1. @jacob What a great question? Hell if I know but could it have something to do with part of the plant seeking light? Oh my, I answered your question with a question…

  2. @jacob A basic answer is that any plants that didn’t would not have survived. The mechanistic answer has to do with the accumulation of plant hormones on the lower side of the root or shoot. That hormone causes cells to elongate in the shoot. That curves the shoot upwards, and levels of the hormone even out on both sides. I am not sure of the exact mechanism in roots, and I imagine it is mostly similar. Of course, there are different mechanisms in lateral branches.

    Charles Darwin did the basic experiments on oat seedlings in the 1870s, IIRC

  3. @jeremycherfas thank you. But does it explain just why the shoot grows out of the seed or why it grows up? I mean that my understanding is that the shoot might grow out first downward or to the side because of the seed orientation but then would curve to grow to the up side. Something must give it direction. So it could be gravity or light (if it penetrates) or something about air underground. Gravity would be the most stable force but I don’t know biology, so left puzzled.

  4. @jacob Gravity is the main signal while the shoot is underground, and probably continues to be the main signal for roots. Once the shoot is in the light, the light reacts with the plant hormone, which sets up the gradient whereby the cells on the darker side of the shoot elongate more.

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