Geotropism
Geotropism is also called gravitropism because plants use the gravitational pull for orientation. The roots grow into the soil (positive gravitropism) in the direction of the gravitational pull while the shoots grow in the opposite direction against gravity towards the light (negative gravitropism). Thus, plants (and also animals) use gravity to orient themselves in space.
If you take a plant out of the soil and hang it upside down, the roots and shoots (stems, leaves) will bend and reverse their growth.
Discover other important plant tropisms:
Gravitropism Perception
How do plants perceive gravitation? Scientific studies showed that in higher plants there are specialized plant cells called statocytes. And these statocytes contain specific amyloplast plastids that function as statoliths.
In animals, a statolith (also known as otolith in higher animals) is a dense granule of calcium carbonate that weights vertically down. Statoliths are in contact with hair cells and stimulate them. This provides animals information about their position in space.
The statoliths in plants are also denser than their cellular environment and their vertical sedimentation indicates gravity. The statoliths are found in root caps and in stems.
Plants in Space
But, what will happen if there is no gravity to which the plant can respond? Scientists have studied what will happen to the plant growth process in space. The results? At first the different plant parts did not know in which direction to grow because with gravity being absent they could not detect which way is down! The plant parts did all grow wild in any direction!
And now the amazing part comes: After a while, the plant growth normalised! The plants were able to compensate the lack of gravity as a reference point and used other external stimuli instead: The shoots, stem and leaves grew towards the light (positive
phototropism)
and the roots towards the water (positive
hydrotropism)!
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