The plant hormone auxin governs directionality of the growing tip (root or stem) in combination with a bunch of other hormones.
Auxin can provide phototropic, geotropic or hydrotropic response, or any combination.
This is a good summary on hydrotropism :
Hydrotropism - Wikipedia , which I believe has mainly been studied in plants with air roots, which can detect and grow towards the higher side of a humidity gradient. This mechanism is very sensitive and can be to a gradient we growers would have no hope of detecting with ordinary instruments.
I have never seen vandas growing in nature, but I often see their smaller relatives like sarchochilus that grow around here. They scramble, the roots nearly always angling back to the growing surface from which they get moisture, nutrient and stability. Some plants have a high percentage of roots growing in the air, particularly when there is a drop-off immediately below the plant, but this is not ideal and is associated with a high risk of desiccation in dry times.
So my view, yes, I would expect them to grow towards the nearest darker or moister surface. I think the response would be both phototropic and hydrotropic, but I believe the latter would be much stronger as a phototropic response seems to me to be just too easily fooled into a really unhelpful growth habit.
And yep, there must be a geotropic response going on as well, evidence by the fact the roots grow downwards, not up.
A mechanical hold is an inevitable byproduct of this strategy.
Coconuts, it’s time for you to do some experiments and maybe you could report your findings somewhere. I would do it but vandas grow so slowly in our climate we would never get to see the results.
Auxin - Wikipedia
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