Site author Richard Steane
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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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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.
EXPERIMENTAL TREATMENTS | COMPONENTS OF MIXTURE | |||
v | NITRATE | PHOSPHATE | POTASSIUM | MAGNESIUM |
Lacking NITROGEN (-N) | No | Yes | Yes | Yes |
Lacking Phosphorus (-P) | Yes | No | Yes | Yes |
Lacking Potassium (-K) | Yes | Yes | No | Yes |
Lacking Magnesium (-Mg) | Yes | Yes | Yes | No |
Lacking ALL (-ALL) | No | No | No | No |
"COMPLETE" (-NONE) | Yes | Yes | Yes | Yes |
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
Procedure
1) Take a piece of plant from the material provided for your team.
HANDLE IT WITH GREAT CARE. Note its name. >
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.