MEMORY CANDIDATE. “One promising candidate, as long as we attribute individuality also to cells, would be auxin canalization, i.e., gradual tuning of auxin transport across cells and tissues, based on their previous experience and resulting in increased auxin flow in cells that already have transported auxin. This phenomenon, which has been proposed as a major factor determining e.g., the topology of leaf venation.”
http://www.tandfonline.com/doi/full/10.4161/psb.4.5.8276#abstract