Growth initiation in the geotropic response of the wheat node.

The geotropic response in stems of Triticum aestivum occurs at the leaf sheath bases. Growth is initiated in the lower halves of these organs after orientation into the horizontal position. Lateral transport of growth hormone is not involved in the development of curvature. Continuous stimulation is necessary to maintain the response.Growth can be initiated in small pieces of tissue excised from the leaf sheath base but this is entirely dependent upon the precise orientation of the tissue in the gravitational field. Growth occurs only when the segment is orientated horizontally with the outer epidermis facing downwards. IAA is able to initiate growth in nongeo-induced tissue segments of the leaf sheath base but this effect appears to differ from the geo-induced growth.