Lesson overview
Xylem, phloem and water uptake: How does water get from the soil to the top of a plant?
Stand celery stalks in red dye, cut them at set heights after different times to find the route water takes, look at a slice under the microscope and at the root hairs of a radish seedling, follow water from root hair to leaf, and see how the phloem carries sucrose from sources to sinks.
Suggested sequence
- Lesson 1 · Tracing water with a dye. The functions of xylem and phloem; tracing water up celery stalks with a dye; xylem and phloem in a stem section; xylem vessels (Supplement). Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · From root hair to leaf. Root hair cells and their large surface area; the pathway of water from root hair cells to mesophyll cells; xylem and phloem in a root and a leaf. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Translocation. Supplement: translocation of sucrose and amino acids in the phloem; sources and sinks; parts that are a source at one time and a sink at another. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 9 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 10 questions, 15 min, lesson closed.
Learning goals
Part 1: Tracing water with a dye
- State the functions of xylem (transport of water and mineral ions, and support) and phloem (transport of sucrose and amino acids) (8.1.1)
- Investigate, using a suitable stain, the pathway of water through the above-ground parts of a plant (8.2.4)
- Identify in diagrams and images the position of xylem and phloem as seen in sections of stems of non-woody dicotyledonous plants (8.1.2)
- Supplement: relate the structure of xylem vessels to their function: thick walls with lignin, no cell contents, cells joined end to end with no cross walls to form a long continuous tube (8.1.3)
Part 2: From root hair to leaf
- Identify in diagrams and images root hair cells and state their functions (8.2.1)
- State that the large surface area of root hairs increases the uptake of water and mineral ions (8.2.2)
- Outline the pathway taken by water through the root, stem and leaf as: root hair cells, root cortex cells, xylem, mesophyll cells (8.2.3)
- Identify in diagrams and images the position of xylem and phloem as seen in sections of roots and leaves of non-woody dicotyledonous plants (8.1.2)
Part 3: Translocation
- Supplement: describe translocation as the movement of sucrose and amino acids in phloem from sources to sinks (8.4.1)
- Supplement: describe sources as the parts of plants that release sucrose or amino acids, and sinks as the parts of plants that use or store sucrose or amino acids (8.4.2)
- Supplement: explain why some parts of a plant may act as a source and a sink at different times (8.4.3)
Misconceptions to listen 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.
- “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.
- “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.
- “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.
- “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.
Scope
The functions of xylem and phloem (8.1.1) and their positions in sections of roots, stems and leaves of non-woody dicotyledonous plants (8.1.2); the structure of xylem vessels (Supplement 8.1.3); root hair cells, their functions and their large surface area (8.2.1, 8.2.2); the pathway of water through the root, stem and leaf (8.2.3); the pathway of water through the above-ground parts investigated with a stain (8.2.4); translocation, sources and sinks (Supplement 8.4.1–8.4.3). Transpiration is the next module. How far the dye climbs in the experiment comes from a teaching model.