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how do seeds know which way is up

How do seeds know which way to grow?

Plants need a different type of taxi to get their shoots up to the surface and their roots down in the dirt.

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Asked by: Eileen Kennedy, Abingdon

All plants can sense the direction of the gravitational field and orientate themselves accordingly. This is called geotaxis. In mature plants, phototaxis (growing towards the light source) overrides the gravitational impulse for the stalk and leaves, but the roots – and the seed while it is underground – rely on gravity for orientation.

The mechanism is thought to be based on either the protoplasm (the living substance inside a cell) exerting a greater pressure on the cell walls at the bottom, or starch grains within the cells settling at the bottom. One or both of these cues influence the production of plant growth hormones that cause the plant to ‘steer’ as it grows.

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Plants need a different type of taxi to get their shoots up to the surface and their roots down in the dirt.

Science Underground

How Plants Know Which Way Is Up

Imagine you are a seed. You are buried in the ground and it is dark.

You are starting to sprout and grow. But which way should you go? Which way is up?

Well, it ends up there are special cells in plants that allow it to sense the direction of gravity. These cells are sort of like a jar full of water with small rocks inside. If you throw in the rocks and seal the jar, the rocks will fall to the bottom. If you tilt the jar, the rocks will fall to the new bottom. Something similar is going on inside these cells. There are small rocks that fall to the bottom of the cell and tell the cell, this is where gravity is pulling. So, the roots know to go in that direction; and the shoots of the plant know to go in the opposite direction.

Scientists would call this ability of plants to sense gravity, gravitropism.

If you want, you can try a little experiment. Get a small plant, and put it on its side. In a few days, the plant will tilt upwards.

Now, this ability for plants to sense the direction of gravity is fine on earth were gravity is present, but what about on the space station, where there is little gravity? If we want to grow plants in space, in the right direction, we need to give plants other clues to know which way to grow. Fortunately, plants also grow in the direction of light too. And, recently astronauts have grown a flower in space.

Plants are budding with science and with special cells they know what’s up.

Science Underground How Plants Know Which Way Is Up Imagine you are a seed. You are buried in the ground and it is dark. You are starting to sprout and grow. But which way should you go?