We looked at plant roots this week, partly using a lesson from Mystery Science called “do plants eat dirt”. Learning about how plants grow is a great topic to cover in February as it makes it feel as though Spring really is just around the corner.
A fun way to find out about what plants really need to grow is by starting off a small hydroponic growing system. Ours is from IKEA as I wanted something that worked at the dark end of our long kitchen, but if you have a bright sunny spot you can make one very simply from a container such as a plastic tub or a milk jug and a few net pots of the sort pond plants are grown in. An even simpler and faster option is to place plant parts into a jars of water and see what grows. For example the little side shoots (called slips) from a bunch of carnations will root and provide a set of plants for your garden, and the base of a lettuce will root and re-sprout to produce a small second crop of leaves.
I ignore the IKEA instructions to grow seeds in rockwool, and instead sow onto perlite. I discussed with the boys how roots serve two main purposes for the plant. The first is to ‘suck up’ water and nutrients dissolved in the water, and the second is to anchor the plant so that it doesn’t fall over. The perlite provides no nutrients and is simply there to allow the roots something to anchor into as they grow. The boys are able to add water and liquid plant feed to top up the tank and lift the pots at regular intervals to inspect how the roots are growing. I told the boys about Jan Baptist van Helmont’s experiments in France in the 17th Century which showed that there was no reduction in the mass of soil in a pot in which he grew a willow tree for five years, although the plant itself had gained considerable mass. The boys were able to draw the conclusion that the plant couldn’t therefore have ‘eaten’ the soil. They decided the growth of the tree must have come from the water. I praised this idea – this is what van Helmont had thought, and then asked them if they remembered anything about photosynthesis which we looked at briefly some time ago. In a lightbulb moment Ollie remembered about plants needing ‘something in the air’ to grow too.
Once we had covered what plants need to start growing and keep growing, we wanted to see it in action. Sowing some seeds that sprout quickly, such as cress, onto damp cotton wool enables a way of seeing how roots grow from a seed over just a few days. Bigger seeds are easier to see, but take longer to sprout, so you may decide to experiment with a range of different sizes of seed. I downloaded a free animation app onto my phone which means that the boys can take images of the seeds every day and then run the animation at the end of the week as an easy home made version of a time-lapse film such as the one here from BBC Bitesize. If you want to set up this activity so that a time-lapse can be made very quickly, you could sow one pea seed every day for a week into something like an ice cube tray with each cell lined with damp cotton wool. You will then have a succession of seeds in different stages of growth to photograph in one go.
If you want to see mature roots without growing anything from scratch or disturbing a garden plant we
found that a pot of parsley from the supermarket is ideal. These supermarket herbs are actually a whole group of small plants squeezed together and it is easy to tease them apart to have a good look at the structure of the root, stem and leaves.
Some method of magnification is useful when looking at small structures such as roots, i.e. a magnifying glass, magnifier app or microscope. Different plants have different root forms, so it’s worthwhile drawing attention to this. You could compare parsley to it’s relative the parsnip and talk about another function of some roots, which swell to store carbohydrates.
The boys are at an age where it is enough to handle, observe and have a go at describing the roots of the plants we have looked at, but as they get older this kind of investigation lends itself well to extension into measuring plant growth under different conditions, producing tables and graphs of data, as well as tables or Venn diagrams of the benefits and drawbacks of growing in soil versus a hydroponic system. It could even take them all the way into space science and science fiction as they investigate how plants could be grown on a voyage to another planet.
The investigation this week concluded with making lunch out of the hydroponic experiment. Next stop food chains!
For more information:
Van Helmont’s experiment – http://www.bbc.co.uk/education/clips/zpgb4wx
More detailed info on root biology – http://facweb.furman.edu/~lthompson/bgy34/plantanatomy/plant_root.htm