Big Idea 3, #2, 21. Ethylene, Tropism. Ethylene is an odorless, colorless gas made of hydrocarbons. It's often used in agriculture to "force-ripen" fruit to sell, like these grapes. Tropism is the turning growth of a plant in response to a stimulus. As you can see with this tree, it is bent to put it in a position where it will get more sunlight.