Hopefully you have already found out that the stem is responsible for transporting water and nutrients from the roots to the rest of the plant. I would like you to actually be able to see this if you can.
If you were able to access celery and or flowers and food colouring, I would like you to set up an investigation where you place the stem (with the leaves or flower still attached) in a cup of food colouring and water. You need to observe this over a couple of days and hopefully you will see some changes to the flower or leaves (if you are using celery). If you don’t see anything, perhaps add more food colouring or slice open the celery and look closely at the tubes that run through the stem.
Send in photos of your investigation.