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After cutting my stem plants today and planting the tops, I wondered how does the planted top know that its bottom stem is in the substrate to begin rooting there. Does anyone know of any scientific explanation of how it works on a biochemical level? What are the hormones that get activated/accumulated in the bottom part of the stem? How does it get activated? If anyone has any leads, please share them!

Best,
Kev
 

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That’s a great question and I’m sure there’s a straight forward answer for it, but without googling it, I’d guess they use light to figure it out. Plants reach for the light to photosynthesize, and either stretch for it when there isn’t enough, or avoid growing towards it when there’s too much. I think of it like an equilibrium. Just like the fluid in our ears help us keep balance and orient ourselves, sense what is up and what is down, they probably use light like a compass, they know what is up and what is down, and down is where the nutrients are, so roots will seek it out. Plants are living, just like us. Evolution! Haha. …oh yeah maybe gravity comes into plays as well haha.

Now I want to grow a plant suspended in a nutrient jelly in 0 gravity with a light source that completely surrounds it and see what happens.
 

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Auxin is the primary hormone involved. The very quick and dirty explanation is that auxin is produced in large amounts at the shoot tip and is transported down to the roots, with the concentration and directionality of the auxin triggering a bunch of developmental stuff along the way. But when you sever a shoot from the roots, auxin builds up where the cut is because it can no longer get to the roots, and this increase triggers adventitious root formation.

Auxin is also the primary mechanism behind gravitropism and phototropism, respectively plants' ability to grow opposite gravity and towards light, but this is a different process and it deals with cell elongation. (IIRC, phototropism beats gravity every time, so if a plant needs to grown down to reach light it will do so.)
 
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