This is a Nutrient Pack filled with a gel with special nutrients and surrounded by a special membrane that protects the roots but allows for good gas exchange. Because the Nutrient Packs are little (2 inches cubed) a number of them can be placed side-by-side in a typical chamber as the one pictured here that carried four packs planted with seed into space. Here you see the plants after 10 days of growth in microgravity.

An advantage to the transparent gel is that it allows scientists to watch the root structure.
Can you tell which of the above was grown on STS-51 and which on the ground?