Lesson 1
Gravity cells in the root cap
The very tip of a root wears a little cap, called the root cap. In the middle of the cap is a column of cells called columella cells. Many of them are statocytes.
Each statocyte holds a handful of heavy grains packed with starch. These grains are called amyloplasts. Because they are heavy, they sink to the bottom of the cell, like sand settling in a jar of water.
Try thisPress Find down on the 3D model to light up the heavy grains resting at the bottom of the cell.
Lesson 2
Telling the root which way is down
The cell can tell where the grains are resting. That side is down!
- If a root gets tipped on its side, the grains slide to the new low side.
- The cell sends a signal back up the root.
- The root grows unevenly and bends until its tip points down again.
Growing down helps roots find water and hold the plant tight in the soil.
Try thisSprout a bean in a clear jar with a wet paper towel. When the root is a few centimeters long, turn the jar on its side. Check over the next day or two: the root tip will curve down!
Wow!Astronauts have grown plants in space. With almost no gravity, roots can't tell down, so they wander in different directions!