How a seedling knows "up"
 Q:
How does a seed know which is the right direction to grow, which is always up? -Nicole, aged 4½
A:
Gravity affects plants, as it does you and me. The plant senses gravity and tells its root to grow downward and its stem to
grow upward. The process is similar for both, although we don't understand all the details for the stems yet.
Right: [NASA, Jeffery D. Smith] Growing "up"
The root tip or "cap" has many special sensor cells, which contain heavy, tiny, starch grains (big black dots in Figure 1).
Gravity pulls the starch grains to the bottom of each cell because the grains are denser than any other bit of matter in the cell.
The sensor cells live at the bottom of the root cap. Figure 1. The heavy black arrow shows the down direction. Normally,
gravity affects all the sensor cells the same and they signal for the same growth on each side of the root. (Solid lines in Figure
1.)
 Left: Plant tipped over, dense grains falling
 Suppose you tip the plant over on its side. Oops! The plant has to adjust. Figure 2. The heavy
grains that were at the bottom of each cell now are at its side. Gravity pulls them to the bottom
again. The movement of the settling grains seems to trigger a chain of events: the calcium flow
in the cell becomes unevenly distributed. The uneven calcium distribution changes growth-hormone flow and this causes an uneven hormone distribution, which tells only the cells along the top of the root to grow.
(Dashed line in Figure 2.)
Right: New cells growing on side opposite the fall direction
Since only the cells along the top of the root grow, the root curves-downward, as roots should. Figure 3. .
Essentially the same process works for the stems, except the stems grow in the direction opposite to the falling starch grains.
The stem grows up.
What I've just described is the current theory. Researchers, both in space and on Earth, work toward understanding the process better and debates rage.
(Answered by April Holladay, science correspondent, August 8, 2001)
Further Surfing:
NASA: How plants sense and respond to gravity
|
