Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

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In this investigation, you will set up an experiment to which you will refer again during the next few weeks. The aim will be to demonstrate why plants need certain mineral elements, also known as inorganic ions. This experiment could very well be used as a way to investigate the effects of environmental variation on clonal plants.

This is another example of a control experiment, in which you will attempt to find out the effect of a single factor, which usually works in connection with others.

You will place plants into different liquids (solutions). Each liquid will contain an almost balanced mixture of elements, but some solutions will lack one specific element, as shown in the table below. The "controls" will either lack all, or none.

The class experiment can also include different plants, as long as each gets the same 6 treatments. Each "team" of 6 pupils will manage an experiment involving a single plant type.

After allowing the plants to grow for some time, you should be able to see the effects of each element, by making comparisons between the plants grown with and without it.

Lacking NITROGEN (-N)NoYesYesYes
Lacking Phosphorus (-P)YesNoYesYes
Lacking Potassium (-K)YesYesNoYes
Lacking Magnesium (-Mg)YesYesYesNo
Lacking ALL (-ALL)No NoNoNo

What basically is the liquid in the "lacking all " treatment?
> Distilled water

What other substances ought to be added to every treatment?
   >Trace elements

Why is it important that each experiment is confined to a single type of plant?
   >To enable a valid comparison between experimental treatments (minerals) rather than between types of plants

Why do you think we are using cuttings, not plants in pots?
   >There is no soil involved - which would already contain mineral salts


1) Take a piece of plant from the material provided for your team.

2) If necessary, prepare it by removing lower leaves from its stem. Each member of the team should standardise their preparation, so that each plant ends up looking similar.

3) Measure (and record) the length of the stem. >

4) Count (and record) the number of leaves easily visible. >

5) In conjunction with others in your team, decide on which treatment you will do, and mark a boiling tube appropriately. Note the treatment here, too: >

6) Fill a boiling tube with the appropriate liquid.

7) Place the cut end of the plant into the liquid, and make sure there is enough stem projecting into the liquid. Put the tube away in the rack or container provided, until next week.


At regular intervals perform the following checks, but do not disturb the plant if at all possible:
a) Ensure that the base of the plant is covered with liquid, and top up the tube as required. Make sure you use only the same liquid you started with!
b) Note (in diary form) any changes, such as growth of stem, leaves and roots, loss of leaves, flowering, etc.

At the conclusion of the experiment, you will count the number of leaves and measure the stem length once again, then all the teams' results will be collected together for comparison.

How would you change the experimental procedure if you wanted to obtain more precise information about the effect of these minerals on leaf or root growth?
   >Weigh the plants at the start and end of the experiment - possibly calculate the (final!) dry mass.
   >Calculate the leaf area by tracing round them regularly.
   >Log the amount of solution needed each time the tubes are topped up.

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