Competitive roots Much of the world's plant biomass exists out of sight underground in the form of roots. Cabal et al. developed a theoretical model and tested it empirically to explain the rules that govern root growth (see the Perspective by Semchenko). Plants adjust how and where their roots grow according to how close neighboring—and competing—plants might be. The model extracts some of the rules about how root balls differ when grown close to neighboring plants compared with being grown in the absence of competition. Science , this issue p. 1197 ; see also p. 1167