Thank you for the very interesting question!

Gravitropism is very important for plant growth because it maximizes the plant’s exposure to sunlight, which the plant uses to produce energy through photosynthesis.

Plants have a type of cell called a statocyte which can sense the direction of gravity. This is possible because of a special part of the cell called the amyloplast, heavy particles that “roll” towards the bottom the cell.

Signals sent by these cells direct the growth of a plant’s roots and stems. The biologist Charles Darwin was the first to demonstrate that roots show positive gravitropism: they grow in the direction of gravitational pull.

Conversely, stems show negative gravitropism, meaning they grow in the opposite direction of gravity. This accounts for why roots are underground and stems are overhead.

Now for the interesting experiment you propose! Should a plant be planted upside-down, it turns out that the amyloplasts will adjust to the new orientation and send signals that reverse the direction that the roots and stems grow typically in. This means the stems will grow down and the roots up! You can see an example of this in the amazing PHOTO I have attached to this message. Notice how the branches of this tree are growing down, meaning the roots are growing upwards.

You can try a variant of this experiment at home by turning a potted plant on its side. Given what we know about plants, we could hypothesize, or expect, that the stem will likely start to bend upwards in response to the pull of gravity towards the ground.

The stem will curl around and grow toward the new up.