Geraniums have been popular garden perennials for a long time, and have lots of devotees. Consequently, there are lots of good sources of information on the internet. A good start is provided by the Open Directory Project's geranium category. For a primer on erodiums, this page from the UK gives a good overview.
Stimulus control is the phenomenon of a stimulus increasing the probability of a behavior (operant response) because of a history of that behavior being differentially reinforced in the presence of the stimulus.
A conditioned stimulus is a neural stimulus that gives the same response as an unconditioned stimulus. The effect requires repetition in order to be learned, and over time, a neutral stimulus becomes paired with the unconditioned stimulus and is transformed into a conditioned stimulus. A classic example of this phenomenon is Russian scientist Ivan Pavlovâs experiments with dogs.