I am a gradute student in the Department of Plant Sciences at Cambridge University. I work on circadian rhythms in Arabidopsis, focussing on the signalling mechanisms linking the molecular 'clock' to plant behaviour. That has probably confused you already! - I will write both a scientist's and a non-scientist's explanation of it at some point to help you out!
One of the main methods I have used thus far is leaf movement analysis - this movie (.avi) is of a 10 day old seedling which has grown in light-dark cycles and then been transferred to continuous light. How cool is that!!
If you want to know more about our lab please visit our group's website, although it hasn't been updated in a while and the layout is horrible - maybe once I've mastered my html skills I will do something about it!
Hubbard KE, Hotta CT, Gardner MJ, Baek SJ, Dalchau N, Suhita D, Dodd AN, Webb AAR (2007) Circadian rhythms in stomata: physiological and molecular aspects. In Rhythms in plants: phenomenology, mechanisms and adaptive significance. S. Mancuso and S. Shabala (Eds.) Springer-Verlag, Berlin. pp 150-172
Hotta CT, Gardner MJ, Hubbard KE, Baek SJ, Dalchau N, Suhita D, Dodd AN, Webb AAR (2007) Modulation of environmental responses of plants by circadian clocks. Plant, Cell & Environment 30:333–349 (.pdf)
Gardner MJ, Hubbard KE, Hotta CT, Dodd AN, Webb AA. (2006) How plants tell the time. Biochemical Journal 397:15-24 (.pdf)