Towards nutrients, including water. They are actually darn good at finding water. They will break through metal pipes just to get to water.
I would say you don't need to look it up, as it is fairly obvious. You understand it completely.
The problem I had with your response earlier is that you don't see turning towards the Sun the same as growing towards the Sun.
What causes a seed to actually sprout?
(Hint: again, it's not gravity)
I imagine temperature and water might do the trick via chemical signals that activate transcription factors that transcribe genes to produce the proper protein signaling molecules to start the ball rolling; but I just say that as a former farmer and current molecular biologist who took botany a LOOOOONG time ago. ;)