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There was an error in the funding statement. The correct funding statement is: "This work was supported by Marie Curie European Reintegration Grant N. 239324 MIPAN (Memory and information processing in assemblies of neurons) within the 7th European Community Framework Program, http://cordis.europa.eu/fp7/dc/index.cfm [^] (MC), Ente Cassa di Risparmio di Firenze, URL:http://www.entecarifirenze.it/ [^] (SM) and EU ERC Project N. 227878 COLLMOT (Complex structure and dynamics of collective motion), http://erc.europa.eu/ [^] (TV). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript."

Interactions between individuals that are guided by simple rules can generate swarming behavior. Swarming behavior has been observed in many groups of organisms, including humans, and recent research has revealed that plants also demonstrate social behavior based on mutual interaction with other individuals. However, this behavior has not previously been analyzed in the context of swarming. Here, we show that roots can be influenced by their neighbors to induce a tendency to align the directions of their growth. In the apparently noisy patterns formed by growing roots, episodic alignments are observed as the roots grow close to each other. These events are incompatible with the statistics of purely random growth. We present experimental results and a theoretical model that describes the growth of maize roots in terms of swarming.