What would happen if you plant a seed downwards. Will it grow in to the ground, upside down? Or will it find its way up? Does a seed know which way is up?
To answer all the above questions, we have picked the best rated explanations from the internet. Read them below.
Plants have cells that can tell which way is down. The roots know which way is down. The leaves/stems decide which way is up by sensing the light.
In the most simple of terms, plants have cells called stratocytes in the tips of roots that respond to gravity. They give a signal that releases a plant hormone that directs the growth of new cells.In the shoots and stems, a similar process occurs which directs growth upward. Mature plants don’t really have this ability anymore and thus use the angle of the light to “decide” where to grow. This is called phototropism.
A seed in utter darkness will know to grow roots downward, but a plant in utter darkness will not know which way to grow upward because there is no light for it to chase after.
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.
Plants can sense gravity because they have starchy components (amyloplasts) in some of their cells that are heavier than the cytoplasm, making them sediment at the bottom of the cell. A hormonal growth signal then emanates in the direction of the part of the cell where the amyloplasts accumulate (i.e. down).
You can see this for yourself by tipping over a fast-growing potted plant. The side of the stem facing down will grow faster, causing the shoot to gradually bend upright again. Plants use the same mechanism – uneven distribution of growth hormones in response to sensory input – when they grow toward light, etc.
It turns out that inside the seed there are specialized cells called statocytes that are mini plant snow-globes. Inside each one there are specialized starch grains that are more dense than the rest of the cell, and they settle toward the bottom of the cells [due to gravity.] Protein networks can sense where they are, and so the seed, and later the plant, knows which way is up. Next time you plant a seed, turn it over and think about the mini snow-globe inside, and then plant it whichever way up you like, because the plant can solve the puzzle.
Part of the reason is geotropism (response to gravity) and the other part is phototropism (response to light)
Geotropism occurs mostly in the root, and it is thought that the settling of starch granules at the bottom of cells causes elongation in that direction (towards gravity, i.e. downwards)
Phototropism occurs mostly in the shoot, where a chemical called auxin is released on the side opposite the light, causing cells on that side to elongate, and the plant grows towards the light
A plant that is healthy knows which way is up and down regardless of whether there is light or not. There are hormones that promote plant growth, IAA, that respond to light as well as other sensory responses. In roots, there are starches that settle to the “bottom” of the cells that tell the cell which way to grow (elongate) and divide. This is how the plant knows up and down. As for when light is absent, the sensory system previously mentioned still works, it’s just that the addition of light makes the plant produce IAA on the other side of the plant so those cells divide, turning the plant away from the production of hormones and toward the light.
I used to work in a lab and we grew our samples in complete darkness and they always grew against gravity (the shoot) there have been studies that show that indole acetic acid makes the shoot grow away from application. The top of the plant was cut off and then on half of the now exposed vascular system was treated with IAA and the plant bent away from the side it was applied to.