Cress and environmental stress (printer friendly)

These protocols are thought to be suitable for S1/S2 pupils but all schools should conduct the necessary risk assessments before performing the experiment.

Objective
To show that extremes in environmental conditions (in this case concentration) reduces growth of the germinating seeds
around an optimal value, by growing cress seeds using water containing a range of solute concentrations.

Context in real-life
Living organisms are dependent on their physical environment, to sustain their growth and reproduction. Changes in the
quality of the environment (pollution) or simply the naturally occurring diversity in environmental conditions (e.g. an estuary
or coastline versus a freshwater pond or a river) affect plant physiology, often through alteration of simple processes such
as osmosis.

Links with the curriculum
‘Give examples of inherited and environmental causes of variation’
‘Give examples of physical factors that affect the distribution of living things’
Kit needed
• plastic cups/trays • cress seeds
• cotton wool • ruler
• salt and water

Description of experiment
1. Prepare the cotton wool in order to provide a humid environment for the seeds to germinate. For instance, lay at the
bottom of a cup/tray and humidify with the different dilutions of water/salt (e.g. start with normal tap water and salty
water).There is degree of flexibility in terms of germinating tray design.
2. Spread the same number of cress seeds of the cotton wool (e.g. 15), making sure they are approximately evenly spaced
(e.g. 5 mm from each other).
3. Within a week the seeds will have germinated and the influence of the water quality can be assessed by (1) counting
proportion of germinated seeds for each treatment (2) measuring shoot length (plant growth).

Likely outcomes and what they mean
Cress seeds left on humid cotton wool for a few days will start growing. Disruption of osmotic processes through changes
in water salt content will affect the growth/germination rates. After one week, the seeds in salty water have not germinated
whereas the ones in tap water have turned into beautiful little plants.

Opportunity for extension
• use distilled water for a higher hypotonicity
• use other solutes (e.g. sugar)
• seeds left to germinate in the dark may have different growth/germination rates (impact of light on photosynthesis) for a
given salt concentration.
• use a large range of solute concentrations to experimentally determine the salt concentration isotonic with that necessary
for seed development.
• try germinating other seeds (beans, lentils). Experience might reveal that different seeds will have different optimal
germination salt content (tolerance or preference to extremes in environmental conditions.)

Note: The potato and cress experiments illustrate the same phenomenon, either at the tissue level (potato stick) or at the
level of a physiological process (cress seed germination).


This activity sheet is one of several developed by early career scientists at the MRC Human Reproductive Science Unit in
Edinburgh. You can download all the activity sheets at www.hrsu.mrc.ac.uk/schools. If you experience problems accessing the
website, please email schools@hrsu.mrc.ac.uk.