RESOURCE GRADIENTS. “It is growth along gradients of resources that lead both root and shoot to eventually meet rich pockets. Exploratory shoots will grow along light gradients to the more intense regions and roots will grow along humidity and mineral gradients. Information transduced by resource sensors thus directs behavioral responses that increase the probability of attainment of richer sources of food in the near future.”
http://link.springer.com/article/10.1007/s00114-005-0014-9