Perhaps you've heard someone insult another person by saying that he can't tell up from down. We determine this so easily that it may not occur to us how we tell this. Pressure on the underparts of our bodies gives us a clue, but much of our ability to tell up from down is due to sensors in our inner ears. Buried in bony cavities within our skulls, small bodies of limestone-like calcium carbonate are attached to hair-like extensions of sensory cells. Attracted by gravity, these bodies tug on the hairs, telling the cells which direction is down. Of course, changes in the orientation of the head or the tugging on the hairs when the head moves suddenly also send important orientation information to the brain.
Plants also need to know up from down, and the general principle is the
same. Small, heavy objects, starch grains in the case of plants, sink to the bottom of
cells, informing the plant that that direction is down. Now, about that person who said
I have rocks in my head....
Listen to the Audio (mp3 format) as recorded by KTEP, National Public Radio for the Southwest.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.