Plant Growth Substances Produced by Micro‐organisms of Soil and Rhizosphere

S ummary : Micro‐organisms isolated from rhizospheres and rhizoplanes of wheat plants, and from root‐free soil, produced growth regulating substances with the properties of gibberellins and indolyl‐3‐acetic acid (IAA). Substances inhibiting extensions of pea plant internodes and lettuce hypocotyls were also produced, especially by bacteria from the root region of seedlings 6 days old. Bacteria producing growth promoting substances were most abundant on roots of older plants. Seedlings grown aseptically with added gibberellic acid (GA 3 ) and IAA, or grown with a soil inoculum, developed similarly and differed in their morphology from those grown aseptically without additives.