statolith

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One of the small movable calcareous particles in a statocyst.
• noun One of the starch grains or other solid particles in a plant cell thought to function in the plant's response to gravity.

from The Century Dictionary.

• noun A small stony secretion or an accumulation of particles of sand contained in an open or closed sensory sac or lodged in a sensory pedicel of some actively locomotor invertebrate (cœlenterate, ctenophore, worm, mollusk, or arthropod), and so situated that it enables the animal possessing it to become oriented with reference to the center of the earth by responding to the pressure it exercises through the pull of gravity.
• noun In botany, a movable starch-grain occurring in the cell sap of some cells, supposed to correspond in function with bodies so named found in the statocysts of animals. See s˙tatoplast.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A specialized form of amyloplast involved in graviperception by plant roots and most invertebrates.