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When a seedling starts to grow covered by soil, I thought directions are clear:

  • Roots grow following gravity (down to the water), and the cotyledons (right word?) grow against gravity, so that they'll reach sunlight eventually.

  • Once the roots "find water" they'll follow it, while the cotyledons grow towards the light with the usual mechanism that the lit side grows a bit slower.

However when watching my paprika seeds grow, I found one case where the roots grew "up in the sky" while the cotyledons were still sticking in the soil. After some time the roots started their way down, however.

It could be that the strange situation was caused by a rather wet environment (a glass covered with plastic foil), however.

Here are some photos I took (not the best due to small size and using a macro lens):

  1. Roots coming out first
  2. Roots growing into the air
  3. Cotyledons coming out, roots change direction to horizontal
  4. Roots still in the air
  5. Cotyledons detail
  6. Roots beginning to grow downwards
  7. Root forked, growing downwards on the outside
  8. Cotyledons unfolding
  9. Cotyledons and roots detail
  10. View from side showing roots (blurred by water drops, unfortunately)

From these observations I might guess that roots need some initial "dryness" to "know" the direction to avoid, and it seems the roots are important to let the cotyledons grow upwards, to that they reach sunlight eventually.

(another interesting question would be: What triggers the seed to start growing? Is it "temperature plus moisture plus sunlight", or is it just "temperature plus moisture"? I had to wait quite some while when the seeds refused to "come out")

Who can explain the details?

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  • $\begingroup$ What triggers seed germination depends a bit on the plant species, usually moisture is required for germination. In addition, some seeds only germinate after a period of cold, or other stimulus. The terms of interest are seed germination (what you call growing), and seed dormancy. Breaking seed dormancy is required for germination. $\endgroup$
    – Niklas
    Commented Apr 30 at 18:39

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Plants moving toward/away from light, water, temperature, and gravity are referred to as tropism.

Phototropism

Liscum et al. (2014) provide a nice review.

Phototropism, or the differential cell elongation exhibited by a plant organ in response to directional blue light, provides the plant with a means to optimize photosynthetic light capture in the aerial portion and water and nutrient acquisition in the roots. Tremendous advances have been made in our understanding of the molecular, biochemical, and cellular bases of phototropism in recent years. Six photoreceptors and their associated signaling pathways have been linked to phototropic responses under various conditions.

Gravitropism

Chin et al. (2022) and Sato et al. (2015) provide reviews.

The physiological process of root gravitropism comprises gravity perception, signal transmission, growth response, and the re-establishment of normal growth. Gravity perception is best explained by the starch–statolith hypothesis that states that dense starch-filled amyloplasts or statoliths within columella cells sediment in the direction of gravity, resulting in the generation of a signal that causes asymmetric growth. Though little is known about the gravity receptor(s), the role of auxin linking gravity sensing to the response is well established. Auxin influx and efflux carriers facilitate creation of a differential auxin gradient between the upper and lower side of gravistimulated roots. This asymmetric auxin gradient causes differential growth responses in the graviresponding tissue of the elongation zone, leading to root curvature

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  • $\begingroup$ I agree that those are the two mechanisms I had suspected, but still there is something unknown: Obviously roots are not phototropic (as an empiric heuristic "water is not where sunlight is"), but the question is: "Are roots gravitropic?" (as I would expect, but the photo arises some doubts). So is there a third mechanism like "hydrotropism", and if so to roots use gravitropism or hydrotropism, or a combination of both? $\endgroup$
    – U. Windl
    Commented May 3 at 7:00

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