Plant sensitivity

Are plants sensitive? Well they might not have a nervous system but they do respond to stimuli, specifically light, gravity and water.  We call these responses Tropisms.
Here we have a geranium that has been left on a windowsill.  The plant has responded to the direction of light; it is said the be positively photo tropic.
The light alters the distribution of auxin, a chemical that promotes growth, so that one side of a growing shoot grows faster than the other.  In this way the shoot bends towards the light.