Have you ever noticed that when you put a plant on a windowsill, it turns to look out at the sun. So you turn it back around so the pretty part is towards you. The next day or two, your plant starts to ignore your wishes and turns around again.
phototropism – Growth or movement of a sessile organism toward or away from a source of light. (dictionary.com)
Well, it doesn’t help when they use words you don’t know in the definition. So–
sessile – Permanently attached or fixed; not free-moving