Is there a plant I can use for a growth study that will show appreciable growth in two weeks from planting seeds? I would like it to grow fast enough to note differences between plants grown under different conditions every couple of days.
There are several candidates for this type of experiment.
It depends on what you are interested in investigating.
If you search our website for references to "duckweed" you will find growing details of a plant that doubles in size every three days under suitable conditions. This is good for investigations of the effect of light and nutrients on plant growth. I have used this with 12 - 14 year old pupils. Water cress (bought from a greengrocer) can also be used for this, but I would prefer duckweed.
However, it does not grow from a seed.
If you are also interested in recording the change in dry mass of the seed after germination and before green leaves are produced (as well as the growth after that), then you would get best results with a fairly large seed like a pea.
If you wish to get results every couple of days, including the root mass, then you will need a plant which doesn't take up too much space - for example mustard or cress, when you can harvest about 10 plants on each sampling day.
If you are interested in monitoring root as well as shoot growth, radish is an interesting plant to grow.