Heliotropism
Heliotropism, a form of tropism, is the diurnal motion or seasonal motion of plant parts in response to the direction of the sun. The habit of some plants to move in the direction of the sun was already known by the Ancient Greeks. They named one of those plants after that property Heliotropium, meaning
sun turn. The Greeks assumed it to be a passive effect, presumably the loss of fluid on the illuminated side, that did not need further study. Aristotle's logic that plants are passive and immobile organisms, prevailed. In the 19th century, however, botanists discovered that growth processes in the plant were involved, and conducted increasingly ingenious experiments. A. P. de Candolle called this phenomenon in
any plant
heliotropism. It was renamed phototropism in 1892, because it is a response to light rather than to the sun, and because the phototropism of algae in lab studies at that time strongly depended on the brightness. A botanist studying this subject in the lab, at the cellular and subcellular level, or using artificial light, is more likely to employ the more abstract word phototropism.