Photosynthetic roots.

[Ray]

Originally Posted by bil

Not the chlorophyl. I think that the photosynthesis is simply the way of the plant 'seeing' the direction and if it is that way then it is interesting that the mechanism is the opposite was round.

Chlorophyll's purpose IS photosynthesis. Phototropism is controlled by hormones like auxins and gibberellins.

On this pic, they are reaching for the shade. There's no moisture there.
If that top root was headed for moisture, it would be going downwards to the moss.

I didn't state my response clearly enough. I didn't really mean "looking for moisture", but more like looking for where it might be. Let me try this way:

Think about that plant in nature, bil. Growing attached to a tree, the drying sun will be in one direction, and the tree - being a host "anchor", shaded by the leaves and protected a bit from breezes - will be the likely place the plant would find moisture, and not be dried out by the heat of the sun.

Maybe that is a function of (negative) phototropism, but again, not due to chlorophyll.

[bil]

Originally Posted by Ray

Chlorophyll's purpose IS photosynthesis. Phototropism is controlled by hormones like auxins and gibberellins.


I didn't state my response clearly enough. I didn't really mean "looking for moisture", but more like looking for where it might be. Let me try this way:

Think about that plant in nature, bil. Growing attached to a tree, the drying sun will be in one direction, and the tree - being a host "anchor", shaded by the leaves and protected a bit from breezes - will be the likely place the plant would find moisture, and not be dried out by the heat of the sun.

Maybe that is a function of (negative) phototropism, but again, not due to chlorophyll.

No, I did say due to auxin production. The processes in the tip include auxins, steering compounds. Perhaps I didn't say it clearly. The tips photosynthesise and grow, but that process is to steer the shoot in the opposite direction to how it works in shoots so that it steers the tip into the dark where, as you say moisture is most likely to be found.

[orchidsarefun]

Originally Posted by bil

No, I did say due to auxin production. The processes in the tip include auxins, steering compounds. Perhaps I didn't say it clearly. The tips photosynthesise and grow, but that process is to steer the shoot in the opposite direction to how it works in shoots so that it steers the tip into the dark where, as you say moisture is most likely to be found.

I also think that the plant can adapt to changed conditions. If your growing conditions have a high ambient humidity, then there is a tendency for exterior roots to grow and thrive. In conditions of low humidity the roots in the pot grow ( more) and thrive. This is particularly noticeably in phals and especially phal seedlings. An extensive root system helps with the 'survival of the fittest' concept. My ambient humidity on my grow stands is consistently in the 50% range. When the humidity levels dropped into the 30's for a few days, all the exterior roots immediately suffered.
Its a balancing act to keep roots healthy. I have problems keeping mounted plants happy in Winter as ambient humidity in my various grow-rooms varies too much even though I have humidifiers.