These occur (a) in animals (eating, sleeping, mating, hibernating, migration, cellular regeneration, etc.
They have even been found in bacteria, especially among the cyanobacteria (aka blue-green algae, see bacterial circadian rhythms).
In some cases, multi-trophic systems may exhibit rhythms driven by the circadian clock of one of the members (which may also be influenced or reset by external factors).
The endogenous plant cycles may regulate the activity of the bacterium by controlling availability of plant-produced photosynthate.
Goldbeter's book[2] provides a thorough analysis of the biochemical mechanisms and their kinetic properties that underlie biological rhythms.