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What size test tubes are these? There are a lot of plants that will grow just in water - think hydroponic systems. Usually there is an inert medium for the plants to root in and an exchange of water baring nutrients.
In test tubes, you could root small cuttings of your local plants: Ivy, coleus, any plant that enjoys water and roots readily. The plants will quickly outgrow your test tubes so you will have a constantly changing show.
