IGCSE Biology 0610: Tracking Dye Uptake Through Xylem in a Cut Flower

A gardener wants to see where water travels inside a cut stem. She stands the cut stem of a white chrysanthemum flower in a jar of water with red food dye stirred into it, and leaves it overnight. The stem, flower head and petals remain attached to each other as one continuous piece of plant throughout.

(a) Name the plant tissue responsible for carrying the dye up the stem, and state one other substance this tissue normally transports in an intact, growing plant. [2]

(b) The next morning, the gardener slices across the stem. Instead of one solid red patch in the centre, she sees a ring of separate small red dots arranged in a circle just inside the outer layer of the stem. Explain what this pattern tells her about the position of the tissue named in (a) in this stem. [2]

(c) She then examines one of the flower's white petals with a hand lens and sees the red dye has spread along thin veins running through the petal. Describe the pathway the dye follows, in order, from the water in the jar until it reaches these petal veins. [2]

Part (a): The tissue carrying the dye

The dye travels up the stem inside the xylem. In a living plant, besides carrying water, xylem also transports mineral ions dissolved in that water. This is the only correct second substance here. Xylem’s thick, strengthened cell walls do give the stem support, but support is a structural role of the tissue, not a substance it transports, so it cannot be credited as the “other substance” asked for in this question.

Part (b): What the ring of dots shows about xylem position

If the xylem were spread randomly throughout the stem, the dye would appear scattered evenly across the whole cross-section. Instead, the dye appears only in a ring of separate small dots near the outer part of the stem. This shows that the xylem is grouped together into a number of vascular bundles, and that these bundles are arranged in a ring, rather than being spread evenly through the stem tissue or clustered as one central mass.

Part (c): The pathway of the dye to the petal veins

The dye first enters the plant through the cut surface at the base of the stem, where it mixes with water taken up into the xylem vessels. From there it moves upward through the xylem running the length of the stem, since these vessels form one continuous pathway rather than separate disconnected sections. The pathway continues into the flower head, where the xylem branches out towards the petals, and finally into the fine veins of the petals themselves, which is why the red colour appears along these veins under the hand lens.

Final answers

(a) Xylem; it also transports mineral ions. (Support is a related function of xylem, not a transported substance, so it does not count as the second answer.)
(b) The ring of dots shows the xylem is grouped into bundles arranged in a ring near the outside of the stem, not spread evenly or centred.
(c) The dye passes from the cut stem base into the xylem, travels up the stem, then into the flower head, and finally into the petal veins, all via one continuous xylem pathway.