem tuber — an underground stem swollen with stored starch, bearing buds known as eyes.
• A bulb is a short stem that produces fleshy scale leaves or modified leaf bases; onion and garlic are bulbs.
• Carrot and turnip are storage taproots, that is, true roots.
• Nodes, internodes, buds and scale leaves mark a stem; roots have none of these.
• Ginger and turmeric are rhizomes and colocasia is a corm — further examples of underground stems.

• Calling an onion a root because it grows underground and is fleshy; it is a bulb built on a compressed stem.
• Grouping carrot and turnip with potato because all three are underground and starchy — the first two are roots.
• Reading 'artichoke' as the Jerusalem artichoke tuber when the vegetable named in a list of gourds is the globe artichoke, a flower head.

Asked as a set-selection item where three or four vegetables must all be modified stems, so a single root in the set is enough to reject it.

• practice — not a real PYQ

  Which one of the following is a modified underground stem?

  • (a)Radish
  • (b)Ginger
  • (c)Beetroot
  • (d)Sweet potato

  Answer(b) Ginger — a rhizome, and therefore a stem; radish, beetroot and sweet potato are all root structures.

• practice — not a real PYQ

  The 'eyes' of a potato are best described as

  • (a)lateral roots
  • (b)axillary buds
  • (c)lenticels for gas exchange
  • (d)scars left by fallen fruits

  Answer(b) axillary buds — their presence is what identifies the potato as a stem rather than a root.