Why do Autumn leaves fall?

What triggers leaves to fall in autumn (fall) and new growth in spring?

• The possibilities seem to be change in day length or temperature, or a combination of these. Or do plants have an inbuilt clock?

• Answer:

  There is a good deal of variety from plant to plant, but on the whole plants have homologous circadian rhythm genes to those of humans and other animals (tim, per, etc. http://www.sciencemag.org/content/286/5440/766.abstract). Plants use this same rhythm to measure the length of the day.  You can imagine a circadian protein being produced only during daylight, therefore accumulating throughout the day.  Another gene is inhibited by the light produced gene product, so as the days get shorter the inhibition drops.  At some level the second protein is no longer sufficiently inhibited and starts a program of dropping leaves, etc. Temperature also plays a role in most plant clocks.  You can imagine low temperatures would trigger a protein that could interact with the same circadian rhythm genes by inhibiting the same light produced protein, thus making the plant think the days are shorter than they actually are if it is still cold.  This is a simplification of the diverse molecular pathways plants use to maintain circadian rhythms, but hopefully conveys the general idea.