Chad Clark's Open Journal : 2007-06-13

1) Plant root growth appears to be a function of whether neighbouring plants
are genetically related or not.

  Dudley and File grew batches of a beach-dwelling plant, the Great Lakes
  sea rocket (Cakile edentula), in pots of four, either with specimens from
  the same maternal family or from several different families.
  
  Those growing with strangers had a greater mass of roots after two months
  of growing than those sharing pots with siblings.

http://www.nature.com/news/2007/070611/full/070611-4.html