Sensing Up and Down

How do you tell the difference between up and down? Vision helps, but blind people still know up from down. We sense gravity mostly through our inner ear. Sensory hairs inside the inner ear are stimulated by small calcium carbonate crystals. As we move, those crystals brush against the hairs, sending electrical signals through our nervous system to the brain. The brain interprets the signals, thus sensing up and down.

We know that plants can determine up and down because, no matter which direction you plant the seed, the roots grow down as the stalk goes up. How does the plant know which direction to grow without a brain or nervous system?

Since ancient times, people have known that plants grow upward while their roots grow downward, but they didn’t know why. Plant growth is controlled by hormones, and the primary one affecting growth is auxin. Plants have specialized organelles known as amyloplasts that enable gravitropism, the ability to sense up and down.

Most plants have heavy amyloplasts at the tips of their roots, where they settle in response to gravity. The amyloplasts trigger the production of auxin. When higher levels of auxin are produced on the lower side of plant roots, the lower side grows downward in the direction of gravity.

A mysterious part of this system is that amyloplasts do not produce auxin. It is produced in another set of cells that are physically distant from the amyloplasts. Without a system of nerves connecting the amyloplasts and the auxin-producing cells, how does the directional information get to the auxin-producing cells? Scientists have not fully answered that question, but they are working on it. The Creator knows.

Auxin is also the major hormone causing plants to grow upward toward light. Phototropins are pigments that sense light and trigger the production of auxin at the tips of the shoots. The auxin is transported to the bottom side of the stem, where it stimulates growth and causes the stem to grow upward toward the light.

Sensing up and down, and many other functions of plants, are carried out without the benefit of a brain or central nervous system. We may think of plants as simple compared to animals, but they must be very complex to accomplish what animals do (including taking in nourishment, defending themselves, and reproducing) without being able to move from where they are planted. The design of living things, both animals and plants, shows that there is an intelligent Designer.

— Roland Earnst © 2026