We would like our students to investigate the effect of planting density on the growth of plants. One suggestion was to sow increasing numbers of cress seeds onto moistened filter paper in Petri dishes. Students would then measure the height of 5 seedlings in each dish after a period of time, e.g. 1 week.
My experience with cress is highly variable. I find that the filter paper dries out incredibly quickly once the seeds have germinated and the lids are removed. Also, just watering the seeds does not provide sufficient nutrients for the cress to grow properly. How could we improve this idea to give us good results? What would be the best seeds to use? Mustard? Radish? Would it be better to use cotton wool in specimen tubes to resist drying out?
John Hewitson says:
Normally, our instinct is to try to produce ideal growing conditions for our experimental material so that nothing is limiting growth.
However, in this case, the whole point is that something (light? nutrients?) should be competed for by the plants. As you suggest, we probably don’t want water to be the limiting factor (this would normally vary from dish to dish unless we are very careful).
If we use a big seed, like radish, then each seed may be providing sufficient of its own nutrients (energy/carbohydrate as well as minerals??) and within a week, we may not detect much difference due to competition.
I would go for a small seed - I think the SAPS team would favour mustard over cress.
A positive point is that you're hoping the results should be visible within a week. This gives time for you to do a trial or two. (A very sparse and a very dense treatment?) I would suggest at least 3 filter papers per petri dish (use lids - filter papers fit inside better) - indeed, cotton wool would hold more water. Try them both! (Do facial pads just fit a small petri dish?) I have found it best to water each dish with a pipette (which can safely and quickly be used by mouth for distilled water). Will you use the SAME amount per dish as the seedlings grow, or will you aim to water each to the same level of dampness? …. discuss! How big a problem is “drying out” in YOUR laboratory (not over the radiator!)?
Of course, each dish should receive the same amount of light and perhaps bright light from an overhead light bank may NOT be the best to illustrate competition (you would like to see some mutual shading).
We would be most interested to hear whether you get a measurable effect within a week. If you don't, then the discussion will be WHY? What further experiments could be done to find out WHAT is (or is NOT) limiting growth as density increases? Teacher, be prepared to "rescue success from the jaws of failure”! Taller when closer together (etiolation) or shorter as growth is limited by light? …. discuss, predict, hypothesise!
I wouldn’t like to have to predict the outcome of these experiments! A true investigation.
Sue Hunt says:
Seed choice: There are fast germination (less than 48 hours) and fast grow commercial varieties of cress and mustard which can be purchased in garden centres.
Drying out: To prevent drying out of the filter paper either use:
- 1% agar about 4 mm depth in a petri dish and sow seeds directly on the top making sure the lid stays on until germination and then removing the lid to allow vertical growth. There is sufficient available moisture in the agar to stimulate germination and growth. There is no available nutrients in the agar.
- use water sodden filter paper. Once again keep the lid on until germination and then remove. Place a wick from the filter paper in the petri dish to a water reservoir. Eg those blue jay clothes cut into a strip going from a beaker to the petri dish filter paper. This will automatically keep the filter paper moist and one beaker can serve a number of pertidishes.
Nutrients aren’t needed as the young seedling are using that in the cotyledons and don’t need any supplementation.
1 week should be sufficient to grow cress, however, competition from density will be poor or nonexistent in one week. Just think of the density of cress growth in the supermarket cartons! Any density effect will be apparent as the seedlings move to plantlets which I feel will take longer than one week.
Extra height may not be a positive correlation as the faster growers may become etiolated and not be able to stay upright. I don’t know the answer to this.
Variables to be controlled: Amount of water, nutrients/cotyledon size/temperature/light intensity/day length/number of seeds/.
For competition to occur there needs to be a stress. In the first week, there is no stress.
There is a lot that can be considered with this experiment and it looks to be a simple platform for a good investigation and test of knowledge and understanding.