Valera, Concepcion L. (Cornell University, Ithaca, N.Y.), and M. Alexander. Nodulation factor for Rhizobium-legume symbiosis. J. Bacteriol. 89:1134-1139. 1965.-Nodulation of excised roots of Medicago sativa was enhanced by an extract of alfalfa seeds but not by several other substances. Coconut water exerted a similar influence upon the formation of nodules on excised roots of Glycine max and Phaseolus vulgaris. Direct evidence for a host factor concerned in nodule genesis was obtained by demonstrating that, although P. vulgaris seedlings developing from embryos devoid of cotyledons nodulated in the presence of an infective Rhizobium strain, nodules did not appear on explants derived from hypocotyls obtained from ungerminated seed. The legume factor, when examined by a soybean hypocotyl bioassay procedure, could be replaced by a water-soluble, thermostable, dialyzable fraction of coconut water, but not by meso-inositol, scyllitol, sorbitol, yeast extract, or a number of amino acids.