Any of a group of hormones that regulate plant growth, particularly by stimulating cell elongation in stems and inhibiting it in roots .
Auxins influence the growth of stems toward light (phototropism) and against the force of gravity (geotropism). Auxins also play a role in cell division and differentiation, fruit development, the formation of roots from cuttings , the inhibition of lateral branching, and leaf fall. The most important naturally occurring auxin is beta-indolylacetic acid.