How do plant seeds know which way is up?

Question

For my molecular biology project, I chose to experiment with pea seeds in order to answer this question. My hypothesis was that the seeds knew beforehand which way the sun was coming from, and the hypocotyl (the part of the plant between the seed and sprout) would come out of the seed at 0 degrees regardless of the seed's orientation.

The other option was that the plant figures it out as it goes along, meaning the hypocotyl and not the seed itself can sense gravity.

In my experiment, I placed the seeds with the vertex facing four different directions: 0, 90, 180, and 270 degrees. For the most part, the seed hypocotyl came ou at either 180 or 270 degrees every time. This meant the seed knew beforehand.

However, not only did the experiment fail to answer my original question, but also it left me with more!

Answer 1

Plant seeds react to gravity.

Answer 2

There's trace amounts of silica in plant cells. These are sensitive to gravity and know up from down. To test this out, take 4 corn kernels (available from any seed and gardening store), put them on tape with the tip pointing up, down, to the right and to the left. This works really well if the tape is athletic tape. Then, put the tape in a shallow dish with a little bit of water (don't soak the seed, just enough to get it wet). Water is needed to break the seed coat and induce germination. After a day or two, rest the tape against the wall of the bowl. Within a week, you'll see that no matter which way the tip of the corn kernel is oriented, the root is coming out and is pointing down.

Answer 3

There are two likely possibilities.
First, there are tiny particles of starch in plant cells that settle to the bottom, due to the pull of gravity. This grouping of starch may tell the plant what it needs to know.

Second, the heavier fluid contents of cells are pulled toward the bottoms of cells, also by gravity. This puts greater pressure on the lower portion of cell walls. This pressure could be the switch that tells cells which way is down.

The two possibilities are similar, since they both rely on gravity to pull heavier cell contents to the bottom of cells. How can NASA help determine which is the correct signal?
In orbit, experiments can be performed without the effects of gravity. Scientists are unable to duplicate this condition on Earth, except for very short periods of time. For instance, a plant container could be dropped down an elevator shaft. While falling, the plant would be in free fall, and not affected by gravity. But we can't build a shaft deep enough for a plant to germinate and grow while falling. The solution is a spacecraft orbiting the Earth. Objects in the craft are constantly in freefall. So the starch grains and heavy fluid contents in plants in space will just float randomly.

So far, so good, but we still need a way to trick just the starch or just the heavy fluids into thinking there is gravity present. Then we can tell which determines the direction of plant growth. A clever new experiment designed for mission STS-107, slated to fly this year, will do just that. And the way it works is by using magnets!
The magnets will play the role of gravity, but just for the starch grains. Starch grains are not magnetic, but they do form a type of magnetism called diamagnetism. This is a weak repulsion in a magnetic field. This weak repulsion will allow the scientists to 'move' the starch grains much like gravity would move them. On the other hand, the heavy fluids won't be affected by the magnets at all.

If the plants grow in a pattern along with the magnetic field, scientists will know that it is the starch grains that tell plants which way is up. If they grow in random patterns, it is likely that cell pressure is the key.

Answer 4

Have you ever gone to a nursery and seen one of those upside-down tomato plant pots? They are suppose to save space or something. Anyway, I used one once and the plant did not grow up, it grew down and around like any plant would if the sky and the ground shifted. Plants grow towards light, dear. Nothing senses gravity, it feels its force. Do you sense gravity? Didn't think so. Plants don't 'know' anything either. They have no brain.

My second example is those twisted bamboo plants you can buy at stores. The gardeners spend time rotating a bamboo plant around a light, shifting its direction every time they make progress towards the light. The plant doesn't grow up, it grows towards a light. Now, the azaleas and rose bushes in your yard grow up because that is where the sun is.

Answer 5

Ach, don't listen to the people who say plants use light to determine which way is up. Plants use gravity (gravitropism). To test this, germinate a number of seeds in complete darkness. Roots still go down, shoots still go up.

Answer 6

makes no difference, when it germinates the growing shoot reaches for the sky

most seed fal on the ground in any direction
there are some seeds that land like arrows ,but stillthey get knocked over by falling organic waste material ,like leaves ,and still the shoots emerge looking for light

Answer 7

This is called phototrophism the plumule move towards the sunlight.

Answer 8

They feel the warmth from the sun and grow towards it

Answer 9

It's just one of those things God designed.