Zones of Cell Division, Elongation and Differentiation in a Radicle

Aim

To identify and locate the zones of cell division, cell elongation and cell differentiation along a growing root (radicle) tip, using the ink-marking method.

Variables

• Manipulated variable: the position of the ink marks along the radicle, measured as distance from the root tip.
• Responding variable: the change in the distance between adjacent ink marks after a period of growth, which shows which zone is growing (elongating) the most.
• Controlled variables: the type and initial length of the seedling used, the spacing of the ink marks, and the temperature and moisture conditions during growth.

Materials and apparatus

• Germinating seeds with a straight radicle about 2 cm long (e.g. mung bean or maize)
• Waterproof marker or Indian ink with a fine brush or pin
• Ruler or a comb/template for even spacing
• Blotting paper
• Moist cotton wool or a petri dish lined with damp filter paper
• Magnifying glass

Procedure

1. Select a germinating seed with a straight, undamaged radicle about 2 cm long.
2. Gently blot the radicle dry using blotting paper.
3. Using Indian ink and a ruler or comb as a guide, mark a series of evenly spaced lines (e.g. 2 mm apart) along the radicle, starting from the tip.
4. Return the seedling to a moist environment, such as damp cotton wool or a petri dish lined with damp filter paper, with the radicle pointing downward.
5. Leave the seedling to continue growing for 24 to 48 hours.
6. Using a magnifying glass and a ruler, remeasure the distance between each pair of adjacent ink marks.
7. Compare how much each gap has increased in size.

Expected results

The gaps between the ink marks change by different amounts depending on their position along the radicle, because different zones of the radicle grow at different rates. Typical results are shown below.

Conclusion

Root growth in length occurs mainly in a zone just behind the tip, called the zone of elongation, rather than at the very tip itself. Near the tip, cells are actively dividing (zone of cell division) but have not yet elongated much, so those marks stay close together.

Just behind this, cells absorb water and elongate rapidly, so the marks there spread apart the most. Further back, cells stop elongating and differentiate into specialised tissues, such as root hairs, so the marks in that region change little.

This ink-marking method successfully maps the three growth zones based on how much each region of the radicle stretches, meeting the aim of the experiment.

Paper 3-style questions

Question 1 (hypothesis). A student wants to find out which part of a radicle grows most in length. State a suitable hypothesis.

Model answer. The region just behind the root tip (the zone of elongation) grows most in length, so the ink marks there spread apart the most after a period of growth.

Question 2 (variables). State the manipulated, responding and one controlled variable in this investigation. Model answer. Manipulated variable: the position of the ink mark along the radicle (its distance from the tip).

Responding variable: the increase in the gap between adjacent marks. Controlled variable: the initial spacing of the marks (also the seedling type, the temperature, and the moisture).

Question 3 (tabulation and graph). How should the results be recorded and displayed? Model answer. Record the original and final gap for each position in a table, calculate the increase in each gap, then plot a graph of increase in gap against distance from the tip; the peak marks the zone of elongation.

Question 4 (inference). The gaps near the tip and far from the tip changed little, but the gaps just behind the tip increased greatly. What can you infer?

Model answer. Most growth in length happens in the zone of elongation just behind the tip, where cells absorb water and elongate; near the tip the cells are still dividing and further back they have differentiated, so those regions elongate little.

Safety

• Handle the fine brush, pin or marker carefully to avoid pricking your fingers when marking the radicle.
• Wash your hands after handling seedlings, soil, and damp cotton wool.
• Keep Indian ink away from your eyes and mouth, and wipe up any spills.
• Take care with the magnifying glass in sunlight, as it can focus light and cause burns or fire.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions