Definition: Phytohormones are chemical substances other than nutrients produced naturally in plants which regulate growth, development, differentiation and a number of physiological processes, e.g., auxin, gibberellins, abscisic acid, cytokinins.
Function of Auxin:
Cell Enlargement. They bring about growth of cells. In shoots auxin is effective above a concentration of 10 ppm while in roots the required concentration is 0-0001 ppm.
Several plant movements are mediated by differential distribution of auxin, e.g., geotropism, phototropism.