Why do plants have roots?

My sweet boy and mother came home from a farmer's market with these sunflowers.

We were sitting at dinner last night and discussing how pretty the sunflowers were and what they need in order to last a long time out of the ground: sunlight and water. Plants' food comes from the sun.  It takes sunlight and convert it to energy (like we convert our food we eat into energy).  Plants also need water, like we need water.

It turns out potted sunflowers around here are a hot item for the neighborhood rodents. All three of our homegrown ones never made it to where we could harvest the seeds.

Anyways, J stated that he wanted to put the sunflowers he got from the market in the ground. We had to convince him that we'll have to wait until next spring to plant/grow new sunflowers in our pots.

Here's our two minute science lesson:

• Plants in the ground have roots, which, besides drink the water and absorb the nutrients from the soil, act like big giant feet to hold the plant up. Without roots, plants tip over and don't get the water and nutrients (like vitamins) they need, which means they won't last long at all.
• If you have the resources, you can try to "plant" a cut flower and have your kid observe how it acts. Why isn't it working like a plant you've grown from a seed?

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We're having Science Saturday in the Park this Saturday, September 29, 10a-noon, in the SF Bay Area (CA). Feel free to contact me for further details. We'll be exploring bones and muscles and making our own functioning elbow models that you can take home. This activity is good for ages 2-100.