Take a bean seed, lay it on top of wet dirt and let it germinate. The root will come out of the seed and start growing down into the dirt. Lift the seed out of the dirt, turn it over, and lay it back on the dirt so that the root is pointed straight up into the air. The root will turn around and head down again into the dirt. Take a root and put it flat on top of the ground and then put it in a centrifuge that rotates the root at such a speed that the centrifugal force cancels the gravitational force (this device is called a clinostat); with as little as two minutes of gravity, the root will bend several degrees downward into the soil How do roots know which way is down?
For many years scientists have suggested that little packets of starch (which is quite dense) in the root cells fall toward the bottom of the cells and tell the root which way to go. Genetic mutations have been produced in plant cells in a plant called arabidopsis recently in which there is no starch in the cells. When tested, these roots still grew downward!
The space shuttle is being used to see if plants in a weightless
condition can still tell which way to grow when there is no gravity to
clue them in. At the present time we still do not understand how plants
know which way is up. Man's lack of knowledge of something as simple
as plant roots growing down instead of up should remind us that the
intelligence that designed and created the universe was far beyond our
own. It is difficult to have much arrogance about your own
understanding when even the simplest of things in the world around you
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