Tracing water with a dye — answer key

1. RQ1 · Practice

Options as numbered on the pupil sheet: 1 — xylem; 2 — phloem.

Model answer. Xylem carries water and mineral ions from the roots up to the leaves, and its tough walls help to support the plant. Phloem carries sucrose and amino acids, made in the leaves, to the parts that use or store them.

A full answer includes

• Transport of water and mineral ions: xylem.
• Transport of sucrose and amino acids: phloem.
• Helping to support the plant: xylem.

Also accept

• water and mineral ions: xylem; sucrose and amino acids: phloem; support: xylem

Watch for

• “Phloem carries water, because it is on the outer side of each bundle.” — Where a tissue is does not decide its job. Water and mineral ions go up in the xylem.
• “Phloem carries glucose.” — Leaves change glucose into sucrose before it is carried. Phloem carries sucrose and amino acids.

2. RQ2 · Practice

Model answer. Stalk D is the control. It is matched to A, B and C and treated the same, except that it stands in plain water. Its slices show what the inside of a stalk looks like with no dye. So any red in A, B or C that is not in D was caused by the dye.

A full answer includes

• D is the control: it shows the slices with no dye, so any red in A, B and C must have come from the dye.

Also accept

• it is a control: it shows the red came from the dye

Watch for

• “D shows how fast celery takes up plain water.” — D is cut and looked at in the same way as the others. It shows what no dye looks like, not a speed.
• “D is not needed: celery is green, so any red must be dye.” — The strands inside celery can look pale or tinted on their own. Only a stalk with no dye shows what to compare with.

3. RQ3 · Practice

Model answer. The red was only in separate dots, in a row near the curved, ridged side of the slice. The rest of the slice stayed pale green. The dots are the vascular bundles, cut across: water carrying the dye travels up inside them.

A full answer includes

• Red only in separate dots, in a row near the curved, ridged side; the rest of the slice pale green.

Also accept

• separate red dots in a row near the ridged side

Watch for

• “The whole slice turns red.” — Only the dots were red. The dye travels in the vascular bundles, not through every cell.
• “Only the outer skin turns red.” — The skin stayed green. The red was in dots just inside it, near the ridged side.

4. RQ4 · Practice

Model answer. Cutting slices from a stalk destroys part of it: it is shorter and has new cut ends, so after it goes back in the dye it may not take up dye in the same way, and its later slices would not be a fair comparison. With a fresh stalk for each time, matched to the others, all the stalks are treated the same except for how long they stood in the dye. So the slices show how far the dye had got by each time.

A full answer includes

• Cutting slices destroys part of the stalk: it is shorter, with new cut ends, so it may not take up dye as before.
• With a fresh, matched stalk for each time, only the time in the dye differs: a fair comparison.

Also accept

• cutting destroys the stalk; a fresh, matched stalk for each time keeps it fair, so only the time differs

Watch for

• “It does not matter: the dye is still inside the stalk.” — A cut stalk is shorter, with new cut ends, so it is no longer treated the same as before.
• “Using more stalks makes the experiment less fair.” — Matched stalks keep it fair: only the time in the dye differs between them.

5. RQ5 · Practice

Model answer. The results show that the longer a stalk stood in the dye, the higher the red reached: faint at 4 cm after 30 minutes, past 10 cm after 2 hours, and into the leaf veins after 6 hours. Stalk D, in plain water, had no red, so the red came from the dye and not from the celery. The results do not give a speed: they show only how far the dye had got at three times. And the red was only in dots, not in the whole slice.

A full answer includes

• Supported: the longer a stalk stood in the dye, the higher the red reached; after 6 hours the dye had reached the veins of the leaves; the stalk in plain water had no red.
• Not supported: a steady speed of 2 cm per minute; the dye soaking through the whole of each slice; the celery making the red itself.

Also accept

• higher with time; leaf veins red after 6 hours; no red in plain water

Watch for

• “The table shows that the dye climbs at a steady speed.” — It shows only how far the dye had got at three times, in a model. It does not measure a speed.
• “The celery made the red itself.” — Stalk D stood in plain water and had no red.

6. RQ6 · Practice

Model answer. (a) The dye went in at the cut end of the stalk, in the beaker. It travelled up the stalk inside the vascular bundles, mainly in their xylem, and out of the top of the stalk into the veins of the leaves, which are vascular bundles too. (b) Water carrying the dye moved along the same route, so the vascular bundles, through their xylem, carry water up the plant to the leaves.

A full answer includes

• (a) In at the cut end, up the stalk inside the vascular bundles (mainly in their xylem), and out into the veins of the leaves.
• (b) The vascular bundles, through their xylem, carry water up the plant to the leaves.

Also accept

• (a) cut end, up the vascular bundles (xylem), into the leaf veins; (b) they carry water up to the leaves

Watch for

• “The dye went up through every cell of the stalk.” — The red was only in the dots, the vascular bundles. The rest of each slice stayed pale.
• “The dye went down from the leaves.” — The dye went in at the cut end, in the beaker, so it moved up.

7. RQ7 · Practice

Model answer. (a) The wide, empty tubes with red walls are xylem vessels: the dye went up with the water mainly in the xylem. (b) The small, pale cells are phloem, which carries sucrose and amino acids. These are colourless, so the dye cannot show them. (c) Xylem also carries mineral ions, and its tough walls help to support the plant.

A full answer includes

• (a) Xylem (xylem vessels).
• (b) Phloem: it carries sucrose and amino acids.
• (c) Helping to support the plant (or carrying mineral ions).

Also accept

• (a) xylem; (b) phloem, sucrose and amino acids; (c) support (or mineral ions)

Watch for

• “The phloem stayed pale, so it carries nothing.” — Phloem carries sucrose and amino acids, which the dye cannot show.
• “The red-walled tubes are phloem.” — In each strand the xylem is on the inner side and the phloem on the outer side.

8. RQ8 · Practice

Options as numbered on the pupil sheet: 1 — A; 2 — B; 3 — C; 4 — D.

Model answer. In each vascular bundle of a stem the phloem is towards the outside and the xylem is towards the inside. So B, the small cells on the outer side of the bundle, is phloem, and C, the wide tubes on the inner side, is xylem. A is the epidermis, and D the large cells in the middle of the stem.

A full answer includes

• Xylem: C, on the inner side of the bundle.
• Phloem: B, on the outer side of the bundle.

Also accept

• xylem C; phloem B

Watch for

• “C is phloem, because the bigger cells carry the food.” — The wide, thick-walled tubes are xylem vessels. The phloem is the small cells on the outer side.
• “A is phloem, because it is on the outside.” — A is the outer layer of the whole stem, the epidermis. The phloem is inside each bundle, on its outer side.

9. RQ9 · Practice

Options as numbered on the pupil sheet: 1 — keeps the tube open and helps to support the plant; 2 — nothing inside blocks the water; 3 — makes one long, continuous tube.

Model answer. Thick walls strengthened with lignin keep the vessel open, so it does not collapse, and help to support the plant. With no cell contents, no cytoplasm and no nucleus, nothing inside blocks the water. With no cross walls between the cells joined end to end, the vessel is one long, continuous tube from the roots to the leaves.

A full answer includes

• Thick walls with lignin: keep the tube open and help to support the plant.
• No cell contents: nothing blocks the water.
• No cross walls: one long, continuous tube.

Also accept

• lignin walls: strong and open; no contents: nothing blocks; no cross walls: continuous tube

Watch for

• “The lignin lets water soak in through the walls.” — Lignin makes the walls strong and keeps the tube open.
• “The cell contents push the water up.” — A xylem vessel has no cell contents: it is an empty tube.

10. RQ10 · Practice

Model answer. A finished xylem vessel has no cell contents: no cytoplasm and no nucleus, so nothing inside it pushes the water, and nothing blocks it. Its cells are joined end to end with no cross walls, so it is one long, continuous tube, and the water does not have to pass through walls from cell to cell. Its thick walls, strengthened with lignin, keep the tube open.

A full answer includes

• A xylem vessel has no cell contents: no cytoplasm to push the water, and nothing to block it.
• It has no cross walls: its cells are joined end to end into one long, continuous tube.

Also accept

• no cell contents and no cross walls: an empty, continuous tube

Watch for

• “Xylem vessels are living cells.” — A finished xylem vessel has no cell contents.
• “Water squeezes through the cross walls.” — The cross walls break down, leaving one continuous tube.

11. RT1 · Apply

Model answer. (a) Water carrying the dye went in at the cut end of the stem, up the flower stalk in the xylem, and out along the veins of the petals, which contain xylem too. (b) The blue will be in separate dots, the vascular bundles, near the outside of the stalk, and within each dot in the xylem on its inner side, not all over the slice.

A full answer includes

• (a) Water carrying the dye went in at the cut end, up the stem in the xylem, and out into the veins of the petals.
• (b) In the vascular bundles, as separate dots or a ring of them, in the xylem of each; not all over the slice.

Also accept

• (a) up the xylem from the cut end into the petal veins; (b) in the bundles (dots or a ring), in the xylem

Watch for

• “The petals made the blue colour themselves.” — The lines are the colour of the dye and follow the veins: the colour came up from the beaker.
• “The phloem carried the dye down from the leaves to the petals.” — The dye went in at the cut end and travelled up with the water, in the xylem.

12. RT2 · Apply

Model answer. Each half of the stem holds its own vascular bundles. Their xylem vessels are separate long tubes, running up the stem to particular petals. Water carrying red dye goes up the vessels in one half, and water carrying blue dye up the vessels in the other, so each group of petals gets the colour of the half whose xylem leads to it. The dyes do not mix, because each vessel is a separate tube.

A full answer includes

• Each half of the stem has its own xylem vessels: separate tubes running up to particular petals.
• The water and dye in each tube stay in that tube, so the two dyes do not mix.

Also accept

• each half has its own xylem tubes leading to some of the petals; the dyes stay in their own tubes

Watch for

• “The dyes would mix in the stem and make the flower purple.” — Each xylem vessel is a separate tube, so water in one does not mix with water in another.
• “The flower chose to take up red on one side.” — Water goes up whichever xylem vessels stand in the liquid; the flower does not choose.

13. RT3 · Apply

Model answer. (a) The red is in the xylem, which carries the water and the dye up the stem. (b) In each dot the red will be on the side towards the middle of the stem. In a non-woody dicotyledonous stem the vascular bundles form a ring near the outside, and in each bundle the xylem is on the inner side and the phloem on the outer side.

A full answer includes

• (a) Xylem.
• (b) Towards the middle: in a stem the xylem is on the inner side of each bundle, and the phloem on the outer side.

Also accept

• (a) xylem; (b) towards the middle, because the xylem is on the inner side of each bundle

Watch for

• “The red is on the outer side, because the lines show through the skin.” — The bundles are near the outside of the stem, but within each bundle the xylem is on the inner side.
• “The red is in the phloem, because phloem is on the outside.” — The dye travels with the water, in the xylem.

14. RT4 · Apply

Model answer. In a normal xylem vessel, thick walls strengthened with lignin keep the tube open, so it does not collapse. In the mutant the walls have little lignin, so they are thin and weak, and the vessels are squashed flat. A squashed tube is no longer an open, continuous path for water, so water moves up the stem more slowly and the leaves get less water than they lose. They wilt, even though the soil is moist. The weak xylem also gives the plant less support.

A full answer includes

• Lignin thickens and strengthens the walls of xylem vessels, which keeps the tubes open.
• With little lignin the walls are weak, so the vessels collapse: the tube is no longer open and continuous.
• So water moves up more slowly, the leaves get less water than they lose, and they wilt.

Also accept

• little lignin: weak walls, the vessels collapse, less water reaches the leaves, so they wilt

Watch for

• “The leaves wilt because the soil is too dry.” — The soil is moist, the same as for the normal plants. The problem is in the plant: its squashed xylem vessels cannot carry the water up fast enough.
• “Without lignin the vessels fill up with cytoplasm, which blocks the water.” — A finished vessel has no cell contents in any case. Lignin strengthens the walls; without it the walls are weak and the vessels collapse.