- Summary 1 Competition usually involves the allocation of limiting resources to non-reproductive functions. Natural selection is expected to favour mechanisms that increase competition with non-self neighbours and limit wasteful competition with self. 2 We used Pisum sativum plants that had two roots and ‘double plants’ with two shoots and two roots that could be either longitudinally separated into two genetically identical but physiologically distinct individuals or left intact. 3 Root development was significantly greater in split-root plants whose neighbours belonged to different plants. Furthermore, root development was relatively greater in the presence of roots of a different plant, regardless of its identity. This discrimination had a vectorial component whereby plants developed more and longer lateral roots towards neighbouring roots of different plants than towards other roots of the same plant. 4 The results thus demonstrate a mechanism of avoiding self-competition that is based on physiological co-ordination among different organs of the same plant rather than on allochemical recognition that depends on genetic differences. 5 The ability to discriminate between self and non-self could be expected to increase resource use efficiency and ecological performance in plants. It could also be expected that tight physiological co-ordination will be selected for between organs of the same plant that have greater probability of being engaged in direct competitive interactions with each other.