I actually researched on this, but nothing was found regarding this. The only thing I found is that auxins grow on the farthest side causing it to elongate. Yes, but how exactly do the plants know where the source is. In the case of water, For example, the roots always grow toward the water but how do they know it is water?
How does a plant know where the water source is?
Some web pages regarding it, but my question has no answer: