Germinating seedlings know which way is up. Perhaps they just know which way is down. They can not see, hear, taste, smell or feel anything like we can. Nonetheless, they know which direction to extend their first root and stem. One thing that they can perceive is gravity. Gravitropism, which was formerly known as geotropism, is how they respond to gravity, or the Earth that generates it.
Positive gravitropism is why the first root to emerge from a seed extends downward toward gravity. Negative gravitropism is why the first stem to emerge from a seed extends upward away from gravity. Positive is toward. Negative is away. Roots and stems that develop after the first, disperse in other directions in response to other stimuli, but never really forget where gravity comes from.
Once a primary stem of a seedling emerges from the soil, it immediately responds to sunlight. Just as it exhibits negative gravitropism to grow away from gravity, it exhibits positive phototropism to grow toward sunlight. Since sunlight comes from above, positive phototropism is compliant to negative gravitropism. Branches will later disperse laterally to avoid the shade of other branches.
While branches are finding their way in the World, roots are doing the same. All of them can not always reach for the center of the Earth. They branch and disperse laterally as they sense that they are sufficiently deep in the soil. Those that venture too deeply sense an inhibition of gas exchange. Those that are too shallow sense if they get too warm or dry. There are a few types of tropisms.
Roots respond to moisture, nutrients, chemicals, temperature and mechanical stimulation within the soil. Branches and foliage respond to humidity, wind, temperature variations and air pollution. In order to function within their dynamic situations, plants somehow coordinate their responses to all of the many stimuli they experience. They are impressively perceptive, as well as responsive.
Even celery, green onions and leafy tops of carrot in a refrigerator can reach upward away from gravity.
Tony Tomeo 
I intuitively knew some of this, but not all of it. After reading your post, I had the impulse to say, “Good morning!” to the veggies in my refrigerator.
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They respond to various stimuli, but won’t respond to that.
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Great tutorial. Nature is so interesting.
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Thank you. I typically describe ‘gravitropism’ as ‘geotropism’ , but thought that I should use the correct terminology instead after noticing the ‘the Laidback Gardener’ wrote about it properly.
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When I was weeding today, I came across many germinating seedlings below the ground (given the area in the garden, probably Acer campestre). I will be very interested to see what they do now I’ve up-ended them! Given that this is a strong native species, growing in a good position for it, I imagine that they will all correct themselves, providing me with even more weeding ‘opportunities’ in 3 weeks time or so! Very interesting post, as always, Tony.
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Yes, they know which way is up . . . or down. As an arborist, I inspect destabilized trees, and can determine how slowly or suddenly they were destabilized. Those that are leaning away from a group of other trees, but that curve gradually upward, likely grew that way just to get away from the shade. Those that destabilized will have a straight lower section of trunk that leans, with a ‘kink’ or an angle in the trunk where it suddenly turns vertical (after stabilizing). I can see it even if the formerly horizontal (but now slanted) lower limbs of a fir tree are long gone. A tree that is is leaning with a straight trunk from top to bottom just recently became destabilized.
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Such a fascinating subject. And your observations are remarkable and stored in what brain I have left!
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I did not know tis. Not the gravity part. Perhaps I don’t remember it from biology class, but…always happy to learn a new thing!
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Gravity is important for when they can not see sunlight. I suppose it is also important for keeping the Earth from exploding too. Yes, that is sort of important.
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That and the spinning.
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Gravitropism seems more mysterious than some other behaviors, like phototropism. Somehow responding to the sun seems more understandable than responding to gravity.
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It makes sense to the plants, and that is more important.
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