Meristems are the growing points of a plant, where cells divide by mitosis. To interpret meristem evidence, separate where cells are made from where they enlarge.
This lesson follows comparing primary and secondary growth in the plant organisation and growth chapter. If mitosis is hazy, revisit sequencing mitosis.
What happens behind a root tip?
Think of a root tip in three regions, read from the tip backwards.
- Meristem zone: cells divide by mitosis and make new cells.
- Zone of elongation: the new cells take in water and enlarge, which pushes the root tip forward.
- Zone of differentiation: the cells become specialised, such as xylem and phloem.
Worked example: ink marks on a root
A student marks a bean root with ink at 2 mm intervals from the tip, making five segments. After 24 hours the student measures each segment again.
| Segment (from tip) | Start length (mm) | Length after 24 h (mm) | Increase (mm) |
|---|---|---|---|
| 1 (0 to 2 mm) | 2 | 2.5 | 0.5 |
| 2 (2 to 4 mm) | 2 | 5.5 | 3.5 |
| 3 (4 to 6 mm) | 2 | 3.0 | 1.0 |
| 4 (6 to 8 mm) | 2 | 2.0 | 0 |
| 5 (8 to 10 mm) | 2 | 2.0 | 0 |
Describe. Segment 2 increased the most, by 3.5 mm. Segments 4 and 5 did not change.
Explain. Segment 2 lies just behind the tip, where newly made cells enlarge. Segments 4 and 5 have cells that are already mature, so they no longer lengthen.
Conclude. In this root, most lengthening happens 2 to 4 mm behind the tip. Check the total: 0.5 + 3.5 + 1.0 = 5.0 mm, which matches the root growing from 10 mm to 15 mm.
The mistake that costs marks
The common slip is to say “the marks moved apart in segment 2, so the meristem is in segment 2”. The marks show elongation, not division.
| Wrong | Right | |
|---|---|---|
| Segment 2 | Meristem is here | Cells enlarge here |
| Meristem | Where the root gets longest | Near the tip, where cells divide |
| Evidence needed | Ink marks | Dividing cells seen under a microscope |
To show division you need evidence of mitosis, such as cells caught in the stages of mitosis at the tip.
Check yourself
A student examines a root tip under a microscope. The cells at the very tip are small and closely packed, and some are dividing by mitosis. Cells 5 mm back are long and narrow with large vacuoles. What does each region show?
Answer
The small cells at the tip with mitosis in progress show the meristem, where cells divide to make new cells.
The long cells with large vacuoles 5 mm back have enlarged, so they show the zone of elongation. This is where the increase in root length mostly comes from.
What to study next
Continue with reading plant-growth investigation data, where you practise describing and concluding from tables. The biological data interpretation trainer gives extra tables to describe.
Log any slips in the mistake log and paper-error review. For a teacher to go through your conclusions, see online one-to-one Biology tuition.