How does a seed know where 'up' is?

Really, this question has been bugging me some time now. How does a seed know where 'up' is? What mechanism is it that tells the seed that are 20cm under dirt that they should grow in some specific direction? It cannot be water, and I suspect that heat also would not do the trick.

Gravity.

Roots go down.. Shoot goes up.

Once the shoot is out, it follows light.

Could you not have googled this? Its simple plant biology.

It has to do with gravity and something called positive or negative geotropism:

Plants can sense the Earth's gravitational field. Geotropism is the term applied to the consequent orientation response of growing plant parts. Roots are positively geotropic, that is, they will bend and grow downwards, towards the center of the Earth. In contrast, shoots are negatively geotropic, that is, they will bend and grow upwards, or away, from the surface.

These geotropisms can be demonstrated easily with seedlings grown entirely in darkness. A seedling with its radicle (or seedling root) and shoot already in the expected orientation can be turned upside down, or placed on its side, while kept in darkness. The root will subsequently bend and grow downwards, and the shoot upwards. Because the plant is still in darkness, phototropism (a growth movement in response to light) can be eliminated as an explanation for these movements.

Several theories about the manner by which plants perceive gravity have been advanced, but none of them is entirely satisfactory. To account for the positive geotropism of roots, some researchers have proposed that under the influence of gravity, starch grains within the cells of the root fall towards the "bottom" of the cell. There they provide signals to the cell membrane, which are translated into growth responses. However, there have been many objections to this idea. It is likely that starch grains are in constant motion in the cytoplasm of living root cells, and only "sink" during the process of fixation of cells for microscopic examination. Roots can still be positively geotropic and lack starch grains in the appropriate cells.

A more promising hypothesis concerns the transport of auxin, a class of plant-growth regulating hormones. Experiments since 1929 have shown that auxin accumulates on the "down" side of both shoots and roots placed in a horizontal position in darkness. This gradient of auxin was believed to promote bending on that side in shoots, and to do the opposite in roots. Confirmation of the auxin gradient hypothesis came in the 1970s. When seeds are germinated in darkness in the presence of morphactin (an antagonist of the hormonal action of auxin), the resulting seedlings are disoriented—both the root and shoot grow in random directions. Auxin gradients are known to affect the expansion of plant cell walls, so these observations all support the idea that the transport of auxin mediates the bending effect that is an essential part of the directional response of growing plants to gravity.

science.jrank.org...

Here you can also read about growing plants in Zero-G and experiments done on the ISS:
spaceflight.nasa.gov...

lol I'm a horticulturalist and I don't know the whole answer

Simply put, the seed, from the start is programmed to know what's up and down.

Same as it knows when to germinate

The root goes down in search of water and the shoot comes up in search of sunlight. That is all. It is really simple.

Its not just gravity.
In the stages of a seed.
It is chlorofil.
Its humidity, temperature, nutrients and air.
That guides the organism, upwards.
If these things were dramatically changed.
The organism(seed) would not grow at all.
And if they were changed in another certain way.
It would grow sideways, upside down etc.
For example in hydroponics, some grow cabinets have the lights on the sides.
Yet the plant still grows upward.
In a propagation chamber it doesn't even matter of the direction of the light.
Only chlorofil.Its a process of evaporation.
Like the slow sucking of the life out of the seed.
In fact the seed, starts to die, just as it starts to live.
Pieces of its cellular structure, decay.

Read the link about growing plants in zero G, it's very enlightening.

Imagine a seedling with roots and sprouts in all directions...

Originally posted by Chadwickus
lol I'm a horticulturalist and I don't know the whole answer

Simply put, the seed, from the start is programmed to know what's up and down.

Same as it knows when to germinate

huh-huh....heh hu eh *cough*....dude...you said horticulturalist.....hehe hehe....you gunna hit this or what?

Could you not have googled this? Its simple plant biology.

Questions like this are great. I think the OP was looking for somehting deeper than a standard yahoo answers response.

Originally posted by HiAliens

Could you not have googled this? Its simple plant biology.

Questions like this are great. I think the OP was looking for somehting deeper than a standard yahoo answers response.

I was, and I think I finally got a sufficient answer as I googled for all kinds of terms found from responses in this thread. Thanks for everybody.

I didn't google it because I didn't know the terms in english.

It is to do with positive and negative phototropism and geotropism.
Roots are positively geotropic and negatively phototropic; they grow towards gravity and away from light.
Shoots are positively phototropic and negatively geotropic; they grow towards light and away from gravity.

I know, I know. Really sad for knowing.

The simple truth is no one really knows. When that cold quiet seedling receives the touch of something infinite and life is born. Sure, they have theories and really good ideas on the subject, but no one really knows just how or why that little spark of life takes hold.

Like an idea, the seed will take hold and then nothing, not even a slab of concrete, can stop it from reaching for the light.

We live in an amazing world.

Sometimes it's enough to just kick back and say, That's really awesome.

[edit on 14-3-2009 by mrwupy]