Write the name of each plant on a separate notecard. You may want to start out just working with plants that are very different in appearance, then work up to similar species.
Glue the picture of the plant that matches the name to the blank side of that plant's notecard. A real picture is better than a drawing, but if you need to you can draw the plant using colored pencils.
Select a card randomly from the notecards. The person with the notecards can be another person preparing for the test or anyone that wants to help the student study.
Have the guesser begin asking yes-or-no questions about the plant. These questions might include "Does it have serrated leaves? Is it yellow? Does it have 5 petals?" As you go, the guesser may want to record the answers on a notepad or even try to sketch the plant. This can continue until the guesser has reached 20 questions, at which point he must indicate what the name of the plant it.
Reveal the plant's identity. If the guesser is correct, then remove the notecard from the pile. If she gets the identity wrong, then keep the card in the cycle of notecards until she gets it right.