You put seeds of any type in soil and they always come as sprouts out of the earth instead of going deeper into the soil.
I'd like to see an explanation at molecular level or any perspective that is convincing and informative.
Just curious, though. ^_^
Thanks.

hey, no problem.
to put it basic,
when a plant germinates the stem will "sence" the light.
then there is a cell in the top of the stem that excretes a chemical called auxin, this moves away from the light and causes cells to elongate, so it causes the stem to "push" the stem toward the light.

you can create an experiment showing this my setting up a jar with wet tissue in it, place some seeds inside it at differnt angles and obseve what happens, when i did this my seeds grew for a while then it got a mold and died

good luck anyway

that is true. However, when a plant is in the ground, there is no light to sense in either direction. the signal that acts here is gravity. stems have negative gravitropism, meaning that they generally grow opposing gravity. how this is done is by the lateral movement of auxin through the stem of the cell. The uneven distribution of auxin is responsible for the growing upward of a stem in the ground.
Assume the stem has grown laterally for example. the side that is down receives more auxin which stimulates its growth. the side that is up receives less auxin, which inhibits its growth. therefore, the stem curbs upwards.
it is interesting to note that the same mechanism makes a root curb downwards. the difference is that the root has a different receptor, which has an optimum level of auxin lower than that of the one in the stem. in the rout, the higher concentration of auxin actually INHIBITS the growth on the side that is down. however, the lower concentration on the up side is enough to stimulate the root to grow on that side. therefore, the rout grows downwards.

So what happens when plants are grown in space?

Good question
My guess is they will grow in whatever direction they happen to begin growing unless they hit an obstacle(and ethylene directs the plant going round it) or there is a unidirectional source of light(case in which phototropism takes over: the stem grows towards the light and the root away from light.

However, something that cannot be explained by the current model of gravitropism(which is not well established, lots of things about it need to be tested) is this situation: the stem grows DIRECTLY down(without being deviated at all to neither side). in such a case, the concentration of auxin will remain the same on both sides and there will be no bending. However, this never happens. so, there is still more to be discovered.

Thanks a lot! Enlightening!