Heliotropism was first described by Leonardo da Vinci (along with geotropism) in his botanical studies. The actual term "heliotropism," though, was introduced in the early 1800s by A. P. de Candolle, for the growth of the stem tip towards the light, which is now called phototropism. Now, however, the term heliotropism is used only for solar tracking, which is distinctly different from phototropism, or helio-directional growth: it is merely a temporary change of orientation, reverting in the dark of night.
Leaf heliotropism is the solar tracking behavior of plant leaves. Some plant species have leaves that orient themselves perpendicularly to the sun's rays in the morning (diaheliotropism), and others have those that orient themselves parallel to these rays at midday (paraheliotropism). Floral heliotropism is not necessarily exhibited by the same plants that exhibit leaf heliotropism.
Some solar tracking plants are not purely heliotropic: in those plants the change of orientation is an innate circadian motion triggered by light, which continues for one or more periods if the light cycle is interrupted.