Cognitive Science: An Introduction/Vestibular Systems
Vestibular systems are used by agents to balance and to know which way is up and which way is down.
Vestibular Systems in Humans and Other Vertebrates Edit
For humans, there are curved tubes in the ear with little rock-like objects that roll freely and touch sensitive hairs along the lining of the tube. The rocks fall to the bottom of the tube because of gravity, and, depending on which hairs are being touched, the mind gets a sense of up and down. When a human head moves, these rocks are tossed around, allowing the head to feel that it is moving in a particular direction.
Vestibular Systems in Plants Edit
Plants use vestibular systems; they it to know where to grow roots, trunks, and branches, for example. Roots must grow down, and branches and stems must grow upward. Where human gravity detectors are only in the inner ear, in plants they are at many places in the stems and root tips. They have cells that contain little balls called “statoliths,” which fall to the bottom of the cell, indicating which way is down. Humans and other vertebrates use a similar kind of thing called on “otolith.” 
- Groh, J. M. (2014). Making space: how the brain knows where things are. Cambridge, MA: Harvard University Press. Page 180.
- Chamovitz, D. (2012). What a plant knows: A field guide to the senses. Scientific American: New York. Pages 92--102.