I've got a riddle for you

Or perhaps more accurately, a conundrum for you to consider.

I recently posted an image of a large Hylocereus Undatus ( purple dragonfruit) plant growing on a trellis in a container. Here is said picture.




Now, what the picture doesn't show is that there is a large steel beam on the right side of the plant which is one of the supports for a pergola (flat roof, 4 beams, you get the idea).

So here's where it gets interesting; do you see that very large tendril stretching out to the right side? It's doing that in order to reach that pole. In fact, that specific tendril is at least twice as large as any other on the plant, and it had to grow at a very unusual angle to even start heading in the right direction. It wants to climb up it. Dragonfruit are semi-epiphytes, meaning they attach aerial roots to the trunks of trees and draw nutrients from the bark of the tree. It's for this reason that you don't want to grow a dragonfruit up a tree trunk--it will, in time, completely take over the tree, and even if you were to cut the vine off at the base, it will simply keep feeding off the bark and send shoots back down to the ground.

I took this picture a few years ago of a feral dragonfruit in a local park. It is slowly killing a gigantic eucalyptus that is about 50 feet high, and some of the aerial roots were as thick as my arm. I don't know how old this vine is, but it must be ancient.



Anyway, I'm getting off topic:

So you might think that there's nothing unusual about such a species growing towards structure it can climb up. But here's where things stop making sense:

We all know that plants grow towards the light. But here's the thing--the pole is shaded. All the time. The plant has in fact grown away from the light (at a very peculiar angle in order to do this) not towards the light, in order to try and attach itself. The crown of the trellis is actually on a slant which points towards the pole, because the new growth is favoring that location.

So the next step is to assume that it is growing towards the shade. But this doesn't make sense either, because dragonfruit have a very predictable habit; that is, they will grow vertically until the reach the canopy/ the top of whatever they are growing against, and then they will cascade outwards, with the new growth pointing back towards the ground. This is how they grow in the wild. This enables them to catch a lot of light, and create shade at the feet--which they also require for optimal growing conditions. Their objective is to reach full sun, and once that happens, their journey is over.

And here's something else to think about; there's also shade at the left side of the plant created by the left pole, but the plant has not grown a massive tendril trying to reach this. It is about twice the distance away from the plant as the right pole--which is too far, as the tendril could not support itself at that distance and would snap off. However, both poles produce columns of shade that reach the plant at different times of the day, so the plant is experiencing shade coming from both sides.

So here's my question (S): How does the plant know where the pole is--and more interestingly, how does it know the right pole is closer to the plant--is at a distance that is feasibly able to be bridged by new growth, when it experiences pretty much the same level of shade from both angles at different times of the day?

I have no idea. I am, in a word, flummoxed.

Thoughts?