Distribution and activation of the Golgi apparatus in geotropism.

We find a differential distribution of dictyosomes in the avascular tip cells of the oat (Avena sativa) coleoptile upon geostimulation. A differential activation (increased vesicle production) of the dictyosomes with respect to gravity also occurs, but only in the tip cells of the lower tissues. Similar differences in distribution and activation of dictyosomes occur also in cells subjacent to the avascular tip (1st and 5th millimeter from the apex) of both the upper and lower half-tissues. When only the outer epidermal cells below the apex are considered, the differential distribution and activation of dictyosomes occur only in the lower outer epidermis. The changes in distribution of dictyosomes begin at 6 minutes, or sooner, from the start of geostimulation, before the onset of geotropism. The kinetics of amyloplast sedimentation and Golgi movement do not appear to differ in the cells of the avascular tip. We suggest that the Golgi participates in, and possibly initiates, the differential elongation of cells of geotropically stimulated coleoptiles.