This is a plant (usually a flower) that turns itself to face the sun as it moves across the sky.
“Helio” is Greek for “sun” and “trope” is derived from the Greek word for “turn.”
However, the effect is more accurately known as “phototropism,” because the flowers are actually tracking light, not the sun. The same effect can be produced with artificial light.
There is a less-common usage for a particular mineral in the quartz family.
Why I Looked It Up
In “The First Man in Rome,” I found this:
Every one of the thousands of faces in The Forum turned to him as heliotropes to the sun…