Sunflowers are among several plant species – especially desert plants – that
have the ability to either grow or move in response to stimuli from sunlight to
maximise the amount of direct solar radiation received for photosynthesis and
growth. Known as heliotropism, the phenomenon can either make a plant move
to face the Sun (diaheliotropism) or away from sunlight (paraheliotropism).
The plant equivalent of a muscle is the pulvinus, which is a specialised organ
found at the base of the leaf. The pulvinus consists of extensor cells (for
stretching) and fl exor cells (for bending), which swell or shrink in response to
changes in turgor (pressure) determined by the amount of water in the cell. As
extensor cells swell and fl exor cells shrink, the leaf can move to track the Sun’s journey across the sky, from sun up to sun down.