The shape of the lesson
By the end of the lesson, learners can
- Where the vascular tissue lies: The lab opens on a stem section.
- Built for the job: Click each feature of the cut-away cells: the xylem vessel elements on the left, then the sieve tube elements and the companion cell.
- Drawing cells, not plans: You may also be asked to draw and label the individual cells from slides, photomicrographs or electron micrographs.
- Root: xylem central, X or star shape; phloem between the arms.
- Stem: ring of bundles; xylem inside, phloem outside, cambium between.
- Leaf: xylem upper, phloem lower in each vein.
- Xylem dead, empty, lignified; sieve tube living, no nucleus, sieve plates.
One picture every idea returns to
In a herbaceous dicotyledonous plant the xylem carries water and mineral ions up from the roots, and the phloem carries assimilates such as sucrose and amino acids from sources to sinks. Their positions differ: in the root the xylem is in the centre, often X or star shaped, with phloem between its arms; in the stem the vascular bundles form a ring near the outside, with xylem on the inner side and phloem on the outer side of each bundle; in the leaf midrib and veins the xylem is on the upper side and the phloem on the lower side. Xylem vessel elements are dead, empty and joined end to end, with lignified walls and pits. Sieve tube elements are living but have no nucleus and very little cytoplasm, with sieve plates between them; each is kept alive by a companion cell with a nucleus, many mitochondria and many plasmodesmata.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Two pipe systems, one dead and one alive.. Water and mineral ions travel up a plant in xylem made of dead, empty, lignified tubes. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Where the vascular tissue lies | The lab opens on a stem section. Project tab 1 of the lab (Transport in plants lab, transverse sections) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Built for the job | Click each feature of the cut-away cells: the xylem vessel elements on the left, then the sieve tube elements and the companion cell. Project tab 2 of the lab (Transport in plants lab, transport cells) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Drawing cells, not plans | You may also be asked to draw and label the individual cells from slides, photomicrographs or electron micrographs. | Corners |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Drawing cells, not plans, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Corners
Put one answer or viewpoint in each corner of the room. Students move to the corner they agree with, justify the choice to a partner there, then hear one voice from each corner before the reveal.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A plan diagram shows the outlines of tissues only: no individual cells, no shading, clear continuous lines, proportions that match the specimen and label lines drawn with a ruler. | Drawing cells in a plan diagram loses marks. |
| Xylem vessel elements are dead, so they cannot actively transport anything; water moves up them passively. | Sieve tube elements are alive even though they lack a nucleus. Mixing these up is a common error. |
Xylem and phloem: structure and distribution: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the distribution of xylem and phloem in a transverse section of the root of a herbaceous dicotyledonous plant.
List four rules to follow when drawing a plan diagram of a transverse section of a stem from a microscope slide.
Relate the structure of a xylem vessel element to its function of transporting water.
Explain how the structure of a companion cell is related to its role in transporting sucrose into a sieve tube element.
Compare the structure of a mature xylem vessel element with that of a sieve tube element.
A transverse section of a plant organ shows vascular bundles arranged in a ring, with the xylem of each bundle facing the centre and a large region of pith in the middle. Identify the organ and give two reasons for your answer.
Answer key and marking notes
- The xylem is in the centre of the root (in the vascular tissue inside the endodermis and pericycle).
- It forms an X or star shape with several arms.
- The phloem is found in separate groups between the arms of the xylem.
- Draw only the outlines of tissues; do not draw individual cells.
- Use clear, single, continuous lines and no shading.
- Keep the proportions of the tissues the same as in the specimen.
- Use a ruler for label lines, which touch the tissue and do not cross each other.
- Dead, with no cytoplasm or nucleus, so nothing obstructs the flow of water.
- No end walls, so the elements form a continuous tube from root to leaf.
- Lignified walls strengthen the vessel so it does not collapse under tension, and are waterproof.
- Pits allow water to move sideways into adjacent vessels or surrounding cells (and around an air bubble).
- Many mitochondria carry out aerobic respiration and make ATP.
- The ATP is used by proton pumps in the companion cell membrane for active loading of sucrose.
- Many plasmodesmata link the companion cell to the sieve tube element, so sucrose can pass into it.
- A nucleus and dense cytoplasm (many ribosomes) make the proteins needed, including those for the sieve tube element, which has no nucleus.
- Xylem vessel element is dead; sieve tube element is living (but has no nucleus).
- Xylem has lignified walls; sieve tube element has cellulose walls with no lignin.
- Xylem vessel element has no end walls; sieve tube element has end walls perforated as sieve plates.
- The organ is a stem.
- Vascular bundles in a ring near the outside (a root has a single central block of vascular tissue).
- Xylem on the inner side and phloem on the outer side of each bundle, around a central pith (a root has no pith and a leaf has a flat lamina with xylem on the upper side).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The plan diagram shows a transverse section of an organ from a herbaceous dicotyledonous plant. Which label points to the xylem?
- AA
- BB
- CC
- DD
Which feature is present in phloem sieve tube elements but not in mature xylem vessel elements?
- Asieve plates with pores in the end walls
- Bwalls thickened with lignin
- Cpits in the side walls
- Dthe absence of a nucleus
An electron micrograph shows a narrow cell with a nucleus, dense cytoplasm and many mitochondria. Many plasmodesmata connect it to a wide neighbouring cell that has no nucleus. What is the narrow cell?
- Aa companion cell
- Ba sieve tube element
- Ca xylem vessel element
- Dan endodermal cell
Why is it an advantage that mature xylem vessel elements have no end walls?
- Athe elements form a continuous tube, so water flows with little resistance
- Bthe lack of end walls lets the cells actively pump water upwards
- Cthe lack of end walls allows sucrose to be loaded into the vessels
- Dit lets water leave the vessel sideways into the surrounding cells
A longitudinal section of a xylem vessel shows dark rings and spirals along its walls. What are these?
- Alignin thickening laid down in the cellulose wall
- Bsieve plates between neighbouring elements
- Cplasmodesmata linking the vessel to companion cells
- Dstrands of cytoplasm lining the vessel
A transverse section of an organ shows: an outer epidermis; a wide region of parenchyma; vascular bundles arranged in a ring, each with a cap of fibres on its outer side; and a central region of large parenchyma cells. Which conclusion is correct?
- Ait is a stem, and phloem lies between each fibre cap and the xylem
- Bit is a root, and the central parenchyma is surrounded by an endodermis
- Cit is a stem, and xylem lies just inside each fibre cap
- Dit is a leaf, and xylem lies on the lower side of each bundle
On a photomicrograph taken at ×400 magnification, the diameter of a xylem vessel measures 18 mm. What is the actual diameter of the vessel?
- A45 μm
- B4.5 μm
- C450 μm
- D0.045 μm
In an experiment, the companion cells next to a group of sieve tube elements were killed, but the sieve tube elements were left undamaged. Soon afterwards the sieve tube elements also died. Which explanation is best?
- Asieve tube elements have no nucleus and depend on companion cells for proteins and ATP
- Bsieve tube elements are dead cells and only appeared alive because of the companion cells
- Cthe sieve plates were blocked by lignin released from the dead companion cells
- Dxylem vessels stopped supplying the sieve tube elements with sucrose
Quiz key
The single central star-shaped block of vascular tissue shows this is a root, so the xylem is the central star at A. B is one of the groups between the arms of the xylem, which is phloem. C is the thick ring around the vascular tissue, the endodermis. D is in the wide cortex. Choosing B confuses the two vascular tissues; choosing C or D places the xylem outside the central core, as it would be in a stem.
Sieve tube elements keep perforated end walls, the sieve plates; xylem vessel elements lose their end walls completely. Lignified walls and pits belong to xylem, not to sieve tubes. Neither mature cell has a nucleus (the xylem cell is dead, the sieve tube element has lost its nucleus), so the absence of a nucleus does not distinguish them.
A companion cell has a nucleus, dense cytoplasm and many mitochondria, and is linked to its sieve tube element (the wide cell with no nucleus) by many plasmodesmata. The sieve tube element is the neighbour, not the narrow cell. A xylem vessel element is dead with no cytoplasm or mitochondria. An endodermal cell is in the root and is not paired with a sieve tube element in this way.
With the end walls gone, vessel elements join into one long open tube, so a continuous column of water can be pulled up with little resistance. Xylem cells are dead and cannot pump. Sucrose is loaded into sieve tubes, not xylem. Sideways movement happens through pits in the side walls, not through the missing end walls.
Lignin is deposited in the walls as rings, spirals or networks, strengthening the vessel against collapse under tension and waterproofing it. Sieve plates are end walls of phloem sieve tubes. Plasmodesmata link living cells, and mature xylem has no living contents. Xylem vessels have no cytoplasm.
Bundles in a ring around a central pith show a stem. In each stem bundle the order from outside to inside is sclerenchyma (fibre) cap, phloem, cambium, xylem, so the phloem lies between the cap and the xylem. A root has no ring of bundles or pith; its vascular tissue is a single central core. Putting xylem just inside the cap reverses the order. A leaf has a flat lamina with mesophyll, and its xylem is on the upper side.
18 mm = 18 000 μm, and 18 000 ÷ 400 = 45 μm. 4.5 μm and 450 μm come from slipping a factor of ten in the unit conversion (1 mm = 1000 μm). 0.045 is the actual size in millimetres (18 ÷ 400), so giving it in micrometres forgets the conversion altogether.
A mature sieve tube element has no nucleus and very few organelles, so it relies on its companion cell to make proteins and to supply ATP through the plasmodesmata; without it the element dies. Sieve tube elements are living cells, not dead ones. Companion cells do not contain lignin. Xylem carries water and mineral ions, not sucrose.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.